Flat Roof Repair 1

[blennon]

I have a flat garage roof (8" pitch over 12'), and want to rip down to the sheathing, install ice dam & felt, and finish the entire roof with aluminum flashing.

It seems to me that this would be superior to any 'asphalt' type roof in terms of durability and maintenance.

Is this a reasonable idea (and also up to code)?

thanks in advance,
bl

 

[TheBlacksmith]

I had a similar question some time ago. Might be of some use to you. See thread507-15174.

Blacksmith

 

[ishvaaag]

Flashing use to be more for line elements than surface. Of course cost is a factor, but also constructability. Wings are made of aluminum and riveted and pass more severe test than your use as roofing, all is a question fo how good detailing and construction one can enjoy.

The good looking claddings all are thick enough to limit the deformations out of plane to some limit. Sealants between parts are fundamental part of damp-proofing in walls made of aluminium, and also will in your case. There may be plaques for wall use being applied with scarce modification to finish hi-tech roofs. Expensive in general, as your solution will be compared to any typical damp-proofing of a roof. At your small slope, you may use interlock and still you will have some need to seal, for wind can make penetrate the water even upwards the slope, not only laterally.

 

[Mattman]

Don't do it. You have a 1/8" per foot slope- water will tavel slowly down it. If you use aluminum flashing over your membranes, it will be mechanically attached. You will have many penetrations through your aluminum flashing "shingles" and slow moving water. This translates into leaks. Even if you use gasketed fasteners, your "shingle" material is not self-healing (as are membrane roofing and shingles, to a degree) or compressible enough to allow a watertight fit (as are single-ply systems made for mechanical attachment).

Even caulking the hell out of all your penetrations with a good sealant isn't the answer- 1) you will create all sorts of nooks and crannies to catch and hold water which will- believe me- eventually find a way in or freeze and force openings, and 2) even a "20 year" sealant won't work more than 10 years in this environment, with long-term exposure to water and an elastomeric surface (ice and water shield) as one of its substrates.

Similarly, adhesive or caulk attachment of the aluminum, in lieu of mechanical attachment, will fail. In this case however, a good ice and water shield over the entire roof may save you (if the adhesive doesn't eat it). It may act as a roof on its own.

With your slope, membrane roofing of some sort is required. Remember, even for the best standing seam metal roofs, 1.5" per foot is the minimum slope, and for the best shingle roofs, 2.5" per foot is the minimum slope. For less slope than that, you need a continuous membrane to keep the water out.

I would recommend modified bit roofing for your project, torch applied or hot-mopped. Follow the manufacturer's instructions, put down a recovery board over the deck (you'll be less likely to damage the membrane during installation and when walking on the roof), a base sheet, and the your modified bit (much thicker and more ductile than tar and felt). As an alternate, believe it or not, a 3-, 4-, or 5- ply tar and felt roof is still a good long-term solution to a low-slope roof such as yours.

Because of cost and the size of your project, some of the better commercial-grade membrane systems are not appropriate for your project, but there are some great systems out there these days.

Of course flashing is key. Do it right.

By the way, I'm a licensed architect as well as a structural guy, so I know a bit about roofing. I design roofing systems all the time.

 

[Ron]

I agree with Mattman....use membrane system of some type and Modified Bitumen is most amenable to your application, since there are torch-applied MB systems available, negating the need for a kettle.

Three or four-ply built-up roof is still best system when done properly. I reiterate the importance of the tie-in flashing.

 

[blennon]

I really appreciate all the replys, especially from Mattman & Ron.
It sounds like my idea was rather misguided (heading to disaster), but here's why I was thinking in those terms:

The roof i want to flash is 22' x 12' over an existing room that was added to the 1st floor of my house. The roof is not leaking now, but i REALLY want to put a deck on top of it and thought the flashing would provide a smooth and watertight surface on which to lay the deck support members. So i figured that i could 1) remove the exisiting sheathing, 2) bolt new (perimeter) 4x4's to the existing rafters (every 5 feet) for the rails/balisters, 3) install new sheathing (cutouts for the 4x4's), 4) put down felt and (rolled) aluminum flashing, 5) run deck supports (joists) along the roof slop, 6) attach the decking to the joists.

I know this sounds overly simple, but I have done some homework here, and have built a few decks before (this potential 2nd fl. deck will match the 450sq ft one I built for the 1st fl). The existing roof rafters are douglas fir, 2x10, every 16" and the foundation and supporting walls are quite up to code (according to the span tables this is a 40 psf roof). In addition, the ceiling underneath this 2nd fl. deck is a suspended one, so deflection is a minimal issue.

That being said, I think i need to get some professional advice, as the issue still comes back to the relatively flat roof which apparently needs to be layered with an asphalt like compound, which will obviously not be appropriate for any weight on top of it.

Again, my thanks and appreciation to all responders.