Tatnic

Being an engineer, I tend to fret over some things that others may not care about. For instance, I don't like the idea of using so damn many fasteners for a flat roof project. You're screwing down the lower layer of tapered insulation, then the upper layer, then you're screwing down the membrane. You've got a lower layer of insulation (and roof deck) with way too many penetrations. Isn't there another way to do this without so many fasteners? Again, I've never done roof work but here's what I'd do if it were my roof (please let me know where I'm going wrong here):

First of all, I'd rather have the insulation in one layer, even if it is tapered. But if that's not possible, then I'd section off the roof in segments that could be done in one day or less. I'd spot glue the insulation to the deck, spot glue the upper layer to the lower layer and then immediately place the membrane and mechanically attach it. That way I have only one set of fasteners and not 3. Alot less thermal bridging and damage to the deck. I'd also try to hit the 2x8's that are 16" o/c., but that's not going to be simple. Am I worried about nothing here?

thanks in advance.

OUTLAW

There are many ways of attaching a singleply roof. Some such as ballasted require no fasteners.

For futher information please refer to The NRCA's Roofing Manual: Membrane Roof Systems. Or go to your local supplier and ask for a manufactors specification manual.

Either can provide a ton of information and hours enjoyable reading.

__________________
http://www.ClarkRoofingAndSiding.com

rwolfe

Use lightweight concrete and fully adhere fleece back system to it. Depending on configuration, LWC could have lower installed cost that tapered iso.

Having said that, you could also secure bottom layer of board then adhere remaing layers in asphalt or low rise adhesive. Low rise gets pretty expensive though.

By the way, with mechanically attached roof assemblies like rubber, tpo and pvc, it is typically not necessary to put more than four fasteners per board. Factory mutual typically tests mechanically attached systems using preliminarily fastened insulation assemblies. I'm not saying this is right. I'm just saying that's the way they test. The idea is that the seam fasteners are going to hold down the boards. I find that interesting considering the wide sheets are now 12 feet wide and insulation is only 8 feet long.

Tatnic

Sorry I didn't explain the roof system as it exists:

Ballast on EPDM, over perlite (1"~3"), over bur, over 1/2" plywood, supported by 2x8's, 16" o/c, 126" span. Max. load is only around 52 psf, so I don't have the libery here in NH to add any more dead load than I can. I'm pulling off everything down to the deck and going up with tapered polyiso and mech. attached PVC or TPO. I'm looking at a live load for snow of 35 psf. So, hopefully this makes my question a little more clear.

rwolfe

I here you. Ballast on a tapered inulation is being removed. Not sure of the weight but I have to think the assembly must weigh around 20+ pounds per s.f. in areas where tapered gets around 4" or so. LWC comes in around 25-30 pounds per s.f. in the same area. So your not too much heavier. May be worth getting an engineer to do some dead load calcs. for the specific building. If you could do this assembly with LWC it may save you a few bucks. Maybe not.

Instead, why not loose lay all of the tapered insulation then install 4 fasteners per board thru the top layer into the wood deck. Then come down with Mechanically attached tpo. Seems like that may make more sense if the LWC is not an option.

One other option, but it can be a little more labor intensive. Tin Cap a heavy base sheet down like a nailable stratavent (or any 75# if the budget is tight) and hot mop the insulation layers into place. Then slap down a fully adhered tpo.

If UL Class A is required you may need a dens deck or fire-out may be needed no matter which assembly you need.

rw

Tatnic

I am the engineer. I've done the rating on the roof and it wasn't built with much factor of safety way back when. That's why I'm trying to reduce the dead loads and gain capacity for the live loads (snow). I will say though, that the roof has been through some pretty harsh winters and has not shown any signs of problems, so that empirical evidence gives me some comfort from a structural standpoint.

My roofing knowledge a couple of weeks ago on a scale of 1 to 10 was around 2~3. Since I've been coming here I've pushed that up considerably but am still learning about the industry. I appreciate the info and opinions, and heated discussions too which can be a learning process as well.

Lefty

Hi,

If the insulation is only one layer, you will have the thermal bridging in all the joints. Staggering multiple layers stops this.

We only screw the top layer of insulation.

The amount and placement of screws are for wind uplift.

Grumpy

If installing multiple layers of insulation, we'd only install one or two fasteners per board in the first layer. After all the fasteners from the 2nd layer will penetrate through the first layer and hold down the first layer. In addition most specifications I have read for mechanically attached systems require fewer fasteners per board than on fully adhered systems.

You can install a roof with almost no fasteners (except at boxes and perimiters) by using adhesives, if your budget allows.


For the record I hate tapered.

__________________
-Grumpy
Chicago Gutters Chicago Roofing

SinglePlyGuy

Keep it simple...

The proper technique would be to use two layers (as Lefty suggests) if you are going thicker than 2.5".

The first layer can be screwed with the second set in low rise foam- such as Olybond.

The membrane can then be fully adhered whther its TPO, PVC or dare I say...EPDM .

If you so choose you could also use low rise on both layers and get rid of the fasteners all-together. typically beads of adhesive are spaced 12" on center in the field and 3-4" on center at the perimeter and corners.

Keep in mind, depending on your wind rating / uplift requirements you may have to install a mechanically attached "peel stop" at the perimeter to add securement but that usually is a manufacturer requirment.

There are many low-rise adhesives that are more cost effective than in the past and can also increase production, depending on the rooftop layout of course.

Hope this helps.

AaronB.

Does anybody know of any sprayble adhesives that we could use over a mechanically fastened flatstock board layer?

I used to love the production of a hot asphalt applied tapered perlite system, but as we all know... the ISO would float and the insurance is ridiculous.