Roof Framing technique
I have a question that’s come up regarding framing a Roof for a customer (Boss are you listening).
(Now don’t jump on me guys, I’m fishing here)
Really quite a simple job.
A 30 foot span with a 7 on 12 pitch.
I have alot of 2x8x20ft left from another job, enough to do this one.
Can I frame this with a support midway between the ends and peak, kind of like a truss.
This will give me a support mid way (at approx 9ft) on either side.
If I do this, do I need to cut another notch (birdmouth) or can I just nail on supports without notching.
Will this be ok for the Vermont Snow loads we get here.
I could always go up in size, but then need to buy the material.
Not looking to be complicated, just practical.
I’ve got my own thoughts on this and am looking for some other opinions other then my own.
Oh, one other thing, I’ll have wall support below.
I don't know about your snow loads or other building code requirements, so it's kinda hard to tell.
The 2X8 rafters may or may not work - You need to check that against a span chart for the species and grade of rafters you have.
The support wall(s) below should help, depending on where they are. How you attach any support boards underneath them depends on where they are, the forces involved, their angle relative to the support walls and rafters, etc.
I was in the grocery store. I saw a sign that said "pet supplies." So I did. Then I went outside and saw a sign that said "compact cars"...
I know the 8" won't span the full 18 ft, the span chart say's 12 ft
max for snow load.
But, with support in the middle, does this become a 9ft span.
That's my thinking.
Looking for someone to say: You'r crazy, it won't work, or that I'm
ok and go for it. Or, here's how to make it work.
I don't want to have the customer call me during he winter and say
the roof feel in.
"I don't want to have the customer call me during he winter and say the roof feel in."
You might look at a thread here named Ballon framing, some of your issues were discussed there.
I too am curious as to whether a birdmouth cut is a good idea at the knee wall. Some one please chime in.
I'm not sure I totally understand your question. Are you supporting the rafter in the middle of it's span and posting to a wall below? Or are you trying to site build a truss?
If you are posting to a wall down below, then you don't need to cut another birdsmouth. When we do this, we just cut the support stud at the pitch of the roof. Nail a plate across all the rafters and nail your support stud into that.
I have a feeling though, that I misread your question.
First, I was curious about the knee walls. Do you build a wall with a pitched top plate or birdmouth the rafter to fit flat on the wall. You said pitch the studs.
Secondly, I understand that if the knee wall takes a load the floor must support it. However, is this true enough if the rafter is sized to handle the load all the the way to the outside wall (sized for the entire span)? In other words, in that case would the knee wall would serve to only hang sheetrock and not be load bearing?
I have always wondered about this since the cape cod or story and half house often have knee walls which typically do not fall above first floor walls. Are they built with stout floors or rafters designed to carry the load to the outside walls?
To answer your second question, I don't know. I'm not qualified to even speculate. I'm sorry I can't give you an answer, but your question really is something that an engineer would have to look at and answer.
Whenever I build a wall like that, I always make sure there is support underneath. We build a lot of bonus rooms over garages. We stick frame and use a structural ridge and then build 4' high walls on the sides of the room and make sure they are supported underneath. This also helps to shorten the span of the rafters.
Hope you get your question answered. :-)
I plan to support the middle of the span down to a lower wall on one
side, and a support beam on the other.
No, I'm not trying to build a truss.
I'm also planning to have support in the center, somewhat.
I know offhand that I have trusses in my garage that have a larger
span with 2x4 material.
Yes, I realise that a truss has alot more design in it with tension
I offered the client trusses also.
Judging from the reply, I don't kneed to cut birdmouth midway. That
makes things easier.
You say you support your knee wall beneath. How? It would make little sense to have a wall directly below the knee wall, as it would give you a little six foot (plus or minus) room between that support wall and the outside wall.
Thanks for your thoughts.
There must be a simple answer to my question as this cap cod or story and half style is very old and quite common. Anyone else?
There are a few ways that we have done it. In the bonus room situation, we have beams underneath the walls that carry the loads to outside walls below. I realize that this isn't making all that much sense, since I don't have a pic.
We have built kneewalls over joists that were sized to take the loads and we usually then will run strongbacks behind the wall nailed to the wall and the joists below. That is usually overkill (because the joists were sized on our plans), but since it partly my company, I would rather overkill than underkill.
Does any of that make any sense?
What you said makes sense to me.
Heavy beam parallel with the knee wall and directly below. Ok, sure.
Strong ties? From where on the knee wall to where on the joist? (I assume all some how in the attic space triangle area behind the knee wall) ??
Strongback. If you had 4' walls that nailed into the rafters and into a plate on the subfloor in a bonus room, or on top of the ceiling joists in an attic, then you could nail 2x12s onto the back of the wall and into the studs and the plate. We only do this to stiffen the assembly after we have followed the directions on the plans. I probably wouldn't do this to somehow make the joists strong enough to support a kneewall. This would be just to stiffen the assembly and make the ceiling flatter for drywall down below.
We do this sometimes in bedrooms if we have some bad lumber. By bad, I mean those discrepencies you see when using longer 2x12 or 2x10s as joists.
Edited 8/6/2003 10:20:02 PM ET by TIMUHLER
Edited 8/6/2003 10:21:45 PM ET by TIMUHLER
Thanks for the thoughts.
not a problem.
In a gambrel roof 1½ storey I built in a high snow-load area, I framed the lower roof with 2x6 rafters as if it were a slanted wall--top and bottom plates, 'studs' cut to the complementary angle of the roof slope, etc. We placed a 5' high knee wall 3' in from the outer edge of the second floor deck. It was framed with a sole plate but no top plates; the studs were cut to the angle of the rafters and joined directly to them. Gyprock nailers were added later. The studs landed on a 2x sole plate; layout placed them directly over the floor joists which were oversized (3x8 BC Fir), under a 2x6 T&G red-pine roof-decking finish floor (and ceiling below--exposed joists in the first floor).
No bird's mouths anywhere. Span on the floor joists below is 10 feet from outside wall to main beam. Load from the knee wall falls at 30% of the span distance from the supporting outside wall. Deflection in floor joists is unmeasurable, even under max snow load.
Note that the design purpose of the knee wall in this particular case was not to give mid-span support to the rafters, but to resist racking moments imposed on the entire roof structure by wind loads.
Your cape could be framed this way if the rafters of the lower roof continued up to a ridge board instead of topping out flat to receive the upper roof trusses. I have lived in one that was arranged similarly, although not with the beefed up floor joists and H-D flooring. You'd need to add collar beams or ceiling joists though, to resist spreading.
'Y-a-tu de la justice dans ce maudit monde?
Looks like your span is slightly greater then mine, so I think I
won't have problems.
I plan collar ties and full span support.
Area will be dead space or storage, not living space.
I suppose it should work.
I spoke to an architect friend off hand and he thought it would be ok
without checking any span tables.
What supports the knee wall? Knee walls in this situation carry alot of load. do you have properly sized joists below? You need to think about the load path all the way to the foundation. You don't want you customer calling and saying that a crack has developed in the ceiling below, or worse yet that the second floor fell in just before the roof came down.