Roofing Contractors & Hot-Mopped Roofs - Beware!

Roofer Mike exposes the dirty secrets of less scrupulous Miami roofing contractors. The focus is on hot-mopped flat roofs and the shortcuts they employ to fool inspectors and cheat homeowners.


People sometimes ask what I have against hot-mopped flat roof systems. Experience has shown that many Miami roofing contractors employ shortcuts which are difficult for the homeowner to detect. Indeed, these shortcuts sometimes get by inspectors as well. Fact is, these roofs are fine when done correctly. Quite a few old 3-ply tar and gravel roofs survived the Cat 5 winds of Hurricane Andrew and many of the hot-mopped replacement roofs done after Andrew soon failed. We'll address three of the more common shortcuts here.


Cap Sheet For Ruberoid – The Switch

A popular system sold by roofers is a Built-Up Roof (BUR) which basically is layers of fiberglass sheets, usually three, fused together with hot asphalt to form a waterproof membrane. Lesser BUR systems call for an aluminum or white elastomeric coating over the membrane for a protective top layer. A more robust system includes a granulated fiberglass cap sheet as the top layer and an upgrade from that is a granulated modified bitumen or Ruberoid cap. Unscrupulous contractors will sell the roof with a modified cap but actually install the fiberglass cap sheet. They both look the same but modified costs twice as much and is far superior.
A little tip: fiberglass cap sheet tears easily while modified products do not.

Jump-start 3-ply 

A typical 3-ply begins with cutting 12" off a 36" roll of fiberglass and mopping it along the eave; then mopping the remaining 24" of the roll over that along the same edge. A full 36" sheet is then mopped on the edge with each succeeding sheet mopped in 12" increments across the roof. This creates the three-ply across the entire roof. The cheaters, and I have personally busted sub-contractors for this, will simply begin with a full sheet and then proceed with their 12" laps which leaves a much weaker perimeter and is almost undetectable. This causes premature failure at the eaves.

Hot Tar Kettle Too Hot

Hot-tar kettles are equipped with a thermostat which allows roofers to heat the asphalt to a proscribed temperature, by Code, in the EVT (Equiviscous Temperature) range. The EVT range of commonly used Type IV asphalt is 425°F +/- 25° at the point of contact so the thermostat should read about 450° - 475° at the most here in South Florida. The cheats will jack up the temperature to 500° or more which comes dangerously close to the flash point and also gives the asphalt an almost watery consistency. The asphalt loses its viscosity. This allows them to work much faster but doesn't put enough asphalt between the plies. The end result is a thinner than optimal membrane, blisters due to delamination and premature failure. I have also seen this with tile roof hot-mop 30/90 underlayment.

Still Want a Roofing Contractor to Hot-Mop Your Roof


Check the workmanship warranty when screening Miami roofing contractors. We warranty our flat roof systems for a minimum of ten years. Stay away from anyone offering much less. 5-year roof warranties are a big red flag. Check their website for signs of pride in workmanship. Do you see detailed descriptions of projects or general marketing “fluff” proclaiming honesty and integrity? You might also want to read ROOFING MIAMISTYLE, also by Roofer Mike.
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