Non-Vented “hot” Roof Assembly

The roof was designed as an unvented “hot” roof. A 4-inch box is built on top of the roof sheathing, filled with foam and a final 3/4-inch roof deck screwed down on top to provide a nailing plane for the shingles. It’s air-tight so any moisture that gets in will rely on inward/outward drying.

First I built a framed 4-inch high box around the outside of the roof. I used cutting jigs and a circular saw to cut 1-1/2-inch notches in the gable ends for lookouts. The eave-pieces were cut sloped top and bottom. so they install vertical.

Next I built 4-inch high boxes for the skylights. Prosoco Fast-flash was used to seal between the sheathing and the boxes. The whole roof box was then sealed on the inside and outside using Prosoco Cat5 liquid flashing.

DensGlass extends the fire-break from the sheathing up to the top of the roof assembly.

I lined the bottom of the roof box with wrap to isolate the liquid flashed sheathing from the rigid insulation. The reason was I’d read some people have reported squeaking noises from roofs with rigid foam as solar heat causes the foam to expand and contract against the sheathing. I suspect it’s not necessary but it should reduce any abrasion of the flashing membrane during daily heat/cool cycles.

Next a layer of 2-inch XPS foam was installed and taped.
The gable-end lookouts (2x4s on the flat) were screwed down over the first layer of insulation back at the first rafter. The eliminates thermal bridging from the lookouts to the roof sheathing. I could get by with lookouts on the flat as the overhangs are short. (For substantial overhangs it would have been necessary to use 2x4s vertically.)
The second layer of foam was then applied and taped down.
I decided against using Rockwool for the roof insulation as I was concerned about it’s compressiblility when the top roof deck is screwed down. The XPS foam is sufficiently dense that it provides a solid base for the top roof deck.
I believe Rockwool could be used for the roof assembly, now that I have some experience working with rockwool for the exterior walls. It would take a little more fiddling around to get the deck screwed down true and level but it should work okay.

Next the eave rafter tails were cut and screwed on to the 4-inch high roof assembly box. It’s not possible to use the actual structural rafters as i) they are too low down given there’s a 4-inch roof assembly built on top of the roof sheathing and ii) the structure is built as a smooth box with the rafters cut smooth with the sheathing so that the exterior insulation can wrap completely around.

The eave overhang is short and the roof slope is working with us so lookouts were not needed. The rafters are screwed into the side of the structure. The gable-ends provide good rigidity to the whole eave structure. If the overhangs were significantly larger then obviously some stronger structure would have been needed.
I didn’t worry about getting all the rafter ends the correct length. Just build them longer than needed, then cut with a circular saw using a string line and a cutting jig.

I then installed the sub-fascias (cut down from 2x6s) and built the bird-boxes at the gable/eave ends.

The top roof deck was then installed over the roof assembly and insulation. (3/4-inch ply – fire-rated on the north-side due to proximity to the property-line).

I used Heco Topix screws to secure the ply through 4 inches of foam through the sheathing and into the interior rafters. These screws are a brilliant design. There are two groups of threads set at the same pitch. This allows any 3/4-inch strapping or ply sheet to be fastened through thick insulation so that the screw head self-countersinks without compressing the insulation. The screw can be tweaked in and out to really dial in the level of compression and get the roof deck super flat.
Heco Topix screws are available from from Small Planet Supply and come in a variety of lengths to suit your insulation thickness.

Installing these screws through thick insulation requires very accurate measurement and marking of the rafter locations on the roof – accurate to within 1/8 – 1/4-inch to make sure you hit the rafter right in the center.
I made a jig to help ensure they went in true. Out of 200 or so screws only 3 missed the rafter – due to a measurement error.

Video of installing the screws. The rafter locations are marked on the ply.
After screwing down the entire roof deck I then went back and retweaked all the screws to get the deck super flat.

I then ran a router around the ply perimeter to get a constant 3/4-inch overhang that will cap the fascia.

Finally I liquid-flashed all the plywood seams and also around the junction of the underside of the ply deck and the sub-fascia to make the roof assembly air-tight, then applied Cat5 on the roof deck to water-proof it.

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Bill Dickens

I grew up on a hill country farm in New Zealand, then lived and worked in Hong Kong. I moved to Portland, Oregon in 2009. I'm presently renovating my 1929 "English Cottage" house in the Irvington Historic District.

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