Sloping a flat roof...with tiered insulation?
I have a flat roof in need of replacement, and aside from deciding on the right material (lots of opinions about that,) I need to have it sloped correctly. The house is in Albuquerque, NM and does have some ponding. The ceiling is already insulated, although it may be only about 10 inches of fiberglass. I have a variety of proposals from various roofers. (Most of the bids are for TPO, but some hot mop 3 ply bids are out there too. Also, a spray foam is noted below.) Here are some of the options:
1. Tear off existing 2 roofs (hot mops) and build a ¼” per linear foot wood structure with “rippers” and 7/16” OSB. This contractor would also bust out two canales on the north of the house so that the slope would be symmetrical from the center out to all sides. This is the only "structural" approach.
2. Tear off existing 2 roofs and build up with tapered insulation. Those suggesting this will use the canales/drains on the South, East, and West sides, sloping from the center North of the house. (Is it important for the tapered polyiso to have multiple layers in order to have staggered joints?
3. Build right over the existing old roofs OR tear off to the decking, and then slope with tiered standard polyiso. For some reason, this roofer/contractor is suggesting 1/8” per linear foot of slope. I did an internet search on sloping with NONtapered insulation and found very little discussion of it. I did, however, find an article from the NRCA describing it: Minimizing ponding using flat insulation. http://docserver.nrca.net:8080/technical/275.pdf Are people doing this still? Is it that much cheaper than the tapered insulation? Will it end up being a lumpy roof at the seams between the various thickness insulation pieces? For that matter, if I have ponding already, are all the insulation based sloping solutions bound to have some lumpiness given that we are starting on a roof that is not flat and instead likely has some sag…certainly on the current roof, and maybe even at the level of the decking? (Correct me if this is not the case.)
4. Tear off the existing roof and spray foam roofing, building it up “by eye” to obtain good drainage. (This proposal is not yet in, but that is the gist.)
So my main question is about using flat insulation, but I put some other questions in as well, if anyone is willing to answer them. Thanks.
1) This seems like the nicest solution, as it will give you a sloped surface that you can apply flat insulation. Tapered around here is about 15% more expensive, and maybe more if you consider the additional labour.
2) Typically 2" is about the minimum thickness you want in a piece of insulation, so it depends how much R value you want if you can have multiple layers. It is better to have two layers, but again, it depends on the R value you are going for. Compared to polyiso, expanded polystyrene gives you less R value per inch but more play with the slope due to the increased thickness.
3) I would not suggest building over the two existing. I'm not familiar with the tiered flat insulation method, but I can see why you'd only have a 1% slope (otherwise your tiers would be too big). You could smooth the lumps with a cover board, but then proper adherance and wind uplift becomes an issue. I'm not a big fan of this suggestion.
4) I don't have any experience with this type of roofing, but I know the quality depends greatly on the applicator. They seem to require a new waterproof covering every couple years as well, so factor that into the cost.
The roofer could fill in any dips in the deck with an additional sheet of cover board. A good roofer would give you a nice straight surface.
1. Stay away from the spray solution
2. It sounds like you aren't worried about R-value so no need to worry about staggering. If you are going for more R-value, a layer of flat 2" polyiso can always be installed under or over the taper staggered.
3. Yes you want to go with a taper system. 1/4" is minimum you want to go.
4. If you go with single-ply, EPS taper is a cheaper system than polyiso and just as good. Don't know the availability of it in your area. I'm lucky to have the plant right down the street from my shop. If you go with BUR, polyiso is ideal.
5. You can go with the structural framing but it doesn't sound necessary. By the time you get labor and materials you are into it more than a taper system. Taper systems give you flexibility and ease of installation, a big plus.
+1 to both of the posts above.
Have you considered metal roofing?
Stay away from the spray foam. They're called SPUFs around here and can be a nightmare. A complete tear off and tapered insulation system is the way to go. It is normal and advisable to stagger the joints. If the panels are adhered, the staggered joint makes a good solid foundation. If you go with an asphalt system, you need to use iso board insulation. The oils in asphalt will disolve styrene insulation panels. If it's hot asphalt the styrene will melt and then disolve.
Thanks for the advice fellas. I did not know to be so skeptical of the spray on roof. Although I had not yet investigated it, I thought it was another high quality option. It sounds like that may not be the case.
I tend to agree with you OldNBroken, that to make a wooden frame atop the existing roof seems like overkill when you can use tapered insulation. You also lose the insulation benefit, unless you add flat insulation to the top of the new structure. I am leaning away from this now.
I was hoping that someone would say that it was just fine to use flat insulation in a tiered way in order to achieve a relatively smooth taper. I guess it is just a reality check that this is not the case.
Thank you ShazApple for telling me a little bit more about my insulation options.
OldNBroken, you are right that although some extra insulation sounds good, the fact that I already have at least some R value already makes it a less important aspect of the job. They are to be mechanically fastened and then the TPO would be mechanically fastened over that. (To the decking I presume.)
Although I don't have anything against the BURs, in Albuquerque, I think it is a little more of a selling point to have a membrane roof, whether or not it is superior. The two seem like they could have similar life expectancy.
I have not seen any metal flat roofs in Albuquerque - at least on residential homes- so don't know that I would go for that Windows on Wash.
I appreciate all the input.
Tear it off and install tapered insulation and tpo.
If you use eps and want a adheared system you will have to use a cover board. No adhesives at all can be applied to it.
And god forbid theres ever a fire eps turns into greek fire. Very bad if your deck isnt sealed 100%.
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