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-   -   Low slope roofing – underlayment question. (http://www.houserepairtalk.com/f34/low-slope-roofing-underlayment-question-17496/)

cybergoofy 03-26-2014 05:14 PM

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.

oldognewtrick 03-27-2014 11:52 AM

First off, :welcome: to House Repair Talk!

Cost is always a consideration in any roofing project. But most important is, are the $$$ you are spending, being spent wisely. Cutting corners effects how the new roof system will perform and how long it will last. My recommendation would be to mechanically attach the Liberty MA base sheet over a clean roof deck, follow up with the mid-ply and then cover with the cap sheet.

Why would you not use #30 roofing felt as a underlayment? Because it is not as strong and you run a very real possibility of wind lifting and the cap sheet will not seal to the organic base material like it will to the mid-ply. Staples will NOT hold as well as a nail. We use screws and plates to secure the base sheet in the field and around the perimeter. It's important that you use a 3X3" drip edge over the base sheets and then the cap sheet is embedded in roof cement around the outside edge. All lap seams should also be embedded in roof cement. Asphalt metal primmer should be sprayed on the drip edge prior to the roof cement.

Liberty is designed as a "system" to be installed, not a pick and choose what parts you want to incorporate into something you're trying to modify and shortcut. As a roofing contractor, what I see happen more often than not is premature roof failures by improper installation.

My advise, install a proper system, then you can expect the maxi mun life cycle out of it and get a better value for the $$$ you spend.

Just my:2cents:

cybergoofy 03-31-2014 05:32 PM

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.

Wuzzat? 03-31-2014 06:34 PM

Quote:

Originally Posted by cybergoofy (Post 102756)
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?

It depends on what it does.

This is way out of my field but I hope this helps. . .
"Separation of the polymeric film support 3 from the bituminous waterproofing adhesive layer 1 by the intermediate impervious metallic film is further desirable since it prevents oils present in the bituminous waterproofing composition from contacting the polymeric film 3. Contact of the oil and synthetic polymer film can adversely affect the film, for example, adversely affect the dimensional stability of the film after adherence to the roof substrate."
from
http://www.google.com/patents/US4396665

There may be other reasons but there may also be disadvantages to this extra layer and there are probably newer patents explaining the interaction between layers.

You seem pretty conscientious.

cybergoofy 04-01-2014 12:40 AM

Quote:

Originally Posted by Wuzzat? (Post 102760)
You seem pretty conscientious.

I grew up with a DIY dad rebuilding cars and remodeling/repairing houses. Probably the most important thing he taught me was to learn from others and then do the job RIGHT. None of my friends are into DIY at the level that I am. They all think I'm crazy to take on a re-roof at my age (65). But I love the challenge and the satisfaction. Just a glutton for punishment I guess.

oldognewtrick 04-01-2014 03:28 AM

I'll look this afternoon for the install specks for you on how to terminate at walls and penetrations. It will be a lot easier than me trying to describe. You will need "Cant Strip" where any horizontal surface transitions to a vertical plane.

And don't worry about age numbers, you only have me by a couple and I deal with this stuff every day.

oldognewtrick 04-01-2014 04:20 PM

http://www.certainteed.com/resources/RPG_Flintlastic-SA-Brochure.pdf

this should give you all the details for installing SA products and flashing details. Hope it helps.

cybergoofy 04-01-2014 05:07 PM

Quote:

Originally Posted by oldognewtrick (Post 102773)
I'll look this afternoon for the install specks for you on how to terminate at walls and penetrations.

Thanks oldognewtrick. Looking forward to the information. The abutments and penetrations are the most challenging part of the job.

cybergoofy 04-01-2014 05:39 PM

Quote:

Originally Posted by oldognewtrick (Post 102792)
this should give you all the details for installing SA products and flashing details. Hope it helps.

Thanks. This is EXCELLENT information. Lots and lots of detail and pix. It will help a LOT.

A question: What do you think of using an 12 inch or so strip of non-SA cap sheet to tuck up under the "drop flashing"? Nail it to the wall and secure to the main roof SA cap sheet with troweled adhesive. I am looking at page 8 of the brochure "Concrete Wall Termination with Surface Mount Flashing" and adapting to my situation. Seems like it would solve my problem with fighting the super sticky SA cap sheet as I try to do the tuck under.

nealtw 04-01-2014 05:44 PM

Perhaps a photo of that problem area.


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