Expert: Ron Haynes - 5/2/2006

Question
I have owned an office building for 30 years, part
of the roof is flat with hot tar and rolled roofing.
The front part is a series of steep v's and valleys.
This part has comp shingles and metal flashings in
the valleys.  My son saw on the internet some type
of spray foam that swelled up like "Great Stuff" &
got very hard.  Do you know what that product might
be or what other sggestion would you have for me to
fix the leaking?  It has been a constant problem
that the roofing compoany here has not been able to fix.  Thanks Bob Barnes P.S. The total square footage of the
slab of the bldg. is 1900 sq. ft.


Answer
Bob,
Not knowing the exact layout of your roof I'm not sure what might be best suited for performance and appearance but I'll offer the following for information.

There is a spray polyurethane foam roof system that can be applied over existing roof materials.  It is referred to in the industry with the acronyms SPUF, SPF, PUF.  The foam covers the roof area, insulates, and theoretically provides a suitable uniform surface for the recommended waterproof coating that needs to be applied.  While you can do wonderful things with the foam (it was used on Planet Hollywood restaurants), it requires special equipment and a specialty contractor to apply.  Where your roof might be highly visible, you definitely want an experience person applying the material so that you avoid an ugly, lumpy, uneven finished surface.  It is best suited for drier climates from an application standpoint.  Wind and humidity during the application can affect quality, especially when an inexperienced laborer is applying the material.  Some of the recognized manufacturers of these materials, in no particular order, are Dow Corning, BASF, GE, Gaco Western, North Carolina Foam Industries, among others.  

Also because you mentioned your son noticed a spray roof that gets "hard", this may be a spray-applied polyurea roof.  This is a relatively new system that I have no firsthand experience with.  I do know the system is spray applied and has a very fast cure rate so that you have a roof that can handle the weather almost as fast as you can spray the material in place.

With any roof system that you apply over an existing roof you do have to be careful that you are not trapping moisture either on the existing roof surface or trapped in the existing roof materials, especially if you building has a wood deck.  Any entrapped moisture can lead to rot and cause problems with the new overlying roof.

Other cost effective alternatives might be installing a separation board like a moisture resistant silicone modified gypsum board (such as Georgia-Pacific DensDeck or a similar US Gypsum product) or a high density wood fiberboard and covering with a single-ply roof membrane on the flat roof portion of the building and re-shingling the steep areas.  This would work well if the flat roof portion has no insulation on a wood deck.  It could also be used on an insulated deck if measures are taken to ensure there is no moisture in the existing insulation, or remove and replace any wet insulation.

Best options, but more expensive, might be removal of the the existing roofs and installing new.  In cost order from least expensive to most expensive, the new flat roof coverings might be single-plies (EPDM, TPO, PVC), modified bitumen which can be a one or two ply system and can be installed in hot asphalt or carefully applied with a torch, or a new "tar and gravel" system.  If you are in a high wind zone, such as the southern coastal regions, and you choose a single-ply system, I would recommend a fully adhered system as opposed to a system that is intermittently screwed down with screws and plates.  

For the shingle roof area, if you reroof with shingles, I would highly recommend lining the valleys with a peel & stick membrane prior to new valley installation.  This will provide a good secondary protection to water instrusion, and if in a cold climate it will provide protection from damages caused by ice build up.

I have my own approach and system preferences but budgets are typically the driver in selecting the approach to be implemented.  Just remember the lowest cost alternative may also be the most short lived.

If you want to see if there is a SPUF contractor in your general area, go to the following link:  http://www.nrca.net/consumer/find/ .  Select "spray polyurethane foam" for the roof type, enter you zip code, and select a radius in miles for the search area.  Some SPUF contractors are also polyurea roof system applicators.

Good luck!

Ron