Roll Roofing questions

[TomR]

I have a small roof area over a garage that I am thinking of having re-roofed with roll roofing. It is a small flat roof area (about 14' x 17') with a low slope (about 3/12). The plan is to tear off the existing roll roofing down to the plywood deck, and then re-roof it using roll roofing. I have a contractor friend who can do the work and who has done regular asphalt shingle roofs etc. in the past. But, he has not done roll roofing and he and I are not quite clear on a few of the details on how roll roofing is done.

I've been doing Internet searches and I can't quite seem to come up with instructions that are clear about exactly how roll roofing is done. Here is one link that I found:

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My friend was thinking of doing the tear-off, then putting down tar paper (felt), and then the roll roofing on top of that. The above link shows the roll roofing going directly on the plywood decking with no tar paper/felt underneath. Is either way okay? Or, is one method better than the other?

For sealing the laps where the roll roofing overlaps itself, I have seen a number of different terms mentioned for which product to use to do that. I have seen "lap cement", "roof cement", "roof adhesive", "asphalt plastic cement", etc. When I go into Home Depot, I see a bunch of different products that all seem similar but I don't know which one to pick.

So, one question I have is, if I buy regular roll roofing at Home Depot, what product should I buy to seal where the roll roofing overlaps? -- "lap cement", "roof cement", something else?. If you happen to know the exact Home Depot product name, that would been even better so I get the right product.

Also, we were planning on using the "Application Parallel To The Eaves" and "Concealed Nail Method" described on the above link. If we do it as shown in the link -- without any tar paper/felt -- would there be any advantage to also just gluing down the whole width of the roll roofing to the plywood deck (in addition to the concealed nailing), rather than just "gluing"/sealing the overlaps? If so, what product would we use to adhere the entire width of the roll roofing to the bare wood deck? -- lap cement; something else?

Any suggestions or links to good instructions or a good YouTube how-to video would be appreciated.

Thanks.

[Doug]

No and I don't mean to hijack your question here but I've got 2 questions of my own that come to mind for you. Is this the same thing they do on many residential roof tops in NYC as it sounds similar?

2nd, why do you want this instead of the shingled roof? I'm sure you have your reasons, just curious.

[clare]

Put down blueShield or equivalent first - or better yet a "torch down" membrane - then the roll roofing for protection.

[clare]

3/12 slope is too flat for normal shingles.

[TomR]

I don't know. I am not in NYC. I'm in Southern New Jersey.

The slope is just a little to shallow for a shingled roof. I wrote "3/12" but it's more like "2/12" for the slope.

[fred.flintstone]

The instructions are right on the rolls of the roll roofing.....

Roll roofing is crap. If you want a roof that will last, use rubber or steel. It costs more, but you'll be replacing roll roofing every 3 to 5 years, and that is costly and time consuming. Do it right the first time, and forget it for many years.

Your roof is nearly the same size as my tool shed. It had roll roofing when I bought this place, and was in bad shape. I priced roll roofing and dound the cost would be about $125 for materials. I bought steel roofing (pole barn ribbed steel) and spent $220. ($95 difference). That steel roof will likely outlast me, with roll roofing, I'd be spending $125 in 3 to 5 years again, and having to do the job over again.

The steel was easier to install too. No messy roofing cement to cope with.

However, if your roof is completely flat, you cant use pole barn steel, so you'll need a costlier steel or rubber roofing. But I'd still consider getting prices for a better roofing material.

PS. I'm not sure what the slope is on my roof, or how to calculate it, but it drops 27 inches in 14 feet. Sufficient for the pole barn metal.

[Doug]

Thanks Tom. I bet it is similar to NYC because the roof tops I had in mind there are essentially flat ... tho they probably have a slight pitch in reality. Good luck.

[TomR]

Thanks, but for various reasons, this is going to be a roll roofing job -- and not a rubber, torch-down, TPO, steel, or any other kind of roofing job. I do know the pros and cons etc., but in this case all I need is info on doing it as a roll roofing job.

[Pete C.]

Metal roofing may be nice, and ok for a separate structure, but it sounds like this garage may be attached to a house so the roofing may need to blend in better.

27"/14'= 1.9285714 or roughly 2/12 slope i.e. 2" rise for every 12" of run.

[clare]

You are about as close to 2/12 as you can get. 2 inches in a foot would be 28 inches over 14 feet.

[fred.flintstone]

Thanks

What is the formula used to figure this? I have to admit being an idiot when it comes to mathematics. But I can plug in figures if I know the formula. The shed itself is 10 foot high in front and 8 foot in the rear. It's 12 feet wide. But there is one foot overhang on both front and rear, thus 14 feet of roof. Therefore the roof is higher in front and loer in back from the actual building walls.

[Gomba]

Look into GAF Ruberoid mop down, cold process. No doubt you have to buy it through a roofing supply house, do not believe you will find it in your typical box store.

It's very flexible/pliable, only about a 4" over lap. You can bend the stuff in half, and not crack it. You mop down the adhesive, on the overlap you trowel down Karnak or similar lap cement. The nails are concealed. Good stuff, at least it was about 15 yrs ago.

[Gomba]

Around here, torch down is no longer allowed, cities won't allow a permit when it finds out it's a torch down. The insurance companies will no longer insure torch down methods. Been this way at least for 10 years.

Guess there were too many fire bugs!

[clare]

Torch down membrane is still OK here, but hot tar is just about done.

[knuckle-dragger]

Oh, nonsense. I replaced my roll roofing just after I moved into my house 35 years ago; I replaced it again about 10 years ago. Since technology keeps improving, I guess I'll get 15 to 20 more years out of it. Nowhere near 3 to 5 years.

Single sheet rubber is supposed to be even better but the problem is that it costs about 3 times as much and is usually only done for large commercial premises. The cost is in the labor and transportation. It's all special order and lifting it to the top of a building (even 4 stories like mine) requires a crane. It also requires lots of skill and experience to lay, not just a couple of day laborers.

[Harry K]

Yep, I have a low slope 'shed roof' over my back porch. Did roll roofing some 10 or 15 yiears ago and it shows zero signs of damage.

Of course if one doesn't apply it correctly...

Harry K

[krw]

I had "half-lap" on the shed dormer of my house in VT. The house was eight years old when it started leaking. It was utterly gone. I had it replaced with 30yr shingles over ice and water shield. Worked great.

...and only you know how to do that, right?

Never again!

[bucketagogo]

I live in Salt Lake City. My garage roof has rolled roofing on it now. It does not leak. If I were to replace it, I'd use asphalt shingles. However, Allstate wants to cancel my home owners policy because of rolled roofing. Are they serious?

[Stormin' Norman]

Rolled roofing for flat roofs, in some jurisdictions, is not code compliant. Modified bitumen would be a good alternative. This could be why Allstate wants to cancel.

Here is a little info on the topic.