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Question DetailsAsked on 7/6/2016

I live in south florida and have a flat roof. what is the best covering? Hot mop or peel and stick?

choices are:1. tin-tag 1 ply 43 lb base sheet, new eave drop metal, hot-mop modified white granule. 2nd choice-1 layer of 75 lb base sheet, 1 layer fiberglass mid ply, 1 layer insulated deck, another layer fib midply, 1 lay mineral surface cap sheet 37 sqs, 3rd-#75 fiberglass base sheet felt using 1 1/4 ring shank nails, 1 layer GAF rubberoid 2o modified interply, 3x3 factory drip edge metal, 1 layer GAF Mineral surface cap sheet to be set in Type IV hot asphalt.
Thank-you for any help in understanding and deciding

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1 Answer


Note you have a mixture of insulated and uninsulated options here, and one says white granule but other two do not, so the options are not comparable if these are quotes from different vendors - if these are options you can choose from all from one vendor, then fine - but need to clarify the solar heating protection on each.

Assuming all systems will be certified to meet the South Florida hurricane standards (and Dade COunty standard if in Dade County), here are my thoughts:

- Option 1 is a single-ply hot mop roof - not ever a good idea in my opinion, and I am not sure this could meet hurricane code - maybe so. Base sheet is pretty light-weight for a one-layer system anyway, and does not say what kind of base sheet - felt, impregnated fiberglass felt, rubberoid or synthetic, or what.

- Option 2 you did not say how the layers are to be applied, but I assume this is peel and stick or sprayed/rolled adhesive - because a lot of layers to bitumen adhere ? In which case you would need to check about perimeter edging, which both holds the edges down against peeling up and also provide the dripedge. Otherwise asphalt tar-down cap sheet ? If the base sheet is not asphalt impregnated then this too is basically a one-layer system as far as the waterproof layers go. Also - nothing was noted about fastening system for the insulation ? Nail-down or glue-down ?

- Option 3 does not indicate how the GAF Ruberoid is being fastened down - cold adhesive or hot bitumen ? With a 75# base sheet and asphalt adhesion of cap sheeet I would presume bitumen. Not sure which (or both) meet applicable Florida or Dade County (as applicable) standard - but certainly needs one or the other or you would have a flapping roof.

Note one of the most common options was not stated - don't know if you ruled it out or what, but multi-layer hot mop felt with granule covering was not listed - basically option 1 but with multiple layers of hot mopped felt with a final hot-mop granule covering. Provides a lot more leak protection and somewhat more wind-driven object penetration protection than a single-layer system.

You did not say if the intermediate fiberglass midlayers are bitumen-impregnated or not - makes a big difference. I am assuming here they are just the normal buffer layers of fiberglass mat like GAFGLAS - not waterproof layers.

Personally, while I would recommend 2) if it had two waterproof layers in it (cap sheets tend to get punctured easily, especially in hurricane/tornado/heavy thunderstorm areas like yours and also crack from sun exposure, but a self-sealing waterproof layer under the insulation (assuming proper drainage and slight slope to liner) can provide a lot better protection against puncture or cracking leaks.

As you stated it, I would choose #3, #2 in that order - I would not consider #1 at all.

As to hot mop or peel and stick in general - hot mop has the advantage (in most systems) of multpile layers of the waterproofing media whereas peel and stick is commonly a single layer of protection. Hot mop does however form pockets and holes through it due to gas bubbles, and cracks with exposure to the weather. Peel and stick is generally more readily punctured, sometimes has adhesion problems causing lifting and tearing in heavy winds, and also (if the only one layer) has weathering cracking issues.

Answered 3 years ago by LCD

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