On the dormer job I’m working on (my own place) I’ve already built a strongly reinforced floor where there were only crappy 2X4 joists over the kitchen and now I’m just completing the walls of this new room. I’ve built these walls one by one and stacked them off to the side. The ordered windows have just come in and I’m also building a small one, and I’m getting close to finally busting out up through the roof to erect these walls and come across with the new rafters.
I was strongly advised here to give the new dormer roof at least SOME slight pitch — “anything but totally flat”. Now, I want to come across with a series of 20′, 16OC rafters that’ll be slanted at a smidge over about only 1 inch/ft. Very slight indeed. I REALLY would like to use only 2X4’s for this. As it won’t be a ‘deck roof’ or anything taking any more weight or activity than a normal roof, would 2X4’s 16″OC spanning about 17 or 18 feet and dropping about 2 ft over that span be OK? I DONT want to go to 2X6 or 8 but thought I’d throw it out there first and see if it gets pounded down or not.
Thanks,
Terry
Replies
Terry,
I don't know the area you are building in but here are a few considerations;
1. Asphalt shingles are not allowable In NC for roof pitches under 2"/12". You must use metal panels or built up roofs.
2. 2000 International Residential code states that 2x4 rafters on 12" centers has a maximum span(horizontal distance) of 11'-6" using Douglas Fir SS (I think "SS" means structural select) with a live load of 20 psf and a dead load of 20 psf, a deflection limit of L/180 and the ceiling not attached to the rafters. Rafters 16" OC with the same constraints will allow 21'7" for a 2x8 Douglas Fir SS, 19'5" for 2x8 Douglas Fir #1 or Southern Pine #2 .
Check your code book or call your local inspection department they are generally pretty helpful. If you get into something very unusual they will tell you to hire an engineer but your questions seems to be straight forward.
Good luck and keep us posted
Clark
"Asphalt shingles are not allowable In NC for roof pitches under 2"/12". You must use metal panels or built up roofs."
Clark
Could I use a 'roll-roofing' on these low-pitch dormer roofs? Next step up from there would be epdm ... considerably more expensive!
Terry
terry.. roll roofing is going to leak at that pitch..i'd probably use epdm
AND i'd use 2x8 at those spans
Mike Smith Rhode Island : Design / Build / Repair / Restore
Edited 5/4/2005 12:26 pm ET by Mike Smith
You are correct; "SS" stands for Structural Select.
Good luck finding any, though, LOL....
Generally, #1&2 SPF (Spruce, Pine, Fir) is the best grade I can order from my yard for framing lumber.
Dinosaur
'Y-a-tu de la justice dans ce maudit monde?
The idea that 2X4s could span 17 or 18' is ridiculous.
"The idea that 2X4s could span 17 or 18' is ridiculous."
I know .. it's obvious. But the original rafters up there are 2X4's 22"OC spanning 14'. I figured that 12" or 16"OC ... since it wont be required to carry any load more than the roofing materials ... may be ok. But that's why I tossed it out here ...
What I think I'll do is run a cross-header off the top of two of the new dormer walls to mid-support on those rafters. That'll mean they're supported at mid-point and require only apprx. 9 ft. spans of them. (in re-calc'ing I see that the actual load-span is slightly short of 18 feet.) The thing that strikes me is how these guys in the 1920's built so many beautiful houses around here ... Early Craftsman Bungalows ... with ONLY 2X4's. And they are absolutely rock solid! If there's a way to avoid cramming more and more heavier wood into the job then I'm definitely interested in doing it that way. I'm working solo on a high roof -- I'll leave the 20' 2X8's and 2X10's to the other guy if I can help it.
Thanks!
Terry
Terry,Why can't you extend the the rafters on the front roof by nailing a microlam into the existing ridge and then nail on short rafters. That will create a higher ridge and give you a streeper pitch in the back. Then you nail ceiling joists back to the microlam. I've done this plenty of times.
Terry,Is this you back in 4/14/03? http://forums.taunton.com/tp-breaktime/messages?msg=29611.16
Edited 5/4/2005 7:43 pm ET by Framer
Hey Joe,
Yes, I clearly recall that late night ongoing exchange. Went till nearly 3 in the morning, my time, which must have put it at nearly SIX your time!!! And yes, it's the same project which, after other concerns and affairs interviened, I'm now seriously returning to. But that whole idea of extending the ridge upward into a new one is, it feels like to me, way to 'advanced' for me. I wouldn't wanna do that without someone on it with me who knew their stuff better than I do. But I DO agree, theoretically, that it's a viable approach.
Realistically, for me, I'll either have to bite the bullet and go with heavier 'timber' ... or support the span mid span with an underlying header as I mentioned earlier. What's more, unless there's something 'fishy' about this plan that you or someone else can enlighten me on, I'm going to tie in to the existing rafters a couple of feet DOWN from the existing ridge. Again, I'm 'relatively' inexperienced and it makes me nervous to mess with the central spine of the roof-structure that's stood the test of over 80 years and two or three strongish earthquakes and still looks good.
When this squally, windy, tempermental weather stabilizes some I'll be ready to bust up through the roof and pop my pre-built walls up. It'll be a first for me. I'll try to post some 'here's how it went' pics. [wish me luck]
Terry
Having had a house with a shed dormer, PERSONALLY, I like the look of gable dormers much better. Then you could use shingles. But the diminsions on your dormer might be out of the question for gables.
Some good points in here.
Which gives me a question, is there any reason to lock yourself in to bringing the flat shed dormer to the existing ridgeline?
Like, could you but a clerestory window at or near the existing ridge, and then use a "good" slope down to where your existing wall is?
Maybe you could have a 'doghouse' dormer that cuts through (perpendicular to) the ridgeline? That would make the dormer geometry a lot simpler. The "other side" of the dormer might not be in the "new" room, but would balance the look on the outside.
That whole line of thought came from just wondering if a SIP would do as the "whole" roof, then as pieces-parts for the dormer.