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When a curved stairway is freestanding, its up to the stairbuilder to incorporate all means available to make it structurally sound.
The inside stringer is not much of a problem. It has a steeper pitch and a shorter run than the outside stinger.
The outside stringer is the “bad boy” to make it strong enough to support the dead load of the stairs and any live load.
Further complicating this is when the outside stringer is of the open stringer style. Then you have the stringer being weakened by the riser/tread cutouts.
I have compiled a list over the years of structural techniques that increase the strength of this outside stringer. These techniques have been gathered by personal experience, visits to other stairshops, and questions I have asked of others.
This subject fascinates me and I know there are ideas in this forum that are NOT on my list.
I will give 5 milkbones to good ideas offered that are on my list.
Ten milkbones to any good ideas that are not on my list. I will be honest.
Now, to the one that offers what I feel is the best idea, or ideas, I will send a Stan’s Stair Shop hat as pictured.
Replies
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Stan, I haven't built any curved stairs yet, but I hope to, and have been thoroughly impressed by the ones you have posted (are you looking for an apprentice?). That said, I would think resisting the torsional forces would be paramount. If I were to do a curved stair, I would want to tenon in the top and bottom of the stringers to the available framing, and I would want the inside stringer to help resist the live load so I would dado and glue the treads and risers together, and dado (and wedge, if covered later) both into a solid stringer. If the stringer is notched, I would screw (from above w/ bungs, or pocket screw from below) the treads and risers to the stringers, so the outside stringer pushes against the inside stringer. If the stair has balusters I would idealy dovetail them into the treads before the tread returns go on, and sandwich the tops into the handrail so it is locked securely to the stair assembly. From a structural point of view, a solid rail formed out of framing lumber and thin layers of plywood would form a very strong beam. Of course laminating the stringer, with all plys going lengthwise along the stringer, is the strongest way to make an open stringer.
I realize these are probably basic suggestions, but as I have not yet built a curved stair, they are all I can offer....
MM
*Mike: You are off to a good start. I would swear you have built one by the ideas you have presented.You had 6 methods that are all on my list. Very good ideas. That gives you 30 milkbones. For those interested, I will go over Mikes suggestions. I learn so much from reading others posts, its my turn to contibute what little I know: Tenons or extending the stringers into the framing is a big plus. This gives much needed leverage to resist torsional forces as Mike so well put.The dadoed risers and treads is very strong. In my case, I use pocketscrews to attach the risers to the treads. This I believe is as strong as it gets.The dovetailed balusters is definately on my list. It is a time-proven method, and is originally one purpose for the tread returns. These covered the dovetailed balusters. To be honest, I have never done a whole stairway with this method. One reason is economics, the other is I use bolts that screw the balusters down tight. If it were not for the time involved, I would vote for dovetailed balusters.The laminated stringer is no doubt the stongest. There are several techniques that I will save for someone else to mention that are very important in laminating to increase the strength.Good start, Mike.
*Stan, I have yet to build a freestanding curved staircase, but look forward to it very much.Your work and posts are impressive and interesting, keep it up. I believe the deep riser( that goes from tread bottom to plaster ground in one piece) and extra-thick riser(1"-2")and also the thick outside stringer are all sensible ways to strengthen a freestanding stair.I think all of these are covered in George R. di Cristina's book.I have seen old illustrations that show diagonal blocking between stringer/riser junctions like joist blocking on its side.This would seem to help stop any sway and divide the loads accross the stair.Hope I have at least brought up some things to think on.I love stair work and hope to work up to this someday.
*Stan,We make our outside stringer out of 7 lams of luan glued w/a good yellow glue and shot together with narrow crown staples. The ripped strips of luan are just shy 16". Is that similar to your method?Allen SchellP.S. We try to build a giant pitch block (2 or 3 ft.) into the lower end to add strength.
*Like Allen, we lam the stringers only difference being 9 insted of 7 slips. The idea in a fast explanation is that similar to a bridge, the load is spread over the entire length of the rise with the rail and balisters being part of the entire support structure. The top and bottom mounting points being key. Material used is a large factor as to size and load capacity. I guess the inside sringer could be looked at as a stabilizer, while the outside one being the load carrier. Another key point to remember is all joints have to be perfect and of a cut to accually support when being pulled, rather than being pushed as in a supported staircase, if not it will only add weakness or be a weak link.By the way, explaining through typing is a hell of a lot harder than showing someone.
*never have stan...but both of my suggestions involve composite construction....for those jobs where you are pushing the envelope...1) two of my outside laminations would be epoxy resin in long fiber oriented matt..similar to snow ski lamination..also along the lines of snowski lamination.. the "cap" ski technique which i think makes the ski into a modified box beam...but i bet you 've already got some of that ..2 ) another strengthening technique which i have used was in a large mulled window install..Andersen provided thin steel bands about 3 inches wide, the full height of the window.. these had holes in them and were "captured" by screwing thru the jamb , thru the band , and into the abutting jamb.in efffect this made a vertical flitch plate out of the mulled jambs...those windows never move.. and this is a high wind waterfront install.....size 7 3/8....
*Mike...I did one of those huge window mulls...Marvin custom....xmas tree stack of 2x2 windows ($15,000!!!)...came mulled in three side by side groups that killed four of us to lift, move and install...Little metal bands...And did the same for the stacked windows of the Faux Wright hillside cantilever home in Weschester next to Susan Sarandons shack.I say epoxy too if though the oldtimers had not...near the stream,ajStan...Ya gotta hold the next Fest at your stairworks and we'll build as many of your stairs as we have beer left...Then you sell em and we gots an edumacation.Gotta go ski now...fresh falling...
*Mike,What kind of fiber mat is that? I'm not familiar w/that but it sounds interesting.allen
*allen... any of the marine supply houses carry woven roving... matt & finish cloth in various widths,,..i've used rolls of finish cloth , for instance, glassing the chines of a rowboat... the tapes come in various widths... 6"....8" etc..if you lay it up full length the long fibers will be aligned with the stringer... and the short fibers will be cross-grained...a second lamination could be 45 degrees out of phase with that.. and a third could be parallel again.. ..laid-up with the wood laminations .. i would think this would develop tremendous strength..but it would be trial and error unless some of teh marine architects or fiber mfrs have developed the engineering for it...
*Allen/Chris: Basically I lamimate my stringers the way you guys do. I did have one that was 12 laminates. Typically I have 7 or 8, yellow glue. However, I always clamp them together in one huge glue up with 3/8 bolts and 24 inch clamping cauls.I know this makes a nice strong stringer. I do not want to use any staples or metal fasteners, as I route the treads and risers into my skirtboards if its a closed stringer. If its open, I would not want to be hitting staples cutting the miters. Do you guys lay them out so as to know where not to staple? Mike: The steel bands have been tried also with success. I have placed a welded 10 gage x 11 inch and glued it in the stringer. Then I still laminated more wood on the interior so as to be able to route in my treads and risers.One thing I always do on a freestander is scarf joint all my laminates. Also the laminates are solid boards milled down. A stringer made up of solid laminates scarfjointed is very strong. If the stairs are not freestanding, then I use luan, butt joint them, and face them with solid wood scarfjointed. Poplar if its paint-grade, or oak, hickory, etc.You guys are coming in with some good ideas, but I need to steer someone towards a tensioned cable system. AJ: I would have you over to ski, but two problems, no snow, no slopes. I am a flatlander.
*Sailboarding on the closest lake, vball and tennis then!... Stan...near the stream with my priorities straight,aj
*Stan, Beautiful stairs!I was just wondering, have you ever done an "s" shaped free standing set or a "y" shaped free standing set? And if so could you post pics? And what kind of special difficulties did they present?
*Stan, do you let a cable into the underside of the outside stringer? If so, I would think tensioning it would tend to pull the laminations off the inside. Maybe you could cut a groove in the lower half of the outer side of the outside stringer (huh?), let in a cable, and cover the groove with a strip of wood. Send the cable through the framing at top and bottom, and us a turnbuckle to tension it. This would tend to make the stairs want to straighten out, but would stiffen the stairs. Another option would be to run the cable in a straight line, from the bottom of the outside stringer to the top of the outside stringer, under the treads and risers. I would think the curve of the stairs would make the cable "sag" along its length, so when it's tensioned it would straighten out and lift the stairs. I'm not sure how much this would actually add to the stiffness, though. Seems like the cable would have to be pretty hefty and really tight to do anything. Am I on the right track?MM
*Stan , I am just building my first set of curved stairs , not free standing . I cheeted and built my stringers out of 1x12 pine three ply yellow glue and screws , then I cut triangle blocks for the treads and glued and screwed them on the stringers. I plan to cover the stairs with walnut when I'am all done .As far as stressing cable I think that 1/8 " cable would be stong enough . Route a groove about 3/4" from the bottom of the stringer, and one more about in the middle where you will not cut it . the grooves run from top of the stringer to the bottom in the next to last lamination . lay in the cable with a foot or two extra on each end,cover with the last lamination. Then you could fasten one end and pull the other to tighten. Any where close to what you had in mind?
*Sorry I have been away from the posts here.Mr. Styles: Thanks for the compliments. No, I have never done an s-shaped stairs-I have done s-shaped railing however. I have never done a y-stair--sounds like it would be interesting.Mike: Let me clarify myself on this cable idea. I am pursuing methods of using cable tensioning to help support the outside stringer. To be honest, I have never attempted this method. I am in the r&d phase of it right now. I have taken a cable and went like you suggested from the top corner of the outside stringer to the bottom corner of the outside stringer. If one puts a standpipe midspan to let this cable triangulate tension into an upward thrust, some success has been achieved. I am seriously seeking some new ideas along this line. The biggest problem is the rolling moment of the outside stringer as to it just sagging.Don: Good luck on your first curved stairs. Sounds like you will come out fine. If I can be of any help, I will be more than happy by e-mail or in this forum. The cable following the curved stringer, in my opinion, would not give much upward thrust, but would tend to start pulling the inside laminates apart as Mike is suggesting.Heres the last freestanding stairs I just finished last Monday. There are no cables in this stairway.It was the toughest freestanding I have done. 19 treads, 70 inch outside radius, 18 inch inside radius, two changes of pitch sections as the stairs starts with 4 flared treads, 11 curved, 4 straight at the top. This stairway prompted me to start this thread, as I would be tickled to have some cute cable tensioning method that works. Otherwise, many of the ideas mentioned in these posts here are utilized in this stairway, along with several more that have not been mentioned.
*Heres a side shot of the same stairs.
*Notice on the picture in post #16 that the base of the stairs has a large footprint on the floor. This is one very important part in strengthening this stairway. There are cantilevered supports hidden in the belly of this stairway as well.
*tan , You can't be too far from me depending on your location from Champion, I live just south of Streator . I would like to know where you get the fine lumber that you use? Around here we have a limited supply of hardwood from the small saw mills or the finger joint stuff at the lumber yards. By the way if my stairs turn out just half as good as yours I think I'll be happy , you do fantastic works of art. As for supporting the outside stringer wouldn't tianglating off of the inside stringer be easier than running cable? On the cable issue , what about if you could fasten to the top of the stringer at both ends and at the bottom in the middle . You would need to build some kind of support to keep the center from pulling in and twisting but if used with the cable down the middle that you described it might work as you would need less tension . Just a rambling thought . Don
*A while ago in FHB, there was an article about curved stairs. The guy built an s-shaped by securing 2x4's to a plywood template, top and bottom, using 2xsomethings as risers, laminating three 3/8 plywood or osb on inside and outside of left and right sides (stringer and railing were one and the same), and cutting away the parts below the bottom of the stringer and above the top of the rail. In essence, the weight was carried by laminations of ply about 4' deep.Did best I could describing it till I find the article. By total coincidence, it's exactly the dimensions we need and I was thinking of using his design and technique. I'd think the deeper stringer (what's it called when stringer and raling are the same thing?) would lessen the forces that might cause twisting and wracking. Wondered if you have any thoughts on it or design improvements?
*Don: I live in Paxton, Il. 45 miles straight east of Bloomington, Il. 30 miles north of Champaign. Thanks for the compliments.
*Jim: Heres a 4 ft walk under I built several years ago similar to what you may be building.
*Don: I usually get my wood from a supplier in the Amish country. Lots of nice woodworkers there, they know good sources. This area is the best place I know of for red oak, white oak, hickory, maple, walnut, cherry and poplar.That freestanding stairs I posted had pecan treads that I had to get out of the southern states. One thing for certain, the price of a curved stairway is more labor than anything, and one might as well use the best wood you can find.
*>Heres a 4 ft walk under I built several years ago similar to what you may be building.Yup, that's the idea. Nice work, of course, just like your others. The one in FHB was s-shaped, but same general appearance. Was the technique for this like the ones you described above--essentially laminated stringers, to over-simplify--or like the FHB article I tried to describe? Or are they really the same thing.I wanted that built here in Asheville NC and went to the shop recommended by several other contractors as the best on this side of the state. Total disappointment. The sales guy kept redesigning it as a squared off stair with a landing and no walk-under. Wouldn't listen when I said curved. This in a house with more curved than straight walls. Guess I'll build it myself--the FHB article gives lots of details. I have built a straight one--no-riser, mahogany treads mortised into 2x12 solid mahogany risers--but this would be the first curved. So I'm trying to pick up any and all tips from the FHB article and this thread.
*Stan,I've been thinking about this cable idea, and when mentioned it around the shop, our "go to guy" kinda' gave me the raised eyebrow look. Not to say he thought the idea was crazy, he just did'nt understand at first. When we talked about it and penciled a few ideas, we found that the load would be pushing sideways, in effect trying to make a straight line out of itself under pressure. When using a lammed stringer, all cutting a slot would do is reduce the ammount of strength by the ammount of slips cut (I.E, Depth of cut).How about "building in" the slot when stacking and gluing slips to create a trough(sp?) for the cable to be running through, then use finish wood on the outside. Start the cable (bottom of stringer run) at the top of the stringer, at mid point on the run, the cable would be at the bottom half of the stringer and back to the top half of the stringer at the top of the run. Kind of like a "compound arc" I don't know what you would call two arcs on the same plane, and am having a hell of a time explaining whats in my head, and don't have anything to draw it with. !!!see my next post for the best picture I can do.
*Jim: I don't recall the stair article, I must have missed that issue. Could you tell me what issue it is in? Anyway, heres basically whats in the inner stringer of the stairway pictured in post #21. That stairs was 14 feet floor to floor. It was in an 18 foot diameter addition built just to accomodate that table you see. I built a curved stud wall and glued and screwed laminates to both sides. Each side had 3 laminates of 1/4 inch bc plywood. All the joints were staggered. So what I had was a curved 2 x 4 wall with 3/4 inch of laminates each side, making this stringer 5 inches thick. I then layed out the top and bottom lines and cut through this stringer leaving it self supporting. I had careful measurements taken from several vantage points to see if anything moved when I cut the stringer out. I could find no change anywhere. This stringer is very stout as it is 4 ft in cross section. I had to play with the top cuts due to the fact that the inside of this curb is a steeper pitch than the outside of the curb. Steel studs work perfect for the framing the belly of this stairs. They easily twist to their different pitches that each stringer has.I see no reason why this type of construction would not work for a serpentine stringer.
*Chris:; This cable idea I am experimenting with is in a straight line between corners of the stairway. I never even considered running it inside a stringer as I know it would just bust out if any kind of tension was put on it.I have tried putting a standpipe to give this cable some depth below the stairway, a basic triangle, then tensioned up with turnbuckles. I have had some success, but nothing worth writing home about. Every curved stairway I build, I stand inside it trying to imagine a way that a cable system could be used to help support it. Heres a picture of a curved stairs under constuction in my shop where you can imagine some kind of system to support the outside stringer. This particular stairs is not freestanding.
*Jim; If you need any help with that s-stairs, I am just a few clicks on the keyboard away.
*Stan,Sorry I did'nt get back sooner, my computer decided it wanted to totally crash. A couple of rescue discs and a major pain you know where later...here we are.The idea I have is as follows: Picture a jumprope in comparison to the curve of the stringer, now rotate both ends of the "jumprope" clockwise in relation to the stringer to overemphasize the effect. When the cable under pressure tries to make a straight line of itself, it will "push" up, or put stress on the wood in an upward motion. The key to making this work would be hiding the ends, due to them being at the upper end (vertically) of the stringer. Am I making any kind of sence?
*Stan; I use wacky wood for the glue up stringers, benderboard, both kinds, easy to use and gives you both grain orientaions. How do you cut the mitres on your open curved stair stringers, what tool do you use?I use two skill saws, a conservative one, and a lefty, on specialy made radiused shooting boards. I know a stair fairy that handsaws his mitres on the stringers, but I like power tools, especialy when working with pecan. I bid stairs without support starting at about 25 grand, for the small ones, it culls out the people that aren't serious about pretty stairs. I like building round stuff, no more rectangles please.
*Chris: Thanks for the sketch. I see what you mean now. I experimented with that once, and the problem I found was that the cable is bowed much more horizontally, and is only bowed a little vertically. So that most of the force when tensioned is trying to squeeze the stairs together. Technically you are right, there would be some upward force, but very little, and most of this is being consumed with fighting friction.
*Brisketbean: Freestanders are fun, aren' they? Youre right on the price. It takes a lot more thought to get these solid. The inside stringer is easy, as we all know it is at a steeper pitch, shorter, and the vertical cross section is greater, making the stringer less weakened by the tread/riser cutouts.I cut the mitered stringers with my little cordless Dewalt saw. I love that thing. I clamp a guide of thin laminate to the stringer and cut a 3/8 deep 45 degree cut for the miter. Then on the inside of the stringer, I route just to the bottom of this cut. This leaves a rabbeted, mitered joint for the riser-stringer joint. The riser is milled leaving a 3/8 lip. which is then mitered.This makes a nice strong joint to glue, and pocket screw from inside the stairway.On the face of the riser I put a screw at the top where the scotia trim will hide it, and one at the bottom where the tread hides it. This way gives one a mitered joint, lots of surface area in the rabbet to be glued, and it is all screwed together with no visible fasteners. I am a nut about minimizing visible fasteners in stairways. I pocket screw and glue the risers to the treads, the tread returns are pocket screwed on the underneath, the risers are screwed to the treads.
*>I don't recall the stair article, I must have missed that issue. Could you tell me what issue it is in?June/July 97, I think. I can't find that issue--in one of our moving boxes from 6 months ago--but found this in the back-issue listings.FRAMING CURVED STAIRS ON SITEby Scott PaschalA stud wall, not a laminated stringer, shapes these stairsBut the one that really, really blows me away is cover of Aug/Sep 83. That one is awesome.And thanks for the offer of help. Very kind.
*Jim: The twisted tree trunk stair. I had to run over to my FH collection. That is one awesome piece of work! Thanks for the July 97 tip. I had that issue but for some reason I never read it. Now I have a new stair article to mull over tonight. That was really bugging me because I read every stair article I get my hands on, and I would not have forgotten that stairway.As far as thanking me for advice, its my pleasure to talk shop with another stairbuilder. I had a rep in today from a nation wide stair company out of Chicago talking shop with me today. He was checking out my little operation. They build over 1000 a year. I don't think he will lose any sleep worrying about my operation.
*Stan, I've been thinking about this problem all week, and I haven't gone grocery shopping yet in hopes that I can win the milkbones...I see two forces, the bending load and the moment, or twisting load. To resist the bending load, increase beam stiffness, represented by base x height^3. In other words, to make any beam stiffer, the height is more important than the width by a factor of 3. For calculations, you can only use the part of the stringer below the notches, so say a 2x10 will yield about 3-1/2" of beam after the notches are cut, for a relative stiffness of about 64in^4. A 2x12 will yield 5-1/2" of beam, or a stiffness of about 250in^4. Two inches of depth makes this stringer almost four times as stiff!Another thought I had was that it would be nice to cantilever the stringer wayyyyyy into the floor framing, but since this isn't practical, a welded steel bracket at top and bottom would provide a similar resistance to bending. The longer the legs of the bracket the better, and the heavier the steel the better. Design-wise you probably don't want to increase the depth of the stringer, so I think the best way to strengthen the stringer is to use the steel brackets at top and bottom. Tried to think of a good way to use a cable, but they all turned into Rube Goldberg devices. MM
*Mike: You have a lot of the right ideas. On my cable idea that I have not been successful with, I was talking with a nationwide outfit the other day, and they gave me an idea that I am going to try on my next freestander. The beam coming out of the floor is a workable idea. I have had a post pocket at the angle of the stairs so that after the stairs is set, a post could slide down into this inclined post pocket in the subfloor. This gives some strong cantilever action. The best way is to have some iron foot brackets welded into a triangle and bolted to the floor inside the footprint of the stairs. Basically what you had mentioned. Thats the way I just finished my last one.There is still a way that works real nice, that has not been mentioned. It is a truss system inside the stairs.
*Here is a picture of my first attemt at curved stair
*another shot
*Here is another shot of my cheater blocks , I will wrap it all in walnut when I am done.
*Don: Nice work! Do you have some finished pictures? Curved stairs have always fascinated me, and especially freestanding ones. Thats real impressive work. and especially for your first attempt.
*Stan,Do you try to have all your runs the same if possible in order to avoid having "kinks" in your rail? I understand the problem of having enough room for a full layout, I hate to put those kinks in the rail if I can avoid it.Allen
*Stan, I have not finished yet . I do have 2x risers and 3/4 ply runners onso that I can use the stairs. I will finish them when the construction is done up stairs. these aren't going to be free standing ,I will have a wall under the outside stringer and will put raised panel walnut on the curved wall . Don
*Allen: I try to have the run of the treads the same all the way up to avoid those change of pitch kinks you mentioned. Sometimes we don't have a choice though. I particularly do not like the curved stairways going up into a straight section at the top. On these type, I try to have the run of the inside curved stringer the same as the straight section it goes into. But usually there is a change of pitch like the attached photo of one I just finished. Notice how the curved inside rail is steeper and goes to less inclination on the top 4 treads. Just the opposite on the outside rail. It is less of an angle then goes steeper. I would have designed this stairway to be curved all the way to the top. But I had to work with the architect on this and this is what he wanted.
*Don: Heres that last stairway I just posted with the stringers being worked on in my shop. It looks a lot like your shot, doesn't it?
*Heres another picture of the same stairway a little further along in my shop
*Stan , I notice that you always build your stringers on the outside of the temp. wall . I should have done that,but the 2xs gave me the exact space required for the opening . It was a lot harder to push the boards into the wall then to pull the tighter inside radius.Shure hope mine turn out half as good as yours. Thanks for the help with this and other post . It is amazing how many differant ways somthing can be done . Also when you are done with somthing and you see how someone else did it better or easier. I have learned a lot from this site. I hope I have helped others also.Don
*Don: What I have found over the years is that we all have good ideas. I like to try various methods and stick with what works best for me. I like to look at pictures like yours to see how someone else does things. Many a time I see something that I see is an improvement, and then go try it.I will look forward to see your finished stairs.Heres another shot of a stairs that is long gone out of my shop.
*Stan, by any chance does your cable truss system go from the bottom of the run of the inside stringer, to the center of the run of the outside stringer, to the top of the inside stringer? I could see how this could be used to keep tension against the lower edge of the outside stringer.A strut against the inside stringer would help with the problem of how to attach the cable to the outside stringer.MM
*Mike: I really don't have a cable system that works good enough to satisfy me. I just wondered if anyone by chance had used some type of cable tensioning that would give a lot of uplift to the outside stringer. I know how to make a freestanding stairway with other techniques mentioned so far, and several not mentioned. I just thought it would be nice to figure out a cable method. One method is to attach the cable to the header very securely at the top of the outside stringer, the other end at the bottom of the outside stringer, and put a little kingpost in the middle, bowing the cable down. Then when the cable is tensioned, there is some uplift force. However, it just isn't enough to brag about.
*Stan , the last picture of the housed curved stringer reminded me that you said in another post that you use a bearing guided bit on your router to make the housed stringers . Why not use a guide bushing and not have to worry about the bearing maintenance?
*Don: I just have had such good luck with the ease of making routing templates with this method. A guide bushing works great also, I just haven't used this method much.
*We share a lot of information via our web site. Our customers can click on the customer page link and watch the stair being constructed. Generally every other day we will take a new digital picture and post it. our address is http://www.stairsandrails.comWe have used the services of a professional structural engineer many years ago. What is important is the header and the footer. Keep these two areas secure with doubled up LVL beams and doubled up joists at the footer. Burying steel plates in the stair stringer of free standing stairs will help insure solid and movement free staircases. We have built 180 degree free standing stairs with open risers with no more than 3/16" of movement on the outside stringer. Next time you build a free standing stair drop every other riser the full depth of the stringer. The "drop risers" contribute to the overall strength and stiffness.
*Chuck: Nice wwbsite. I probably will be contacting you sometime for stairparts.
*Stan,When you pre-fab a circle stairs that goes against a radius wall does your wall stringer stand off the wall the thickness of your stringer system or do you notch the wall to recieve the stringer? By the way thanks for all the stair talk, theres always more to learn!Allen
*Allen: The outside of my wall stringers are the same radius as the face of the curved drywall. When I build my forms in the shop, for say, a 126 inch radius drywall face, I build my stair forms right at 126 inch radius, and then also make some 126.5 inch radius wall plates that I go onsite and layout for the contractor to install.My outside wall stringer is typically 1.25 inches thick, and then is routed 1/2 inch to receive treads, risers, and wedges. Attached is a picture of a stairway that has the back side of its outside stringer formed at a 126 inch radius.
*Allen: Here is the same stairs installed showing the 126 inch radius drywall.
*Sometimes we have placed 1/2 -3/4" deep groove in the stringer, on the wall side, to accept drywall. If the contractor has skilled hangers they can slip the drywall down into the routed "drywall drop". Makes for a clean appearance
*
When a curved stairway is freestanding, its up to the stairbuilder to incorporate all means available to make it structurally sound.
The inside stringer is not much of a problem. It has a steeper pitch and a shorter run than the outside stinger.
The outside stringer is the "bad boy" to make it strong enough to support the dead load of the stairs and any live load.
Further complicating this is when the outside stringer is of the open stringer style. Then you have the stringer being weakened by the riser/tread cutouts.
I have compiled a list over the years of structural techniques that increase the strength of this outside stringer. These techniques have been gathered by personal experience, visits to other stairshops, and questions I have asked of others.
This subject fascinates me and I know there are ideas in this forum that are NOT on my list.
I will give 5 milkbones to good ideas offered that are on my list.
Ten milkbones to any good ideas that are not on my list. I will be honest.
Now, to the one that offers what I feel is the best idea, or ideas, I will send a Stan's Stair Shop hat as pictured.