I’ve got an old (90+) house that has a walk up attic that I want to finish. The floor joists are 2×6’s on 16 inch centers. I plan on finishing the attic but the floor doesn’t seem up to the task. Some parts of the floor are ‘bouncy’, to say the least. How can I stiffen the floor without making it higher? The finished stairs wouldn’t line up with deeper floor joist.
I also was looking for a good way to suck out tons of old rock wool insulation. I plan on a spraying the rafters with foam.
Replies
What's the span of the roof?
The longest span is 16 feet. The house is built around a center chimney using it as a support.
"The house is built around a center chimney using it as a support"
Nobody mentioned it before, so I will. On a 200 year old house, sometimes you do see the chimney used as a structural support. On a house your vintage, this should not be the case. On a modern house this is not allowed at all. If your chimney is loadbearing as you say and if you are actually asking for advice here, you need an engineer to come and look at the situation. If you make alterations, new work is supposed to meet current building codes. If you are altering the floor structure and it is currently supported by the chimney, that adds a considerable wrench in the works. Get advice from somebody who can lay eyeballs on your situation.
I'm not making decisions based solely on the advice I get here. It'd be foolish not to seek out new angles to approach problems from the experience that is sometimes represented here. I have had the pleasure of working with and for experienced contractors that only see things one way and only have one solution and all the others are crap. Life is simple for them.
The house is tied into the chimney structure (it is massive - about 4'x8' at the base and 3'x5' where it intersects the attic floor. This is common in the area on this style and age of house. There haven't been any major modifications or structural problems with the house.
OK, if you're not concerned about the chimney and are willing to go in from underneath, here's another possibility. Kill several birds with one stone. Put down a couple layers of plastic sheet in the room below. Start tearing off the old plaster. When you have enough debris to handle, roll it up in the sheet and heave it out the window. You'll want a dumpster out there. As you pull the ceiling, the old insulation will fall and get tossed at the same time. Easier to pull stuff down than to suck it up.
The ceiling/floor joists will not be too flat probably. Use 2x4 flatways on the bottom of each joist. Make a T beam. Glue and screw. Will give you way more than enough stiffness. Run and shim 1x strapping crossways for the new drywall. If you want to insulate, do it first. The T will hold the insulation without wires or staples. Easy. You'll lose 2 1/4 inches of ceiling height. If everything is flat and you don't need strapping, only lose 1 1/2 inches.
Use 2x4 flatways on the bottom of each joist. Make a T beam. Glue and screw. Will give you way more than enough stiffness.
Bob, I'm glad I read your post befor I posted mine because I was going to say that there's a builder that I used to frame for and he was also a framer now I think he's and inspector who told me that whenever he did an Add-a-Level with existing 2x6's as a ceiling the engineer that he knew would design it where he would srew and glue a 2x4 on a flat on top of the 2x6's but they couldn't be splice thay had to be one long 2x4 going from the outside wall to the center bearing wall.
I don't remember the span right now but I'm curious and I will call him and ask.
When he first told me this I thought he was nuts but he said that the guy was an engineer and said it was alright and he framed alot of houses that way and never had any call backs.
Still sounds crazy to me but nothing surprises me anymore.
Joe Carola
Ends up being very much like an I-beam joist. The center part of the joist contributes less to the strength than the top and bottom edges. Putting the T on the bottom gives the most strength for downward load. Top is in compression and has the added strength of the attic floor. The 2x should be a single unspliced length from wall to wall. A 2x6 with a 2x4 flat should be almost as strong as a 4x8 beam. Could also sister a 2x8 or 2x10, but that costs more and any wires or whatever drilled through the joists will be in the way. It will deflect more than a 2x10, but you'd need dancing elephants in the attic for that to make much difference. You'd also need an engineer to calculate the official allowable span and loading. Just like trusses. Depends a lot on the lumber grade, screws, glue, etc. But, if it didn't fall down with 2x6 it sure ain't going to fall down with this and it will be a lot less bouncy.
Bob,
It's sounds okay to me and I'm glad it was brought up and doing what you said by screwing them on the bottom would be better for Sweetlake because he doesn't have to mess with the stairs.
Next architect or engineer I talk to I will bring this up and see what their thoughts are.
Joe Carola
Its much like the design of TJI or engineered trusses. Top chord usually resists the compressive forces and bottom for tensile. Somewhere in the middle is the neutral axis, subject to bending but neither stretched or compressed. Depending on a lot of stuff, the compression member often fails by buckling rather than exceeding the compressive strength of the material. Bottom chord for solid lumber will likely fail in tension because of knots or other flaws. For these reasons, I'd be inclined to put the flat T chord on the bottom rather than the top. More material to resist tension where it is most likely to fail. As far as stiffness and strength for a given amount of material, it gets complicated. A single 2x10 will be stiffer than a 2x6 with a 2x4 on the bottom. Probably a lower ultimate yield point, but it will deflect less up to the point where it fails. However, he already has a 2x6 so the cost of adding a 2x4 is less than sistering a 2x10. Also easier in logistics. So in this specific case it looks like a good idea. If he wanted to rip gluelam into 2x4 strips to have a full-length laminated bottom chord it would be stronger than solid 2x4, but more expensive. Probably not necessary here.
As a general rule, I'd not recommend building your own trusses or TJI substitutes. Too much labor and too many details. But in this particular case, I'm confident it will be stronger that the existing. How much stronger I can't say. A wild guess would be 1/2 to 1/4 of the current deflection for the same load. Probably something like 3 times the ultimate yield. For details like the correct numbers and whether this meets code, a real engineer needs to be consulted. I used to know more of that stuff, but it's been a lot of years since I hauled out the slide rule.
Finally tore down the old ceiling and glued/screwed the 2x4's like described in the thread. After removing the the tacky tiles, furring strips, plaster, lath and rock wool (a good 5 inches) the sagging middle of the span rose 1" over night. It was at least a 1000 pounds. The floor was a lot less bouncy than before. After the 2x4's it feels plenty stiff- I jacked it up another 1/2 inch before screwing the boards tight. Ready for drywall. Thanks for the idea.
Ted
That rockwool insulation is pretty heavy. I replaced mine with fiberglass twice as thick that weighed half as much.
I've seen that onsite wood I-beam construct solution a few times around here..
Excellence is its own reward!
Piffin,
Have you seen it in a way Bob and I described?
As I mentioned befor when the buider told me about this I thought he was nuts but look how strong I-joists are with 1/2" ply vertically and this guy is screwing and glueing down 2x4's on top of 2x6's and as Bob metioned doing the same but on the bottom of the 2x6's.
I will annoy the next architect or engineer about this one because now when they see me they automatically hand over their pen so that I can draw for them.
Joe Carola
On the bottom for handling loads.
One job that comes to mind because it was indeed engineered is where a kithchen floor was framed normally for the span. ( I got all this second hand from the owner when I was remodeling a portion of the house ) it would have been adequate for normall situations but then they chose tumbled limestonme for the floor, Granite countertops, and a couple of monster stove/ranges and a refer so big you could almost walk-in.
It was apparent to the builder that all of that added so much weight that he needed to do soemthing about the floor. He called in an engineer who simply had him add 2x4s falt along the bottom of the joists as a chord, with glue and nails. It made for a solid floor. I don't know all the nubers on it though.
Excellence is its own reward!
"I also was looking for a good way to suck out tons of old rock wool insulation. "
I have heard that it is possible to hire some sort of vacuum truck for this, but have never looked into it.
What I do is shovel most of it into double bags. Rock wool shreds plastic bags with its millions of little shards, but double bagging seems to work pretty well.
The residual is the problem. A shop vac filter will clog instantly.
I use a modified 5 gallon bucket as a water filter. This is similar to the ones marketed for drywall dust, but adapted for a 2 1/2 inch vacuum hose. PVC intake pipe goes within an inch of bottom. Outlet pipe feeds from top. Fill with about 5 inches of water. When water gets too thick to filter anymore solids, move cover and hose assembly onto next 5 gallon bucket (for efficiency). I dump all the waste through a plastic mesh bag (lawnseed bag) to capture the solids for disposal.
Wear a N100 respirator. Full face, or halfmask with ski goggles. Your eyes would take a beating from this dust. I once pulled a rock wool splinter out of my eyeball with tweezer! I mean it was sticking through my cornea! (do not try this at home). A little pre-misting with a tank sprayer will lessen dust a bit for shoveling and bagging part.
Best of luck.
Hometime this year did an attic, and they used a leaf vacuum with a garbage can attachment to suck out the old insulation. think this is the attachment they used:
http://www.amazon.com/exec/obidos/tg/detail/-/B00004R9TJ/104-7042229-1325534?v=glance
lol sounds like what tommy chong was selling
You can add 4x6s in between joists, that's usually easy enough.
One of my favorite tricks in attics is to turn the kneewall into a box beam - unbelievably strong. That usually cuts the joist span significantly. Just make sure you have a positive bearing path down through the walls to the foundation and you can span great lengths with box beams, easy to build in an attic too.
You'll gain strength if you sheet the floor in a staggered pattern, too. Turn it into what engineers refer to as a "diaphram".
"One of my favorite tricks in attics is to turn the kneewall into a box beam - unbelievably strong."
Now that is a cool idea I have never heard of. If the rafters were up to the task (or could be beefed up), could you not essentially 'hang' the box beam/kneewall from them, transferring the load to the outer [structural] walls?
"If the rafters were up to the task (or could be beefed up), could you not essentially 'hang' the box beam/kneewall from them, transferring the load to the outer [structural] walls?"
I guess that's theoretically possible, csnow, but it would take some hurkin' mamma jamma rafters to do what your asking. I've only ever used posts, or existing walls to support the box beam, which in turn supports rafters and joists. They can carry unimaginable weight if designed and built properly, sweet tool for remodellers.
Of course, you can do a lot with glulams, microlams, flitchplate and other built up beams for shorter spans, too. But it seams like this desire to not increase the height of joists comes up a lot, usually something to do with headroom or like in this case existing stairs, and there is often any easy fix already there in the form of a kneewall.
I agree with you about 4x6's (or 2x6's).
If anyone takes the time to "run the numbers", 2x6's or 4x6's with the proper spacing will span 16'. If they are installe dproperly they will provide proper vibration control.
This is a poor plce to find information. Too many people say too many stupid things.
"This is a poor plce to find information. Too many people say too many stupid things. "
Like you maybe?
Maybe you could tell us what you propose to do to 2X6 or 4X6 lumber that will make it span 16'?I'm not as think as you confused I am.
Maybe [he] could tell us what [he] propose to do to 2X6 or 4X6 lumber that will make it span 16'?
I think I have it.
I don't have the formulas, but the span tables for 30 lb live load (sleeping room) for Hem-fir give these spans for select:
24" OC 9'4"
16" OC 10'9"
12" OC 11'10"
So, by extrapoltaion, I don't think that even if we pack 'em in continuously side by side we'll be able to get to a 16' span, but it occurresd to me if we double 'em up, top to bottom, and plate 'em together, we'll have a function 2x12 and can easil;y do it, even at 24" OC.
{G}
_______________________
Albert Einstein said it best:
“Problems,” he said, “cannot be solved at the same level of consciousness that created them.”
Your mileage may vary ....
Thanks to Norm for the idea on the clean up, I searching the phone book now.
I had no intention of getting the regulars so worked up, just wanted a few opinions on beefing up the joists.
I asked the insulting contractor because he's a retired general that's been in the area for a long time and is familiar with the old houses in the area and as an icynene dealer seen his share of attics.
I must not have been clear - the attic has a floor (T&G pine) that's been there from day 1 (1905) that had rock wool blown in under the floor sometime in the 70's. It also has a poorly engineered 2x4 raftered hip roof (which is, BTW, straight as an arrow - even here in snow country). The floor has had everything from furniture to christmas decorations on it and is only bouncy in one area, I suspect a cracked joist or something but have to rid the place of insulation before I can see it.
I'd build up the floor if I didn't have the original stairs (very nice and original) ending at the current floor level. I'm also looking at going in from the bottom (bedroom ceiling) and resupporting the floor from beneath losing a few inches from the top and replacing the cracked plaster and lath ceiling.
Thanks for all your comments.
Look up the formula, run the numbers, and figure out the spacing.
We use #2 DF or SYP for joists around here.
And when you're done eat crow.
"Look up the formula, run the numbers, and figure out the spacing."
You're the engineer. (Or so you claim) Run the numbers yourself.
While you're at it, why don't you figure out the vibration in Hz for the floor?
Seriuosly - Why don't you back up that advice with some numbers?
Never argue with an idiot, they drag you down to your level and beat you with experience.
I did run the numbers. And I did consider vibration. So I know that 2x6s are acceptable.
You made the statement that 2x6s would not work. I did not see your numbers nor did I ask for them. Your statement was simply wrong.
I guess your demands and your unsupported wrong advice might be viewed as arrogant by some.
I did run the numbers. And I did consider vibration. So I know that 2x6s are acceptable.
So, what kind of spacing with what species did you come up with?
_______________________
Albert Einstein said it best:
“Problems,” he said, “cannot be solved at the same level of consciousness that created them.”
Your mileage may vary ....
If you ran the numbers (assuming you actually know how) then I'm sure you won't mind posting them.
You call my advise "unsupported". So where's your support?
I've posted a great deal here about floor design and vibration. All you've posted is a bunch of B.S. about how you know more than everyone else, but you're not willing to back it up with anything.Bumpersticker: Watch out for the idiot behind me.
I will not be prodded into teaching you engineering.
No one else has posted numbers or been asked to post numbers. I think I will keep up that tradition.
In any case, a person (like you) who cannot derive the proper spacing is not competent to judge if the spacing is proper.
I will not be prodded into teaching you engineering.
I don't know about BH, but I'm not asking you to teach me any engineering, and I don't think he asked either. I'm just asking you to tell us what kind of joist spacing one has to use to span 16' with 2x6's. You say it can be done and that you can prove it with numbers, but you don't actually do so.
No one else has posted numbers or been asked to post numbers. I think I will keep up that tradition.
Actually, I posted numbers form a published span table, but that is irrelevant. You claim you have run the numbers and it can be done. The only "tradition" of any concern here is that people support their claims.
In any case, a person (like you) who cannot derive the proper spacing is not competent to judge if the spacing is proper.
But someone here probably will be, so you can feel to post your calculations and someone with the necessary knowledge will stumble along to double check your work.
Or are you just going to dance around, avoiding the question?
_______________________
Albert Einstein said it best:
“Problems,” he said, “cannot be solved at the same level of consciousness that created them.”
Your mileage may vary ....
"I will not be prodded into teaching you engineering."
What makes you think you know more than me, anyway? The engineering stuff I've learned has come from knowledgeable people that I have a lot of respect for. Not from some amateur like you.
I'll make you a deal - You post whatever numbers you came up with, and I'll do likewise.
How about at least you provide some proof that you're an engineer? So far your advice is so poor I doubt you have much construction knowledge at all. You certainly haven't demonstrated any so far.
I see you've set aside this special time to humiliate yourself in public.
I will let the forum operators deal with you.
Ouch! Now ya done it. Hit a sore spot.
Who ever invented work didn't know how to fish....
Thanks for the idea, that's why I'm here. I called the contractor that I work with and thought it was brilliant. Anyone want to watch my 8 year old twins while I destoy their ceiling? I guess that will help me keep at it until its done. Thanks again. Ted
Well, glad I had one halfway decent idea today. Now for the bad news, which may be good news for twins, depending. I pulled down a chunk of ceiling in my house and found a nice assortment of snake skins, snakes, mouse skeletons, wasps, and enough critters to start my own natural history museum. Plenty of plastic, hardhat, gloves, and long sleeves are a must. You may want to do the demolition when the wife is out of the house. It is definitely the fast way, compared to hauling bags of crud down from the attic, and the new work is mostly where you have more room to work. If you find a lot of old cloth or knob/tube wiring under there, take the opportunity to replace it before you close it back up. Also, if you have balloon frame construction, you may expose an inch of the wall stud bays at the top. A good opportunity to dump some insulation or add firestops if any are lacking. Have fun.
Maybe you did run some numbers. And maybe you just come here to spectacularate.
I was shocked once to have an enginneer return a headroom solution detailing that I use doubled 2x6 sisters laid in at 12" OC for a floor. It worked quite well at 11'8" span. He said it would go up to slightly over 12'.
By that copmparison, your numbers come up 4' short.
I keep getting surprised by your recommendations here to do some rediculous sounding things. Hope not too many folks are listenning to you without getting your stamped sealed drawings and a copy of your insurance coverage for errors and omissions.
maybe I or someone else could learn something from you about stretching the envelope, but you never give logical reasonning or sourcing for your statements. That makes your outlandish claims sound as outlandish as a cow jumping over the moon without draggin her teats..
Excellence is its own reward!
The guy who blows cellulose around here just bought a sucker vac that rebags old stuff like that.
You've had some good responses but there is another concern. These may never have been intended to be floor joist buit rafter ties only. I can imagine that they are only tacked into place with a couple of spikes. That fact would add to the Tigger bounce factor and would also increase the risk of failure when simply adding sisters and more weight to the system.
So be sure they are fastened well and/or blocked to top plates for diret support.
Excellence is its own reward!
Good suggestions, thank you. An insulation contractor was by today and suggested plywood sandwiched by 2x6s for sistering. would that work?
Should I use adhesive?
I like the wet filter idea - i'll try that, along with the shovel.
There's no structural value to construction adhesive, but it will help keep vertical members from "rolling", which is how they fail.
Hey neighbor (CSnow) - nice to know you have experience with this! I'm tackling the attic right after I get the #@$%^ porch done! Thanks for the safety tips (your eyeball? ouch!) Bill.
"An insulation contractor was by today and suggested plywood sandwiched by 2x6s for sistering. would that work?"
Nothing you do to a 2X6 will make it span 16' in a floor and perform well.
You probably need the depth of a 2X10 if you're going to use it as a floor.Having been married for some years, I've learned not to question stuff that works. [Andy Engel]
I'm with Boss - depending on grade/type of lumber, in most csases you're going to need 2x10 to meet modern span table requirements.
_______________________
Albert Einstein said it best:
“Problems,” he said, “cannot be solved at the same level of consciousness that created them.”
Your mileage may vary ....
Sounds like you and Boss haven't done much remodelling. There's more than one way to skin a cat.
That's a stupid and irrelevant comment.
You can't make a 2X6 span 16' and perform well, remodelling or new construction.
Maybe you haven't done much engineering.Teach a man to fish and you feed him for a lifetime. Unless he doesn't like sushi - then you also have to teach him to cook.
"That's a stupid and irrelevant comment."
"irrelevant"? I disagree. I think a person's experience dealing with similar situations is relevent.
"stupid"? I guess you're entitled to your opinion.
"You can't make a 2X6 span 16' and perform well, remodelling or new construction." You can if you reduce either the span, or the on center spacing, but I agree, you could devise a much more efficient way to use the 5+1/2" in available height than filling it with 2x6s nailed together (which is exactly how many many roofs in commercial buildings in Tacoma Washington are framed, roofs that span 50, 80, 100 feet).
Not trying to get into a pissing match with you Boss. I just read your earlier post to say that anything under a 2x10 provides an inadequate joist, and I disagree with that. In fact, if you want to say you meant over a 16 foot span, I'd say a 2x10 16" o.c. would be an overspan too.
Sounds like you and Boss haven't done much remodelling. There's more than one way to skin a cat.
No, I haven't done a whole lot, but I have inspected a lot of houses which have been remodeled.
Making the knee wall a box beam is a great idea, but it isn't going to affect the span of the floor joists unless you rig up some sort of way to firmly and effectively attach the joists to the bottom of the box beam.
And, although I've never seen a boxbeam kneewall, I've seen lots of knee walls added in attics which weren't intended to be living space. Usually in those homes the roof rafters are also undersized by today's standards (and they often have 2-3 layers of shingles), and the main thing the kneewalls do structurally is transfer some roof loads to the already undersized joists, and there's almost always a nice crack running down the living room more or less under the kneewall location. (This is what I've seen in hundreds of capes built in the late 40's and 50's)
Tip, that's the basis for a little HI trick - walk inside, unobtrusively check the LR ceiling for that crack and announce "I see we have a finished second floor."
"Whoa! How'd ja know that? You haven't been upstairs!
_______________________
Albert Einstein said it best:
“Problems,” he said, “cannot be solved at the same level of consciousness that created them.”
Your mileage may vary ....
I agree with Jim that this CAN be done, but with Boss that it is far more practical to give up some space to use properly dimensioned lumber. The engineering and labor to gain that four inches can be astronomical. You point out the results of assumptions gone wrong.
The humourus part of this discussion is the homeowner taking structural advice from an insulation contractor, IMO. Maybe the roofer can do the wiring in the attic too!
;).
Excellence is its own reward!
Edited 11/7/2003 8:06:45 PM ET by piffin
The humourtus part of this discussion is the homeowner taking structural advice from an insulation contractor, IMO. Maybe the roofer can do the wiring in the attic too!
Good point. Just so long as he takes his political advice from, oh, say, ahh ...... never mind {G}
_______________________
Albert Einstein said it best:
“Problems,” he said, “cannot be solved at the same level of consciousness that created them.”
Your mileage may vary ....
Ok, everyone, I can't believe that this thread has gotten THIS far without one of you (who should know better!) saying the obvious and most important point:
GET A STRUCTURAL ENGINEER to tell you what is safe/wise to do in YOUR case. No one here can see what you have, or is in a position to properly advise you on structural matters. We can all hazard a guess, and an experienced builder can make a great attempt at it, but having a lisenced engineer go over it for you (only costs a few $100 for an hour or two of their time) and advise the best solution is the wisest course of action.
If you follow the advise of someone on the internet, you might be fine, or you might ruin some key structural component of your roof. It might take 50 years, or 5 minutes for that to become apparent after you are done.
Sistering joists is pretty straightforward, and there are many options depending on what load-bearing walls exist below (which impacts what span you are really working with), and what the roof structure is like (whether any load can be 'hung' from it or NOT). Once you are done, an engineer can tell you what sq ft loading the final structure is safely capable of supporting (like, if you could consider using it for storing heavy items, or just holiday decorations and empty stereo speaker and TV boxes, or whatever.
Regarding the insulation, look for a cleaning company that specializes in "Catastrophe clean-up" or something like that. In Ann Arbor, MI there is such a company and they brought a 100 cu ft bag vacumm truck and sucked out the whole attic of my house (vermiculite) in two hours. A very wise investment before doing any major work in a very old attic with loose insulation (varmit droppings notwithstanding!).
Norm "the voice of caution" Kerr
My two cents,
use 4x6's! Stop by a sawmill and have them saw them for you out of white oak. Put them between your 2x6's and your floor will be plenty stiff.. Around here they would cost you $25.60 each. Put them up green and while they will be plenty heavy, they are realitively easy to work in that condion.
I like the box wall idea as well, when doing a closet in my house I went underneath and screwed the plywood subfloor to the bottom plate of the new wall and then screwed everything together.the added resistance to pull out really stiffened up the too weak floor. basically transfering some load to the ceiling.
the exisiting 2x6's would add only a moderate amount of stiffness but every little bit helps. 3/4 inch plywood with staggered joints would add a fair amount especially if you were to screw it into place rather than just nail.
as for sistering joists, I doubt you'll gain as much as if you were to put them in the middle of the existing bay..
Regarding structural engineers.. unless they have experiance working with exactly the problem you have they won't be much good.. I tried to hire a structural engineer to look at my plans and in a 15 minute conversation it was obvious that he didn't have any experiance with timber frames so he would just look up the information.. (a quick glance around his office didn't show any books on the subject) Information that I had carefully researched. (If you've seen the pictures that have been posted you'll realize why I didn't need one)
I think the deciding point is are you capable of doing the research needed to do a safe job. Is this your first attempt? Can you find a copy of the UBC? or span charts for various woods? It's kind of like follow the money. In that you follow the load path.. when you jump up and down here, that transfers load to the floor, which is carried by the joists and then to the walls below which carry it to the ......
"Making the knee wall a box beam is a great idea, but it isn't going to affect the span of the floor joists unless you rig up some sort of way to firmly and effectively attach the joists to the bottom of the box beam."
I agree 100%. And I also block between the joists, directly under the box beam, to help keep those joists from rolling.
I also agree that it is essential to establish a positive load path down throgh the building to the ground.
Jim,
What would be a good way to secure the joists to the underside of the box beam? I really like that solution and would like to pass it along.
And, any ideas for hope to do the side that the stair comes up? I'm thinking two box beams, with posts down below in the basement on either side of the stair well.
_______________________
Albert Einstein said it best:
“Problems,” he said, “cannot be solved at the same level of consciousness that created them.”
Your mileage may vary ....
> What would be a good way to secure the joists to the underside of the box beam?
I'd start by looking at:
http://www.strongtie.com
-- J.S.
How long is your room going to be. If the room is to long then the box beam won't work. I did use that method once to stiffen some rafters once. Hmmm, I thought I was a genius for coming up with that idea. Anyway, would it be possible to get some glulam beams up there. You could set the beams directly on the joist and tie them with straps.
Also you said the span is 16', well how much of that is going to be room. If you end up with a 10' room, and were able to work two beams up into your attic, then you could double the joist between the beams only, and that would save some money and labor. You just have to make sure the ends of the beams are properly supported and that the joist are tied well.
One more thing, It sounded like from you original post that the fireplace was supporting the roof, If that is the case, then the fireplace may have to be beefed up in order to support the weight of the room.
How can I stiffen the floor without making it higher? The finished stairs wouldn't line up with deeper floor joist.
Are the stairs the only reason why you can't go any higher?
Do you have the height to sister 2x10's and add the 4"?
I've done this on a couple jobs in your situation where the stairs couldn't be touched so we added the 2x10's and headered off around the stair platform keeping the existing 2x6's and just had a 4-5" step all around and never touched the stairs. It might be an option for you.
It might not be the most practicle way of doing it but it works.
Joe Carola
You could also look at adding 5" deep steel channels thru bolted to the existing 2 x 6's creating a flinch beam. The flanges on the channel add stability and strength to the verses a simple steel plate which only adds strength. For simple spans you can run the numbers your self making sure that the load path gets to the ground and that your are not crushing the wood framing or creating horizontal shear forces greater than what the wood can handle. Your best bet is to hire a structural engineer that does a lot of residential/wood framing work. If your use an engineer that does mostly commercial/steel framing you will not be happy.
Hire a structural engineer. No one should have to live in fear with a flinch beam....that's not a mistake, it's rustic