Low slope roofing – underlayment question.

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Old 03-26-14, 06:12 PM
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Low slope roofing – underlayment question.

I am doing a rip and replace on a very low slope (2 in / 10 ft) roof. Four + decades of DIY experience and three re-roofs (but they were all shingles on pitched roofs). This will be my first rolled roofing project (other than a couple of sheds). The project is 2,200 square feet, so cost is a concern.

I really like everything I have read and heard about the self-adhesive (peal and stick) products. They seem to be made-to-order for the DIYer. The peal and stick top sheets (mineral surface) cost about twice as much as the “regular” top sheets. But the savings on adhesive and labor would appear to be well worth the extra cost.

But I have questions about the underlayment.

I see both two layer (underlay + top sheet) and three layer (underlay + intermediate ply + top sheet) approaches. However no discussions about when one is more appropriate than the other. I also see self-adhesive underlayment, but they are expensive (about five times the price of 30# felt) and will add $800 to the project.

I’m not really keen on “gluing” the underlayment to the plywood sheeting (AKA peal and stick underlayment). I can’t give a good reason other than the difficulty that will cause in a re-roof 20 years from now.

What I am proposing is to use 30# felt underlayment (one layer, not two). Stapled at the top edge because I believe I will have fewer problems with “pop ups” using staples than I would with nails. Lapped four inches (rather than the usual 2 inches) and sealed with Henry #203.
Then finish with a GAF (Liberty) peal and stick 90# mineral surface top sheet.

I live in the desert of Arizona, so I don’t have to worry about the special edge treatments for ice damming.

Comments and advice will be much appreciated. Particularly with respect to the underlayment.
 
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Old 03-26-14, 07:09 PM
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Roof pitches are expressed in terms of feet (or inches) of drop in 12'. (not 10')

If the slope is really 2" in 10ft, I don't think your plan will work. That's less than 3/16" per foot. GAF Liberty info clearly says to install their product on roofs with a pitch of between 1/2" and 6" per foot. That's a minimum 6" of drop in 12', also known as 1/2:12 pitch.

I don't think there is much point discussing a plan that is bound to fail.

See the instructions on their own website for details. http://www.gaf.com/Commercial_Roofin...structions.pdf

What you need is either a mop down roof, an EPDM roof or a spray polyurethane foam roof coating.
 
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Old 03-31-14, 05:30 PM
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Additional questions.

I posted my original discussion on several boards and received some informative comments. Thank you to all who took the time to pass on this valuable information. Particularly the comments regarding “scaring” the new roof by standing/working in one place for too long. I would never have thought of that.

There were some commenters who raised the issue of 2-ply vs 3-ply applications. IE: adding an intermediate layer of self-adhesive base sheet between the nail down base and the top sheet.
Question: Just how important is this? Does it add substantially to the life of the roof?
Concerns: This intermediate ply adds about $1,000 to the cost of the project and 25% to the labor hours.

There are four vents and three skylights on the roof. I have done a couple of shingle jobs on pitched roofs in the past. Is there any particular difference in the way you approach these obstacles in a rolled roofing application as opposed to a shingle application? -- Lap each layer up the sides 3-4 inches and apply adhesive between the layers and sealer at the top?

One last item to discuss. The house is “Spanish” style construction, so most of the roof butts against walls, rather than a roof edge as would be that case with most roofs. There is a flashing already installed into the walls that extends down several inches with the roofing tucked up under it. I presume there is also an “L” flashing attached to the roof deck and the wall (I won’t know for sure until I do the tear off). Since the self-adhesive roofing materials sticks so aggressively, I am concerned about just how to do this “tuck under” without the adhesive sticking before I have the roofing piece properly positioned. One thought is to “dirty” the last 6 inches of the adhesive and use troweled on adhesive to secure this tucked up end. Any ideas appreciated.

I have learned a lot from y’all. Thanks so much.
 
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