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Old 06-04-2013, 06:15 PM   #1
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screw spacing on rigid foam roof


I am recycling rigid foam insulation and installing over my plywood roof on my mobile home. I'm not sure the spacing to screw them to the decking.
they are
2ft x 2ft square blocks.

I will be using 6in screws with plastic buck plates

any advice will be helpful.
thanks in advance !!!

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Old 06-04-2013, 07:30 PM   #2
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screw spacing on rigid foam roof


What kind of foam? What is the density? Facer? What kind or roof is going over it?

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Old 06-04-2013, 07:37 PM   #3
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screw spacing on rigid foam roof


1/2 in. plywood
Grace selfadhesive underlayment membrane
4in thick Dow Blue 2lbdensity Styrofoam cut into 2ft x 2ft blocks

i am doing the work above myself then:

lastly- spray foam over Dow insulation by licensed contractor
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Old 06-04-2013, 08:04 PM   #4
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screw spacing on rigid foam roof


Get a bucket of cold process adhesive and stick them down. GAF Matrix 103 or similar, No screws needed
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Old 06-05-2013, 06:50 AM   #5
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screw spacing on rigid foam roof


What kind of roof design and layering?

Not sure I would put down the I/W and then the foam.

Spray foam over 4" of XPS? Why not just run more rigid given that its cheaper.
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Old 06-05-2013, 08:32 AM   #6
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screw spacing on rigid foam roof


This sounds like a recipe for disaster. Sprayed Polyurethane Foam creates a great deal of heat when it is first applied. If there is any moisture below, large blisters will appear in the foam as steam is created below the foam. Let us know how it all works out.
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Old 06-05-2013, 11:16 AM   #7
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screw spacing on rigid foam roof


im glad you told me about the blistering. I havent talked to a sprayfoam installer yet & I would hope they would bring up the same concerns. so im glad im forewarned ! thanks
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Old 06-05-2013, 02:04 PM   #8
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screw spacing on rigid foam roof


The type of insulation your are recycling is often used in inverted roof systems, That is, roof systems where the insulation is placed on top of the waterproofing membrane. In this type of system, the insulation is normally held in place by stone ballast which is spread over a polyester protection sheet. No fasteners are used to hold down the insulation, because fasteners would penetrate to waterproofing membrane which you do not want to do. The problem with using this type of system in your case is twofold.

1, you have no parapet walls to deflect wind and protect the edge of the system.

2. Stone or paver ballast is heavy and will most assuredly exceed the structural capability of a mobile home.

If I was you, I would:

1. Install the foam insulation first with a wood nailer at the perimeter of your trailer the thickness of your insulation. You could stick it in roof cement to hold the boards down till you get the cover board down and screwed in place.

2. Install a cover board board of high density wood fiber board. The board should be screwed to your structural deck which should be 3/4 inch plywood with roof deck screws and pressure plates at the rate of 1 fastener per 2 sq. feet. Make sure you get hard surface plates.

3. Install fully adhered .060 inch unreinforced EPDM. The EPDM should be run out over and down the outside 1.5 inches past the bottom of the perimeter nailers.

4. Install a .032 Aluminum Drip edge all around, and strip in with pressure sensitive EPDM Formflash.

5. Paint the roof with two coats of acrylitop white coating.

6. You will not need another roof.

Please do not think that you are going to screw through ice dams material and have a water tight roof, you are not. Your foam over foam plan might work, but I would not even attempt it, and it is more than likely you will end up with a real expensive mess.
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Old 06-05-2013, 05:48 PM   #9
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screw spacing on rigid foam roof


i love the idea but i have to admit , i honestly believe it would exceed the weight the roof was designed for. i dont know.. i could be wrong !
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Old 06-05-2013, 06:32 PM   #10
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screw spacing on rigid foam roof


What I recommended in post no. 8 does not weigh much at all. Maybe a couple of lbs./ sq ft. Forget the ballasted systems though, too heavy.
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Old 06-06-2013, 08:43 PM   #11
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screw spacing on rigid foam roof


If you are doing cc SPF in small lifts, you can manage the heat.

Do it wrong, and you can burn down a wood structure not to mention ruin a good membrane.

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