Built up vs SPF roofing

elmaur

I have a flat roof- slightly sloped- 3(?)ply built up roof with the coating badly deteriorated- about 2600 sq ft. I was told 2 1/2 years ago I need a re-roof meaning removal and replace all existing material down to the plywood with new Elastek coating.
I just learned of SPF- spray polyurethane foam as a roofing material. What are pros and cons of this vs built up? Is SPF a better insulator against heat? I am in Arizona with 110 degree summer days.

jagans

To determine the number of plies you have you take 36, subtract 2 inches for headlap, and divide by the number of plies. So if you have a three ply roof you have an exposure of 11.33 inches.

Were I you, I would use EPDM and coat it white, or White Sarnafil PVC.

EPDM has a much broader user base, though, so finding a contractor will be easier.

__________________
" A lot of men build things, and a lot of things fall down "

1985gt

Tear off, 1.5" iso, EPDM with white coating or PVC. I would stay away from the SPF roof.

elmaur

I just dug up 2 estimates from local roofing companies that use a 3 ply fiberglass system with hot asphalt between plies and then 2 coatings of Elastek. I could be wrong but I don't think EPDM is commonly used here for residential.
Any thoughts?

jagans

You cant beat a properly installed BUR. Key "Properly Installed"

Whats the substrate, wood?

__________________
" A lot of men build things, and a lot of things fall down "

elmaur

The substrate is sheets of plywood.

jagans

Are you going to insulate this roof? In other words, where is the thermal insulation going to be in this assembly?

__________________
" A lot of men build things, and a lot of things fall down "

jagans

What is the slope of this roof?

__________________
" A lot of men build things, and a lot of things fall down "

elmaur

I do not know the exact slope but it is commonly referred to as a flat roof.
Do you mean insulation as part of the roof system? I don't think thermal insulation was discussed. It is not on the estimates. I think it would raise the cost considerably. There is fiberglass insulation in the ceiling below the plywood.

elmaur

jagans , you did say "thermal insulation as part of the assembly" so again it is not in the estimates. Is this common practice and would it raise the cost considerably?

jagans

As long as you have insulation in the ceiling below the plywood you are fine from the standpoint of energy efficiency, to a point. In other words you have what we in the business call a cold compact roof system. In other words you do not need thermal insulation in your roof system.

You do, however, need a shear plane between the roof system and the wood deck. You can use a glass base sheet nailed to the wood deck with cap nails, and then apply your built up roof, or you can install a layer of insulation and fasten that down to serve as a shear plane. Now, if your roof slopes to a drip edge, you are going to want the roof membrane to break down right at the drip edge so you do not end up with ponding right behind the drip edge. So in your case, I would screw down 3/4 inch perlite insulation with 3 inch metal plates and roof deck screws, then chamfer the edge at the drip edge.

I would then mop 4 plies of ASTM D-2178 Type 4 Glass felt in ASTM D-312 Type III (Steep) asphalt. Flash, but do not flood coat. Apply fibrated aluminum coating (2 Coats) to completed membrane. Recoat about every 7 years.

Do not use Type 6 felt, it does not "Wet" well.

Note: It makes no sense to install a 3 ply roof if you have a relatively small residential roof, when the 4th ply will cost very little.

If you decide to forego the insulation, install a ASTM D-4601 Glass base sheet nailed with cap nails to the wood. Google GAF BUR Installation for help.

Never mop directly to wood.

Good Luck

__________________
" A lot of men build things, and a lot of things fall down "

elmaur

jagans, I appreciate the info but beyond the basics I will have to leave the details of how the job is done up to the company that I hire. Hopefully I make the right choice.
However whether they do a 3 or 4 ply system interests me. The 3 estimates- all in the $6k range-mention 3 ply consisting of a base layer with 2 additional layers. One of the estimates specifically states a fiberglass roof system with a 28 lb base and 2 plies of 12 lbs. Should I inquire about 4 ply? The roof surface is almost 2700 sq ft.

elmaur

As an aside, would I have a skylite installed before or after the new roof is done?

jagans

I would consider a base and two to be a two ply, because the base ply has nails through it. This has been argued ad nauseum in the roofing business, so I would want at least a base and three.

The roofer should install the skylight as part of his work because he has to flash it, and he will need cant strip to flash it.

I would use the ASTM designation I gave you for the sheet goods.

Good Luck.

__________________
" A lot of men build things, and a lot of things fall down "

1985gt

We also do not count the base sheet as a ply.

Jagans again is right on in his recommendations.