Buddy of mine is putting an addition onto his house. Looks like the best design estetically, without ripping the entire roof system off would be to go with a flat roof.
I’ve framed alot of roofs, (truss and handframed) but was surprised when I had to think back and realized I’ve never actually framed a flat one.
I’m looking for any tips advice and any general info related to flat roof framing. I know in reality it is not actually flat, it needs slope. I have framed a number of decks where we have place tapered shims onto sleepers to achieve our slope and ventilation. So this could be one method (with or wthout sleepers). as opposed to sloping rafters with furring the ceiling level.
the addition is about 12′ by 20′ rafters running the short way. the roof deck will not be for walking on. not sure if I need to use 2×10′, I live in Fraser Valley here in BC Canada not much for snowload…. lots of rain. Do guys usually frame flat roofs with 2×10?
anything I should know about the parapet wall design- how tall etc..
The existing house is “L” shaped with gabled roof lines. the addition will fill in the “L” making it a square shaped house.
thanks for any input.
Replies
I have always felt, and I could be wrong here cause I haven't designed a ton of flat roofs, that it is easier to build with the roof rafters flat, lay down your plywood sheathing, then acieve your slope with rigid insulation sloped 1/4" per foot, then rubber membrane roof over that.
Edited 4/7/2006 1:22 pm ET by xosder11
sloping a roof with ridgid insulation sounds hard to me. I'm used to working with wood. how would you exactly slope the inuslation? when I slope a solid deck I rip tapered furring strips then sheathe. your insulation would have to be solid - correct? how exactly do you get your slope, I know whenever I work with that stuff I end up making a big mess.
>>>>>>>>>>>>>>>>>how would you exactly slope the inuslation?You order it with the slope built into it. Take the insulation supplier a roof plan and they'll make it to fit. It works pretty well.Why is it every time I need to get somewhere, I get waylaid by jackassery?
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Roofing supply houses such as would carry the EPDM or modified bitumen torch down roofing would also have the pitched solid roof insulation.but if this is not an easy option in your area, just notch a bird's mouth in the outlying end of the 2x rafter and that 2" will give you enough pitch for this jobor
don't notch it, just build that wall 2" lower than the main house wall the rafter will seet on there.when it comes to parapet wals, you are talking about taking some chances now. Is the intent to surrond this whole thing oir just the 12' end?
if surronding it all, you need carefully planned and detailed scuppers and drainage. Takes a real experty to avoid leaks with this type of detail.
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I've only see flat roofs with flashed parapet wall - aside from the odd shed.
If we dont go with a parapet wall, then the torch on or membrane hangs just over the fascia like regular ashpalt shingles used in conjunction with a standard gutter - is that correct?
part of the idea of the parapet wall is to hide the roofing material from view when down on the ground, but I hear what your saying about the potetial for problems if not done correctly.
a low slope or flat roof leaves nothing to be seen from the ground anyway
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let me rephrase. I'm not sure how the finish detail of the roofing material is where it would meet up with a traditional fascia/gutter system. what I'm used to seeing where I live is torch on, or the older tar and gravel. it runs up the side of the parapet wall and is covered with a flashing cap.
could you explain the finish detail at the overhang as this is the part that is visible.
I just happen to have a detail drawing from a job ort an earlier discusssion on this
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thanks for drawing...I can say that I have never seen that detail before....but now I have.
I take it the red is flashing...2 peices?
what does the top black line represent.
Edited 4/7/2006 10:53 pm ET by alrightythen
The red is the drip edge flashing one piece.The blue is the EPDM membrane.The black is another strip of EPDM (uncured) over the drip edge and it's nails and the two dots are seam sealant tape. I use a slightly different detail. I install the drip edge first, then my manufacturer provides a double sided tape that goes over the drip edge. The EPDM then adheres to the tape and then the resulting outside seam is caulked with the seam sealant. There are pros and cons to both methods and we've discussed them to death. As John said about the scuppers make them big enough to pass a baseketball except I'm gonna add being "palmed by Shaq."Why is it every time I need to get somewhere, I get waylaid by jackassery?
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as CU said
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Yes you need an overhang to get the water past the fasia into the gutter. I am for just shy of the center of the gutter, the number depends on how you're guttering. The sides or rakes I use H flashing, or "slag stop". Use drip edge along the eave, I prefer "F" metal. Prep the underlay as per specs and be paranoid about your edges. For self-stick a lay a swath of muck, elastomeric roof sealant, set my metal, then lay the roofing. If it isn't sticking to the metal, peel back and back butter with some muck. For a simple slope making the outer wall, where the gutter will hang, shorter than the opposite works, you may have to cut you sidewall studs to fit though. Parapets require compound slopes and should be done with the sloped insulation board from a manufacturer once you give him what your dimensions will be. Look at the costs and use what works best.
There's a TV news guy here in LA, Stan Chambers, who's been doing field reporting for over 50 years. Every year when the first big rain of the fall comes around, once again we see Stan on channel 5 doing the flat roof collapse story. Every damn year.
So, if you go with parapet walls, make the scuppers big. Big enough to easily pass a basketball would be nice. The bigger the opening, the more force the water puts on the accumulated crud. The bigger the opening, the weaker the crud spanning it will be. Just like a beam with a long span.
With parapets to hide the roof, you can go for more slope. At least 1/2" per foot, even 1" per foot isn't too much. The steeper the slope, the less the leaks.
We used the tapered celotex for re-roofing on my mother's house, but that was because the roof didn't have enough slope to begin with.
-- J.S.
What they said. You order it sloped. Eliminates the need to furr the ceiling, or mess with shims. Also, as Piffin was saying, parapets are tricky. Pay attention to the scuppers. If they aren't properly, the roof can become a big bathtub, which adds considerable unexpected load to the structure, which could cause big problems.
If you don't have much of a snow load, 2x10, 16" OC should be plenty for 12' span.
For the framing, if you want a level ceiling below the roof, the easiest way is to slope the rafters and have level ceiling joists below. The design load for the roof is much higher than that for the ceiling, so the ceiling joists can be twigs in relative terms.
Next easiest is to go with tapered sleepers (but you're talking about a lot of cutting -- I'd rather spend the money on ceiling joists than the time doing all that cutting).
I can't comment on the tapered insulation as I've never tried that. Maybe that is the best way -- I don't know.
For the parapets, Piffen already pointed out the drainage issue. We have almost all flat roofs here and there are lots of "creative" solutions I've come across. I'm guessing you want parapets so the profile of the addition blends with the gables. Depending on how it all ties together, I'd be inclined to have very wide tall scuppers all along the low side of the roof (like a couple feet wide by 3 or 4 inches tall all along the low side of the roof, just like a porch knee wall). The only way I would be able to do this (maybe there are better ways), would be to tie that parapet into the rest of the structure at the corners. Otherwise it would eventually blow away . . . . (in other words, no free-standing parapet at either end).
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st5ructurally, if I were building it with parrapet walls, I would treat it like a balloon frame and hang the rafters within the walls
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st5ructurally, if I were building it with parrapet walls, I would treat it like a balloon frame and hang the rafters within the walls
I like that one. Make it a lot easier/faster to build too. With just a parapet above the roof line, maybe BI would even let it go without fire blocking; would go up like 1, 2, 3, done.
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i had to get a engineer stamp for my flat roof.16' span,the spec was 2x12 on 12" center. so we laid them up with a 8" drop in 16'. then went in and laid 2x8's level for the clg joist on 24" center.i joked that we must of wiped out a acre of forest to buid the roof. then laided up a rubber roof,so far so good.larry
hand me the chainsaw, i need to trim the casing just a hair.