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Community and Q&A

Walkable porch over a SIP roof

nkurz | Posted in Green Building Techniques on

I’m wondering how best to put a 12′ x 12′ deck over a SIP roof. This would be new construction in the SF Bay area: mild climate, dry most of the year, wet for the winter.

The standard wisdom is that “all roof decks will leak eventually”. I’m presuming it’s because they presume a porous deck and a sealed flat roof? I’m thinking of using a waterproof aluminum system that should function more like a standing seam metal roof that doubles as a walking surface. There are several brands (,,,

They share the principle of coated nonslip aluminum surface, waterproof channels, and fasteners that in theory are out of the water path. All require a minimum 1/8″ per foot slope. This seems like a good system: build a roof with a small amount of slope, waterproof as best as possible, then put the impermeable deck on top. It seems should work as well as any other well sealed but very low slope roof. Which is a concern, as this isn’t usually a recommended practice.

And as I start reading, it seems that the particular concern with SIPs is avoiding condensation on the top (outer) OSB panel. This means that I may not want to put anything vapor impermeable between the decking and the top of the SIP. While good interior sealing should avoid this problem, the room below is a bathroom with a shower. It will be ventilated, but it might be best to plan for some escaping moisture.

What would be the best thing to do here?

I think the waterproof decking is a good idea, and I’m happy with fireproof aluminum, so let’s assume I’ll be using that in some form. And at 12′ x 12′, I can’t cover it with a single panel, but I can get down to only one or two seams. I’ll seal these on the interior to be as vapor proof as I can. Is good taping sufficient?

I could assume that if I seal the inside well enough, exfiltration of humid air won’t be a problem, and I can vapor seal the top of the roof without worry. The combination of a waterproof roof and a waterproof decking should be waterproof, but it fails badly if it gets wet inside by some other means.

I could keep it breathable by putting down felt, screw down the decking, and assume, since the deck is waterproof, the actual contact area of the metal is small, and there is a 1/2″ air gap elsewhere, that I won’t have problems. Other than the very low slope, this seems logical.

I could put down down a waterproof vapor semi-permeable synthetic and add a ventilation layer on top of it (Cedar Breather, Enkamat) or use something that combines them (Delta-Trela, others?) and then put the decking on top of this. This seems to be what R-Control recommends for all metal roofs.

I could make a larger air gap by putting the decking over battens and cross battens, or battens and plywood. Other than the low slope, this seems to be the usual recommended method for . If I do this, what layers should go under the sleepers or battens?

I could increase the slope of the roof and put down sleepers to being it back to almost level. The decking would then go on top of cross battens, or if I’m tricky, I could run the decking perpendicular to the sleepers and slope the surface slightly in that direction. Same layers question.

Or I could decide that using an OSB SIP roof here is part of the problem, and do something else: a metal sip, a foam roof, something else entirely? It’s a gentle enough climate that many things could work.


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  1. GBA Editor
    Martin Holladay | | #1

    If you want your SIP roof to last as long as possible, you need to do an excellent job of air sealing. Interior SIP tape (in conjunction with spray foam at the seams) is the best way to seal SIP seams.

    Because OSB is so susceptible to rot, many SIP installers insist on a ventilated rainscreen gap for walls and a ventilated channel above the SIPs for roofs. This adds cost, of course, but increases resilience and durability.

    I would normally assume that a low-slope roof with a walkable deck above needs a layer of roofing (for example, EDPM) under the decking. But it looks like the products you are considering (for example, AridDek) are designed to be waterproof and include air channels for ventilation under the aluminum decking. That raises to possibility of forgoing the roofing and simply installing a vapor-permeable synthetic underlayment over your SIPs.

    Whether or not this approach will work depends on how waterproof your aluminum decking really is. I would discuss your plans with the decking manufacturer before proceeding.

    For a belt-and-suspenders job (not cheap), you would include these layers: SIPs; vapor-permeable roofing underlayment; 2x4s installed 24 inches o.c. to create ventilation channels; a layer of plywood or OSB above the ventilation channels; EPDM rubber roofing; and then your aluminum decking. This approach will work best if you can figure out a way to ventilate both ends of the ventilation channels -- and that might be challenging.

  2. user-659915 | | #2

    Bear in mind that the aluminum surface finish will get extremely hot and make the deck virtually unusable in sunny weather. We usually achieve a walkable deck by overlaying deckboard panels on sleepers on top of the waterproof finish (the EPDM layer in Martin's assembly above) with no penetrating fixings. The sleepers should run parallel to the direction of slope to allow free drainage and should be no larger than about 8' x 4' to allow them to be lifted for inspection and maintenance. The deck boards can be backscrewed from the sleeper sections to give a very clean finish. The panels will protect the membrane from UV degradation as well as mechanical damage and will enable it to have a good long life.

    Be sure to allow at least 4" upturned flashing at the door threshold. Combined with the thickness of the entire assembly this could well mean that your door threshold is a step or two higher than the interior floor finish. Far better this than a rotted roof SIP panel. You also need to very careful with flashings at the rail posts if their structural connections penetrate the waterproof membrane.

  3. vdh2o | | #3

    James, How would you anchor the sleepers to the house w/o going through the EPDM membrane?

  4. user-659915 | | #4

    The sleepers are part of the pre-assembled panels. Assemble the panels upside down, screw the sleepers to the deck boards and then flip the panel over to lay on top of the membrane. The panels are heavy enough to stay in place with no through fixing. Simple gravity will do the job.

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