Flat roof 2-ply torch-welded: what is it?

I just got my flat roof done. It slopes inwardly (about 1/5) to a central drain. Here is what they did:

  1. Removed old roofing material (tar, paper, gravel - many layers of it)
  2. Added a layer of 1/2" fiberboard over the bare wood
  3. Layed down a rolled membrane 3-4 feet wide, torch-welded at the seams
  4. Layed down a second rolled membrane, torch welded it to the first over the entire surface. The second membrane has a fine gravelly-looking surface coating on it

Here are my questions:

  1. What kind of roof is this? SBS?

I feel a bit of a fool because I just trusted these guys, not doing any research… I’m now trying to understand what I’ve got. Which leads me to question number 2:

  1. They assure me that the blisters I see in it are “normal”, and a result of movement due to temperature shift (“it will blister more on sunny days”). Could this be true, or is this more likely a load of bull to cover up a poor installation?

Any help from you guys would be most appreciated.


hello glabute,
sounds like you got a good system.
i beleive you have a double layer torchdown.
1st layer smooth surface, second layer granulate.
i dont think nobody knows yet how long the material last.
atleast 20 years per layer.
so you got your self a pretty good roof on paper.
i used install one layer of the granual torchdown, over a nailed on base sheet, and thats it. thats how good that product is.

the blisters do concern me though.
it should be pretty much flat .
but sometimes at the end of a roll it can get a little bumpy.
its happened to me before. usually when its cold as hell outside.
and when the roll shifts on ya while applyin, that also can leave a small wrinkle or two.
so a couple of small wrinkles to me is ok.
but in my opion you shouldnt have a lumpidy bumpidy new torchdown roof, most of mine are perfectly flat.
and by no means should you feel a sloshing around , like water under your foot when steppin on theese little blisters.
if you feel water. thats bad.

other than that, as long as all your edges , penatrations, drains, are all sealed good , you should be ok.


Hey Gweedo, wow, thanks for the speedy reply. You’ve comforted me. There aren’t too many wrinkles, and they’re definitely not sloshy. Since you’re in Florida and I’m in Montreal, it probably was cold as hell (on your scale) when they installed it a couple weeks ago.

Would it be rude to ask what’s a typical ball-park cost of the torch-down membranes? Just materials, no labour. I figure it’s about $3-4 per square foot, for the materials. Is that reasonable?


you can go to bradco roofing supplys web site and find out your local
cost for a roll of torchdown.

down here theyre fifty bucks a roll.
so thats 1 dollar per sq ft.
you can get it installed down her for 3 or 4 dollars a sq ft.


Can you post any pics?

I wonder if they used a self-adhered base. If so, I do not think I would trust it over wood fiber. No holding power to wood fiber wiithout a saturation of asphalt to bind.

Did they nail or screw the first layer down?

yeah AaronB, it struck me funny that h/o only mentioned heatin up the feild on the second layer.
did they dry lay the first layer?
that would certainly explain the wrinklin effect.


they dry-layed the first layer, tacking it down every couple of feet along the top edge with large-headed nails. They just welded the seams, up to about 4 inches or so from the edge. Then the top coat was torched over the entire surface.

Unfortunately no pics - we now have a couple inches of snow so I can’t take any for a while.

The wrinkling isn’t really bad, it just isn’t flat so it’s kinda ugly. Let’s say there’s a small (1-foot-long) ripple every 10 feet or so. Maybe I’m wrong there - have to wait till the snow melts.

Also where the membrane goes up the firewall, it just rounds the corner, not sitting tightly at all where the roof meets the firewall. The roofers said it was ok as the stuff is super-tough and won’t tear. Seemed weird to me.

Thanks for the pricing info, that helps.


so what’s this double-layer torchdown stuff made of?

Cerberus probably has a better explanation, but it is bitumen (asphaltic) modified with polymer plasticizers to raise the melting point, lower the harndess, and provide more flexibility to the membrane sheet. A couple two or three layers sandwiching poly scrim for reinforcement.

Hope this helps.

I dont like any wrinkles. I like a fastener every 12 inces on seams and 18" offset rows in the body of the sheet on the base ply, all secured taut with screws and stress plates, but nails can work if you have a ringed shank.

so i guess they didnt want to torch to raw wood, and nailed on the first layer the way they did it.
im shure it will work, but i feel one layer of torchdown over , ring shanked cap nailed (thank you AaronB) fiberglass base, is more than enough.

so basically they used a way heavier base sheet which doesnt lay very flat
unless u heat it up.

so again i think your ok. the material is plenty strong enough.

there is one more reason i dont do two roofs one over the other.
and i should charge for this.
the reason is " roofs have leaks".
roofs have leaks and you have to be able to fix them.
when i have a leak( very rarely) i want it to leak out the ceiling
(in the house), somewhere close to the problem in the roof.
some of the hardest roof leaks ive had to fix ( not mine) were on two layer roofs, were the water would enter the first layer and run 50 ft somewhere else before comin in the second roof.

water and ,im sure ice too, have the most strenght when they can get inbetween somethin.


It’s true what you say Gweedo about multiple roof layers hiding the leaks. That’s exactly why I couldn’t patch the old roof and had to completely strip it down.

But my understanding is that the 2-layer torchdown is equivalent to one layer because it’s completely welded over the entire surface. I hope that’s true.

I suppose there could be imperfections in the welding which would allow water to leak between the layers, but unless those imperfections are in the form of long channels, the water should stay put, right?


PS did I mention that you guys are terrific? Thanks Gweedo and AaronB!

would love to hear from Cerberus about what’s on my roof

your welcome,
and yes that is how they sell it. a completely welded together system.
but in order to acheive that you really have to heat the crap out of the top layer, while tryin to keep it strait and not burn the house down.
this is all very hard to do. it starts to get really sloppy whe heated alot.
so are there spots here and there that mite not be fully torched? probably so.
are they somethin to worry about? no.

youll be just fine.


just saw www.ornithopter.org
hats off to you mr. gweedo
you are one interesting character
and thanks for the roofing advice
you are obviously someone who uses his head
I feel much better

Somethings missing in your description, and I’m not quite sure what it is.

First of all, if they nailed the first layer of membrane it is possible they installed a modified base sheet and not a smooth-surfaced APP modified bitumen roofing membrane. If that is the case, then it doesn’t make sense as to why they torched the side laps. If it is a smooth-surfaced roofing membrane, then the roofing contractor should not have nailed the first layer; it should have been torched down or adhered. Of course, the contractor may have just used a smooth-surfaced membrane for a base and torched the seams to provide a pseudo 2-ply modified bitumen roof system. If you can tell me who the roofing material manufacturer is I could probably help you better.

Now, lets talk about your problem. Do you have blistering or wrinkling? If you have blistering that is not a good thing and likely won’t go away depending on where the blisters and located and what caused the blisters. If the membrane was installed using a cold-process adhesive, then the blisters WILL go away in time. However, if the system was torched down as you say I suspect the blisters are between the two roof membranes and likely won’t be going anywhere. If you have wrinkling instead of blistering, this could be an application problem, or is could be caused by material shrinkage. Once again, I’d be curious whose product you have so I can know if it contains a fiberglass or polyester scrim, or both.

As for what you have. It is most likely an APP modified bitumen roof system; however, there are some torch-grade SBS systems. Still, my bet would be on APP (Actactic PolyPropylene) as the asphalt modifier.

Anyway, I’ve got to go watch the Steelers play the Browns. I hope I’ve at least started to answer your questions. In the meantime, take a look at what the contractor proposed, any wrappers or materials they may have left behind, or anything else that will tell you whose material it is and maybe I can help you more after your next post.

One last thing. You say the roof drains to an internal drain. Is there only one drain, and are there any emergency overflow scuppers? If there are scuppers, how high above the drainline are they located???

Anyway, get back to me and I’ll try and answer any other questions you may have.

I just thought of something. Is it possible the contractor nailed a base sheet, torch-applied a smooth-surfaced modified bitumen roof membrane, and then torch-applied a granule-surfaced membrane? Are you sure there is only two membranes installed and not three?

Hi Cerberus,

Thanks for the feedback. I’ll try to find out from the contractor what the name of the product is. I’ll post again as soon as I have some info.

In the meantime, I can say that there are only 2 layers. Probably the first layer was not torched down because it would not have stuck to the fiberboard underlayment (as was suggested in an earlier post).

They are wrinkles, not blisters. They were there when the first layer went down, and the second layer seems to follow along with them. I can’t swear that the second layer wrinkles are in the same place as the first layer wrinkles, but it’s plausible.

What is an overflow scupper? The roof drains like every other roof I’ve ever seen in this city. It’s a hole at the lowest point, pouring directly into the house’s main stack. There’s a little wire grill-thing to keep out the leaves and other gunk. Should that get clogged (never heard of that happening) then I would get a lake up there until it overflowed off the back end of the building. Probably the roof would cave in first, though.

Just because you aren’t aware of drains getting clogged and causing problem, doesn’t mean it doesn’t happen. If I were you, I’d make sure you keep your roof drain clean and operating properly.

As for the wrinkles, you are probably stuck with them, but I would make sure the wrinkles are in the base membrane and not between the two membranes. You can check this out by pushing down on the wrinkles, and is should be obvious if you are feeling two membranes give or one.

Another thing that is of concern is the mechanical fastening of the insulation. You mentioned is was nailed down to the roof deck, and based on your post it sounds as if it were nailed down in conjunction with the base ply membrane. If this is the case, and cap nails were used, I’d be a little concerned about them backing out and damaging the cap sheet. A minor concern at this time, but something to keep an eye on.

Anyway, gather more information if you can, and get back to us.

Hi folks,

Happy new year to you all; I hope your vacations are going well!

I got some details about what’s on the roof:

It’s Elastomere Membrane by Soprema

1st layer: PS250
2nd layer: Flam250, granulated (grey)

Underneath it all is a layer of 1/2" fiberboard. The fiberboard is nailed down. The first membrane is nailed down (torched at the seams), the second is torched over the entire surface.

So my questions to you are:

  1. is this a quality product?
  2. are the wrinkles that I described a problem?
  3. is $1/sqft/layer a reasonable ballpark figure for the cost of the product? (not including installation)
  4. apart from periodical inspections of the drain is there any regular maintenance that I should be doing?

Feel free to wait for your hangovers to clear up before getting back to me on this :wink:

Thanks a bunch, you guys are awesome!


  1. yes, it is a good quality product
  2. yes,they are a problem.if not now then down the road thay will.especially if you live in an area where temps go below freezing.
  3. yes,that is a reasonable price for that product.
  4. a periodic walk of the entire roof surface for holes or tears in the gray membrane would be a good idea.