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Terence McCann

Wood Shake Roof Question

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One possible source of semi-authoritative info the the Cedar Bureau. Personally, I see wood shakes on plywood sheathing fairly often and it seems to do fine. I could get approval to build a house with skip sheathing, but it would require engineering outside the prescriptive code, as far as I know. Never considered it, so I could be wrong about that last.

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Except that he specifically asked about shakes. By the time each course is interlaced there is so much felt on the roof the plywood is pretty inconsequential. Shingles only perform well over plywood when the slope approaches plumb.

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From the Cedar Bureau:

Solid sheathing is recommended for shakes

and may be required in seismic regions or

under treated shakes and shingles. Solid

sheathing is used in areas with wind-driven

snow. Please note that the only solid sheet

sheathing tested with Certi-label shakes and

shingles is plywood. Check with your local

building official for plywood

thickness/dimensions.

The wind driven snow is certainly a factor here.

Thanks guys.

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Here's where long traditions conflict with new thinking.

It's long been accepted that shakes and shingles are normally applied over skip - shakes with felts interlaced into the roof plane; and that's what all of the home inspection texts say - however, that's not what the roofing industry says these days. The roofing industry has moved on and is now saying that plywood beneath shakes is perfectly acceptable as long as the deck is properly detailed.

The overwhelming majority if shake roofs here in the Seattle vicinity are on skip but every once in a while I find them on solid decks. The Cedar Shake and Shingle Bureau (CSSB) considers solid decks ideal. CSSB recommends installation over solid decking for shakes but says that shakes may be installed over skip. They also specify eaves protection material (IWS) at the eaves to a height of at least 24-inches inside the exterior walls but not over the entire roof. They specify 36-inch wide felt over the entire roof on solid decks.

The use of rigid insulation in certain climates is a little bit of an issue because CSSB considers the use of overly long nails unsatisfactory. In a situation like that they say that strapping is needed. Doing that, you lessen the number of thermal conductors. In areas where ice-damming or condensation can be a problem CSSB recommends a cold roof system along with horizontal strapping and ventilation at the eaves and the crest of the roof. They specify nailing 2 by 4 sleepers on edge in line with the rafters, filling in the space between these sleepers with foam insulation and then installing nailers across the top of these members, giving a ventilated air space and avoiding compression of insulation. Alternatively, one can install a second false plywood deck on top of the insulation and eliminate the horizontal nailers. CSSB warns though that under certain conditions of slope and loading the entire roof above the decking might tend to creep downward and compress the insulation. In those cases they say stick with the vertical nailers.

The Cedar Bureau actually specifies solid decks for humid areas and prescribes four methods for fastening directly to solid decks in humid areas.

1.Apply the shakes or shingles directly to the sheathing.

2.Apply vertical strips over the sheathing in alignment with the rafters below, then place horizontal furring strips on top of the vertical boards.

3.Apply horizontal furring strips to the deck, spaced at the exposure of the product.

4.Use a continuous ventilation product beneath the roofing material.

Knowing the kinds of winters that Terry has in his area, I think a properly detailed solid deck would be perfectly acceptable.

If anyone wants to download CSSB's latest Shake and Shingle Installation Manual, you can do it by clicking here.

ONE TEAM - ONE FIGHT!!!

Mike

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