Rubber roof on a 1 pitch

have a 14’ by 26’ roof . completely open underneath. has T&G 2x6 decking. i want to put 1" xps insulation down and a epdm membrane directly on this. is that acceptable? or do i need some kind of board over the insulation
thanks
james

If it’s heated, you’ll need a vapor barrier and a staggered layer wood fiberboard or polyisocyanurate over the XPS. Staggered layer minimizes thermal breaks, and will accept bonding adhesive.

it is not a heated space it is over a deck. i just want to know if i can glue it to the xps

You don’t need XPS, use 1/2 wood fiberboard.

why do i need fiber board ? cant i put the rubber right on the 2x6 decking? im using the insulation to prevent any condensation on the underside ceiling. if snow is sitting on the roof and we get a unusually warm day. and also to match the thickness profile of the house. so again, is it acceptable to glue a rubber roof directly to xps sheets?
cheers
james

You want to use ISO with a paper surface, XPS is a fire hazard.

xps is used on roofs all the time. do you have a link for the claim that its a fire hazard?

Solvent based bonding adhesive AKA contact cement, will melt the XPS. Water base will work, but you have to apply to both surfaces and allow it to completely flash off

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You can use a construction adhesive to put the foam board down. You can put a Watebase bonding adhesive down to hold your EPDM. That’s complete coverage, fully adhered. I must caution you. An 045 will show every ripple in the joints. I would suggest 060. Why do you want to use a foam board?. I would just screw 7/16 OSB down. Install a fully adhered 060 and not worry about it.

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The reason for screwed down fiberboard is to keep nails from backing out through the membrane. I repaired a DIY EPDM last summer and had to screw through the membrane to secure the plywood, pull the loose nails, then patch all the holes. Just trying to save you that trouble…………

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thanks alot everyone for taking the time to reply. a picture is worth a thousand words so . .


in this photo you can see the porch that needs a roof and the existing house that also has a rubber roof.
the porch has 1.5 inch 2x6 T&g decking on the roof (1 pitch). i want to put rigid insulation on it for 2 reasons, 1. i am worried if the roof has snow and we get a warm day condensation could appear on the underside of the roof. the other reason is to match the profile of the existing house.
my plan was to use 1" xps because its not too expensive, screw this to the deck with the big washer fasteners then glue the rubber to this.
i now know i cant use solvent based glue for this . i thank you for that info it sounds like it would have been a disaster if i tried.
im posting here to try and avoid a disaster as its my first go with a rubber roof.
does using a water base glue over the xps sound like a good plan? the iso is quite a bit more expensive and i would prefer to skip putting sheathing over the insulation if its not really necessary.
thanks
james

R value on fiberboard is 1 per inch. Not to be condescending, but I really don’t think condensation will be a problem

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Open outdoor structure with 2x6 T@G decking…
In my opinion No insulation needed.

I’d install self-adhered ice and water shield underlayment and Certainteed torch-down modified.

@Darkthirty and @roof_lover are right – no XPS, screw down fiberboard or even OSB if you’re careful (to prevent leaks from nail-pull) and glue to that.

Any condensation you see under there (highly unlikely given how open that is and considering the t&g has some R value itself) will:

a) not be effected by an inch of xps
and
b) just evaporate and not cause any damage

Slap fiberboard or obs with cap screws then glue it ,why overthinking?

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