I haven’t framed a dormer, so I am wondering how to do it.
It is the kind that has its front wall back from the mainfloor exterior wall below.
In framing the opening in the rafter scheme, do the doubled rafters sister alongside the outside walls, thus they are outboard, or are the doubled rafters run in the walls, with the walls framed atop?
The dormer in question is shown here.
The ridge of the dormer’s roof is the same elevation as the main ridge it intersects.
The dormer houses a sauna (see the porthole window) and a small deck (note the scupper right over the skylight).
Ground snow load is 85 psf, and I am figuring on #2 SPF 2×10 rafters on 16″ centers, which are spanning 9′-6″ on this side shed roof with the dormer opening. It is probably overkill . . . I didn’t engineer it yet and I can probably get away with either 2x8s or go 24 on center.
Edited 10/30/2005 8:02 am ET by Stinger
Replies
Seems like you could do it either way but I'd be inclined to consider a third way, namely, place the side walls of the dormer on ceiling joists. I assume the ceiling is flat out to the outside wall. The joists would need to be flush (i.e. no taller than) with the sauna room floor joists (at least on the side with the deck). The end wall of the dormer will likely be bearing on the ceiling joists also unless you put a header in the ceiling under the end wall. Looks cozy (especially with 85psf snow!). I wonder just how the ceiling below is to be configured as there are so many skylights right there.
If the floor framing is arranged to support the dormer, I don't think that doubles would be needed and just a single along side of the walls would be suitable.
Additional info would be needed for the design:
Are you saying the shed roof is 9'-6" long and therefore that is the length of the ceiling joists? Is that measured in plan?
Also, what is the length and width of the dormer (including covered deck)? This would be needed to determine the roof load on any of the rafter or ceiling spanning members.
There's a lot to consider here then just the dormer. That's the easy part to frame. I assume this is a new house so your floor joists have to be heavy enough to take the load of the sauna.
I've framed a lot of dormers before and sometimes on new work we have beams that I put in underneath the dormer walls that can support the dormer. Sometimes we use triple rafters and sit the dormer walls on them and use a header vat the bottom to support the gable wall with windows in them.
There's a lot of ways to do this but supporting the sauna and getting proper drain off is the most important one.
If this is an existing house then you have a lot of work ahead of you build the floor up to support the sauna.
Sorry. I just looked at the drawing again and see that the roof your cutting into seams like it's cathedral and has no floor joists. Is this true?
It is a new house. Look at the pic again in my original post. The sauna has two walls paralleling roof rafters. The outside wall that is the far side of the dormer in the drawing, has a wall directly below in the main floor scheme, that can be a bearing wall.
The other outside wall of the sauna, which is the wall between it and the little halfwalled outside deck, has no bearing below, nor does the outside deck wall, which is the other side of the dormer.
Attached is a view of a wireframe model I have made of this house, and you can see the floorframe of the dormer (sized at 2x10 depth) and how it intersects the shedroof rafters (also modeled at 2x10 depth).
I don't think the sauna finish for the room will add any considerable weight. We will need to properly insulate walls, floor, and ceiling, and we'll be using an electric heater. There will be no plumbing, as it is a dry sauna.
The little outside deck will need to have a floor buildup so the membrane-covered deck pitches to the scupper. We'll do mahogany duckboards over the membrane deck cover. I am thinking we ought to put a heat cable in the scupper somehow, to keep it from icing closed. Location is the high peaks area of the Adirondacks, and we are already using snowshoes to climb in the back country.
I listened to some advice over on the JLC site, thought things through, and have now landed here.
The pic attached shows the structural elements of support for this dormer, as I now think it might be.
The pic shows the second floor deck of the house, and the nearside 12w x 6d bumpout is the dormer floor.
As I said earlier, there is bearing below for the near sidewall of the dormer, but nothing elsewhere.
So, a triple LVL at 9.25 depth is place as a common "rafter" right in the farside wall, and to it is clipped a doubled LVL at the same 9.25 depth for the frontwall beam, which is carrying both dormer floor loading, and what remains of the roof on the downslope side of the dormer.
That frontwall floor/wall beam has its near end bearing on the wall shown, and on that near side of the dormer, a doubled LVL at 9.25 depth is carrying the dormer wall and its roofload from atop the dormer.
I used loadings as follows to size members: 85 psf ground snow load (resolved to 66 psf roof loading per ASCE-7), roof deadload at 12 psf, and dormer floor at 40 live and 20 dead.
Don't mean to be a curmudgeon, but I don't know bud... I'm guessing where you live plans don't have to be approved, or maybe they just don't have to be stamped by an archi/eng. Where I live even crummy plans show structural type stuff like what load bearing members are where, what size they are and maybe the load points with # of required jack studs. Are you doing this "on the fly" or are you still in the planing stage? Building a house is a huge investment in material and labor so saving $800 on engineering is kind of a big chance to take. Maybe I'm reading too much into this, and maybe it is a lot simpler than it seams.
Bear with me on this, Matt. The plans are available from a plans seller, and they are of a house already built a few times. The original prototype was done for a private client over on the Maine coast.
I've a potential client who thinks he may want this house, and before springing for the planset (and it is very pricey) wants to know about how much it might run him.
The site selling the plans shows quite a lot, and I have been able to model the structure from what is available there. I can pretty well price it out from what I can develop from the plans at the site, and using my model.
I think I can see my way clearly on structural stuff at this dormer, now that I have kicked it around with you guys here, and someone over at the JLC site. The rest of the structure is pretty straightforward. The one we are in finish on right now makes this one look like a shoebox, in terms of complexity.
Certainly, if it gets built, we will have the planset, but I will have to engineer it myself for local conditions and loads. Our building department (one man) that issues permits is very unsophisticated.
OK - I gotcha... Sucks sometimes on how much work you gotta put in before even getting a signed contract...
I would size the rafters on the shed roof as if they were joists in a floor system, then double them under each side wall and frame the dormer sidewalls up from there. My tables say that would be SPF 1&2 2x8's @ 24" OC, for the 9'6" span (whether sized as rafters or floor joists; the max span is lower for joists, but your 9'6" falls into the 'bracket' for either case). Bumping up to 2x10s wouldn't cost a fortune and would make me sleep better at night after it's done, so I'd do it....
I'd also put a built-up beam sized for the load under the rafters directly under the downslope gable wall and transfer the load via posts straight down to that LB wall you mentioned, making sure the load went straight down to solid at the foundation level. Maybe pour a couple of underslab footings in anticipation of that. (I can't look up that beam because I don't know the span, but at a rough guess it would be tripled 2x8s.)
Instead of 'ballooning' that gable wall, I'd build a stub wall across the rafters over that beam, then sit the dormer floor joists on that and frame up the rest of the gable wall from there. (The sidewalls of the dormer could be balloon-framed or if you're anal like me, you could frame stubwalls up to floor level fading back to zero at the upslope end and then just build the whole dormer on top of the floor like any other bump-out.)
The beam will shorten the span on the rafters holding up the dormer, as well as help hold up the gable wall and the dormer floor system. A triple whammy; so much the better....
How to configure the support for the sauna is a bit trickier. I'm wondering is this just a dry sauna or is there a hot-tub involved? In the first case, I wouldn't worry about it; they are not particularly heavy. In the second case, though, you might find yourself needing steel or monster joists on 12" centers in there. (I am assuming that the dormer floor system can be hung off the structural wall of the 2-storey section of the house on the upslope end.)
One detail you're going to have to really get right, tho, is that skylight directly under the scupper. Seems like an invitation to leaks just by its configuration, LOL....
Just a silly question, though; don't these plan sets your client is considering buying include the engineering for this sort of thing? If not, what's justifying the hefty price tag you mentioned?
Dinosaur
A day may come when the courage of men fails,when we forsake our friends and break all bonds of fellowship...
But it is not this day.
The attached pic shows how things are landing in my mind, and thanks for all your help.
The nearside wall of the dormer is supported all the way down with underneath bearing. The farside wall is supported by the triple LVL shown as a common rafter. Its upper end is posted down to the floorbeam below, which is picked up by posts going all the way down.
The frontside beam is picked up on the nearside by the bearing wall, and its other end is clipped to the triple LVL with the appropriate hanger.
Not shown is the short gable frame atop the frontside wall, which will have a center post going up to pick up the outside end of the dormer's structural ridge. That front wall doesn't go all the way across at full height, because of the little outside porch within the far end of the dormer.
I don't know why, but I can't open those pdf drawings you've been attaching. I've got Acrobat Reader 6.0 and it usually works without any problems, but those files have all come up "Error" once they download.
Can you post them in another format? I can open .dwg or .dwf files
Dinosaur
A day may come when the courage of men fails,when we forsake our friends and break all bonds of fellowship...
But it is not this day.
Here ya go. This one is a .gif.
I wondered about your earlier responses. It is now clear to me that you could not see what was going on with the house.
You probably could see the artist's rendering I clipped and pasted in the first post, but it was not clear to you what was underneath the dormer, in the way of bearing.
View Image
Okay, now it all makes sense, LOL....
From the artist's rendering you pasted into post #1, it looked to me like the whole thing was sitting on the roof; that's why I suggested framing it the way I did. Based on this drawing, tho, it looks like a pretty straightforward deal. The only thing to really wonder about is whether or not there would be any extra beef needed to hold up a hot-tub (if that's in the plan).
I'd still double up the rafters on either side of the dormer, just to provide a little more meat at that point where the roof deck will meet the dormer sidewall. Good flashing ought to keep water out of there without much trouble, but you never know how well HOs will keep up with maintenance over the years. I had a roof repair this past summer where the HO almost lost the whole dormer because of a small leak in the dormer valley flashing which he'd ignored for too long....
Dinosaur
A day may come when the courage of men fails,when we forsake our friends and break all bonds of fellowship...
But it is not this day.
The inside room within the dormer is to house a site-finished sauna, which should not be adding much weight to things. I would guess that the insulation, membranes, and wall finish, plus whatever is on ceiling and floor, adds not much over what gyprock would be. The heating box cannot be that big a deal.
The open porch side of the dormer needs a membrane over its pitched subfloor, and duckboard walking surface atop, so that is a little more than flooring, but just a little.
Have you considered the free Adobe download to get up to V 7.0? That might do it for you.
Unless I'm reading the picture wrong, I would expect that the side rafters will need considerably more than just "doubling up" 2x8's. It does not appear that there are any interior support posts at the first floor level that would be underneath the shed roof area. The dormer roof is spanning side to side isn't it? At 85psf that would be a significant load of the side walls of the dormer. Additionally, does the dormer /sauna have a door out onto the deck or is the door coming from the main portion of the second floor? It seems that you will have considerable load also on the header at the end of the dormer under the gable wall and supporting the short (lower) rafters. This header would deposit a lot of load on the end "rafters".
My guess is that the rafters would want to be 2x12 anyway to get sufficient insulation in. That would also allow 11 1/4 " LVL rafters at the edges of the dormer. I would be guessing doubles or triples for starters there.
From my post over at the JLC site, here is some, but not all, of the engineering:
"Since the rafter beams are picking up the entire dead load of the dormer, plus whatever live loading is coming down from snow loading, I can proceed to size accordingly, except that I really only need one on the open-porch side of this dormer, the other side being supported by a bearing wall below.The facewall beam is picking up half of the dormer's floor load, which in this case has a tributary area of 1/2(6 x 12) or 36 sf times a loading of let's say, 40 live and 20 dead. Thus, the beam, for what it is carrying as a floor load, needs to be able to handle a total load of 2160 lbs over its 12 feet of length, or 180 #/lf. This same facewall beam, having rafters clipped to its downslope side, is picking up half of the total roof load for the 12 wide by 3.5 feet of roof area that is below, so if we resolve the ground snow load of 85 psf to a snow live load of 66 psf (using ASCE criteria), and figure the dead load at a safe 12 psf, we see, after calculating, that the beam needs to handle another 273 #/lf from the roof stuff below.So, we need a facewall beam that can handle a total load of (273 + 180) or 453 pounds per lf over its 12-foot span.Trus Joist's tables tell me that a two-ply 1-3/4 x 9.25 LVL will do nicely here.I'll run through the rest of the calcs, but I'll bet that a triple LVL at 9.25 x 5-1/4 total buildup width will be OK to carry the porchwall side of this dormer."
And I still think it will, with its short span of less than 10 feet.
Edit: After posting this, I went to look at my LVL facts. At a span of 9/6, a tripled 9.25 will handle a uniform loading of over 1600 pounds per lineal foot, thus a total load of 9.5*1600= 15,200# can be presumed OK. That is from the roof snow load table. Snow loadings can be greater than other loadings because they are "durational," which means they come and only stay for a while . . . they're not forever.
The area of both roof and floor picked up by this tripled rafter is 6x6 or 36 sf. It is also picking up half of the rafter loadings from downhill. Add it all up, and it doesn't come anywhere close to what that tripled 9.25 can handle.
To insulate, we would plan to use sprayed urethane and do all the roofs as "hot."
Edited 10/31/2005 7:12 pm ET by Stinger
the one thing that's confusing me is the floor of the dormer section....isn't it at the same elevation as the rest of the second floor, hence it's floor joists would go all the way to the exterior bearing wall, rendering the "wall beam" more or less unneccessary?
My recollection from way back, converting what had been a walk-out set in the shed roof "porch" (which was over the front porch below, internal drains in the walk out above,
we trippled up the existing rafters under the new dormer walls, and voila'
No, if it were that easy, I would not have posted the q.
Take a look at the rendering in the first post of the thread. You will see skylights in the roof that continues below the dormer.
Then take a look at the section view in my post http://forums.taunton.com/tp-breaktime/messages?msg=65186.4
Yes, the floor of the dormer is at the same elevation as the rest of the second level, but there is only straight-down support under one of its walls, and the rest of the floor is over open living space below. The adjacent wall that is outboard the dormer is too low to pick up floor joists. They would poke through the roof.
I saw that in the renderings, but wondered why, from a form or function standpoint, you would design the building "shell" that way
If the shed section had cathedral ceiling below, OK, but if not, then the lower top plate out there does present a problem...just not sure from a design standpoint what it's a "solution" for
I didn't dream it up. Robert Knight AIA, of Blue Hill, Maine, is the architect. The house has been built a few times, originally for private clients, but they released the rights to resell the plans to Knight.
Called "Tapeo's House," the plans are sold by architecturalhouseplans.com
I realized from reading the previous posts that this was a "stock" design.....before you got ahold of it!(just kidding, it's been a long week..TGIF, even though I work tomorrow & won't go home till Turkey time...)
Sounds like a no-brainer, actually. Just build it, LOL....
As to the Adobe upgrade, they have more bloody spyware built into their stuff than about anything I can think of. I need to have the basic pdf reader, but I don't want any more Adobe in this box than that. It is a constant battle with that bloody program to prevent it from downloading stuff automatically. I dare not open any document that calls up Adobe when I'm on line, or the next thing I know, there's a pop-up that appears telling me "UPDATE COMPLETED" without ever having asked permission to download.
I hate when that happens....
Dinosaur
A day may come when the courage of men fails,when we forsake our friends and break all bonds of fellowship...
But it is not this day.