How to Install Torch Down Underlayment

In this video, I wanna show you how to install the base layer of any torch down roofing system. It’s gonna be either a glass base or Polyglass’s Elastobase. Let’s get into it.

What is glass base underlayment?

Alright, so the purpose of the base layer of any torch down system is really for fireproofing. Torch down systems are not fire-rated as far as the cap sheet or the mid-plies, so the base layer is what gives us our fire-proofing. In California and most cities, we need class A fire rating. And in order to achieve that, what we need to do is install two layers of glass base. Glass base is a fiberglass mat, pretty much it has granules on both sides. If you’re using glass base as your base layer, you have to install two layers. In this demo, we’re gonna be installing Polyglass’s base sheet. The reason we like it is it’s thicker than glass base. The backside looks the same as glass, however the front side has a smooth surface that actually has a bitumen surface on it, so it does a great job bonding to the second and third layers of the roof. And on top of that, you just need to install one layer. But essentially both of these, whether you installed glass-based two layers or one layer of this, you still gonna be getting your Class A fire rating. Let’s get into it.

Getting started: laying down your glass base roll

Alright, the first thing you wanna do is lay out your rolls. We laid out our first roll here, you can see it’s just loose laid on top of the plywood. Now you wanna go in the same direction that your regular torch down, or you top rolls, are gonna be going. So in this case, we’re draining to this point right here, in our mock we’ve got a scupper here, so we’re gonna be installing our top rolls this way. So we’ve got our base sheet laid already. Now we’re ready to start nailing.

Nailing pattern and type of nail to use

You can see here, there are these marks marked at 12 inches from the edge, and along these marks we’re gonna be nailing every 18 inches on center.

What we are using is ring shank metal cap nails. So what these do is, first of all we need to have metal, so when we are torching our top layer the metal does not melt and the ringed shank helps it grab onto the plywood.

Overlapping two layers of underlayment

So any time we’re coming on overlaps here, we wanna lap one layer over the next a minimum of four inches, and Polyglass has this marked out so that it makes our job super easy. We just follow that 4 all the way across laid down. Now, on this lap here where we have two layers going over each other, we’re gonna be putting nails in every nine inches on center, instead of the field where we’re doing around the 18, around the edges we wanna do closer together.

Purpose for layering

You can see here along this edge where we have this overlap, we’ve installed nails every nine inches on center. There’s two reasons for that. The first one is, we wanna make sure we have a smooth transition where we have this double layer here. Second, and most important, is we wanna make sure that this is nice and tight so that when we’re torching the next layer on top of it, we don’t have any flames going in underneath that. Because again, remember this base layer is super important for fire-proofing purposes, so we wanna make sure that all these edges are tight so that when we’re torching, we don’t risk getting down to the wood layer.

How to install underlayment over a parapet wall

Alright, so in this mock roof, we have a parapet wall build-out on this side. You can see here that we already have our cant strip installed. This cant strip is pretty much just allowing us to break this corner and have a more round and smoother transition. When installing your base layer on your roof, you wanna make sure that you come up and past the cant strip, at least an inch or two. You don’t have to go all the way up the parapet wall, as that makes it a lot harder. The way we like to do it, and the way manufacturer recommends, is to come up this parapet wall, pass the cant strip, cut it, terminate your rolls there, then come in with a new piece depending on the height of your parapet wall, and install this base layer. And when you install the base layer on your parapet wall, you wanna make sure you come past that cant trip, so this corner you have a double layer.

Final important notes on installing underlayment on a flat roof

Now that we’ve finished installing the base, I wanna show you a few details. You can see first off, this edge right here is not nailed down, and that’s done purposefully. When we install our low rise flashing, we’re gonna be nailing this down. So we don’t want any round cap metal nails bothering the edge right here. So that’s just around the perimeter where we’re gonna have our low rise flashing.

You can see, like I mentioned before, we have nails every 18 inches on center in the middle, and every nine inches on center along the seams. The nails in the center, we wanna have them 12 inches from the edge, and that’s gonna be on both sides here. So you can see we’ve got two rows of nails going down, and pretty much before we start torching, we wanna make sure that there’s no more wood left exposed on the roof. Torching and wood don’t really go hand in hand, so we wanna make sure that we fully covered our entire wood deck prior to torching.

Let’s continue to the next one.

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