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Flat Roofing with Insulation in Miami Springs

Roofer Mike walks us through a flat roofing project in Miami Springs. Ponding areas, the use of tapered insulation and the difference between GAF Ruberoid and “cap sheet” are discussed.

In Miami Springs many homes have had additions built with flat roofing which can get pretty hot in summer if the ceiling is not sufficiently insulated.  Such was the case with this roof and the owner wisely chose to include insulation in the roofing design. There were also a couple of ponding areas to address and since both were fairly close to the eave we included tapered insulation at the perimeter.

flat Roof Insulation

11/2″ composite board w/tapered insulation

screws and Plates

Insulation and base sheet attached with screws & plates

According to Miami-Dade County any standing water which does not evaporate after 48 hours is considered excessive ponding. I have an issue with this standard as a roof could be under water for considerably longer during rainy season when daily thunderstorms are common. A flat roof with a ponding area which takes 36 hours to completely dry could be submerged for many days at a time. I like the idea of the county’s standard but we like to design our flat roofs so they dry completely in one afternoon.  This ensures there will be no ‘algae farms” developing which feed on the calcium carbonate in granulated systems and can lead to a premature loss of granules.

Tapered Insulation

GAF Ruberoid On Flat Roof

GAF Ruberoid cap

The use of tapered insulation is an excellent way to deal with ponding. Here the eaves were slightly elevated, causing rainwater to gather.   We installed 1 1/2” EnergyGuard Composite Board Roof Insulation to within 3′ of the eaves. We use a minimum of 1 1/2” of insulation to ensure a 7.4 “R” Value.  1 1/2” – 0” X 12” x 4’ tapered edge was then installed. This sloped the insulation from 1 1/2” thick to 0 in a one foot span, leaving a one foot flat area without insulation at the edge. Two layers of GAF #75 Base Sheet  and the insulation were simultaneously attached with 3” Drill-Tec plates and #12 screws in a pattern conforming to Roofing Application Standard 128 and its minimum uplift requirements. This flat roof is not going anywhere. 26 gauge galvanized flashings were then installed and primed for good adhesion.

Flat Roof in Miami Springs Complete

Complete with coating

Torching the Ruberoid Torch Granule cap is my favorite part of the job.  I’d torch roofs for my friends and family for free if I hit the Lotto – really!! Tearing the old roof off is their problem . My boy George torched most of this one but I had to jump in and cook a few rolls just to stay sharp.

Cap Sheet or Ruberoid?

Flat Roof In Miami Springs Complete

Flat roof complete

When comparing estimates look at which material is proposed for the top layer. You will probably see some “cap sheet” or “mineral surfaced cap sheet” in some of the estimates. Cap sheet is fiberglass fibers and asphalt with impregnated white granules. It is made much the same way as paper and tears almost as easily. Look for “Ruberoid”, which is much tougher and more durable due to its polyester mat core coated with APP polymer modified asphalt. They both look the same when installed but cap sheet sells for $25-$30/roll while Ruberoid costs $65-$70. And you can’t tear Ruberoid with your hands. Furthermore, cap sheet simply does not perform well under moderate ponding conditions which are present with most flat roofing in the Miami area.

All flat roofing must achieve a Class A fire rating under standards set by Underwriters Laboratories (UL). The UL Listing for this system calls for a coat of Karnak No. 97 Aluminum Roof Coating. I am not a big fan of aluminum coatings and prefer elastomeric roof coatings which are much more reflective.  But, who am I to argue with Underwriters Laboratories?! I’m OK with it as the roof is at least getting a head start on its maintenance regimen.

by Michael Slattery

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