Q&A – Wood roof redo

With cedar shake disintegrating, this homeowner was looking for advice about putting a new roof on. 

Stephane, a homeowner from British Columbia, Canada, recently reached out to our experts with the following question: 

My geodesic dome cabin in the woods (I did not build it) has been roofed with cedar and tar paper for forty years without issues. But the cedar shakes are now disintegrating. I will be reroofing with a self adhesive underlay (breathable) and Enviroshakes composites. The 35x35x35” triangles are sheathed with 3/8 plywood.

Should I add another layer of plywood before putting membrane and roofing on? Sheathing feels solid and in great shape. I am concerned with how little purchase there is for the roofing nails.

We had a couple of our experts weigh in on Stephane's situation. One was Jim from RoofSmart, who said:

Short answer: Yes, add the plywood. Enviroshake's own installation guide calls for 5/8" plywood, and specifically says 1/2". You're at 3/8". Their field shakes take minimum four 1-1/4" ring-shank nails a piece — through 3/8" ply. That's almost an inch of nail hanging out the underside. If your deck is over anything tender, like wires, that's going to do some damage. Another 1/2" over your existing 3/8" gets you to 7/8"...solid, and your fasteners stay where they belong.

There are two things I'd sort out before you buy anything, though. First, slope. Enviroshake is rated 2/12 and up, with a separate procedure below 3/12. A dome doesn't hold its pitch — the facets flatten as you climb toward the crown, and somewhere up there you fall out of spec entirely. That's a call-the-manufacturer-first question, not a figure-it-out-on-the-roof question. You might end up with a yarmulke up there, adding a torchdown or PVC membrane cap.

Second, drying. This is the one that I worry about on domes. Your dome has lasted forty years because it could dry — felt shake liner is permeable and cedar is basically a screen door. The new plan is an unvented deck sealed with a self-adhered membrane. Enviroshake's Gold warranty requires their Enviroshield peel-and-stick over the entire deck, and that's not vapor-open. So if you go breathable, check what it does to your warranty. On a dome you absolutely want the warranty. Most products fail on domes. And either way — the synthetic shake breathing doesn't save you. Whatever is stuck to the plywood is what controls vapor, and it's the one holding the line. When a roof can't dry, you eventually meet Mr. Condensation. He's not a fun house guest.

None of this is a no. It just means the assembly needs to be designed rather than assembled. Domes are a genuinely fun problem. One smart option is to add a layer of 3" polyISO foam before you add the top sheet of plywood. That will be a huge buffer against condensation — like a beer coozy for your house.

Another response came from Brittany of Fields Roof Service. She had this to say: 

Yes, I would recommend adding another layer of plywood before installing the new roofing system.

While the existing 3/8" plywood may still feel solid and be in good condition, the concern is exactly what you mentioned: There simply isn't enough material for the roofing fasteners to get adequate purchase. Enviroshake requires a minimum 5/8" plywood sheathing for proper attachment of their product.

With a unique roof structure like a geodesic dome, proper attachment is especially important. I would bring the roof deck up to the manufacturer's required thickness before installing the underlayment and Enviroshake. That gives the fasteners the substrate they need and helps ensure the new roofing system is installed according to the manufacturer's requirements.

It's great that the existing sheathing is still in good shape after 40 years. In this case, adding the additional sheathing isn't necessarily about replacing a failed deck, it's about creating the proper substrate for the new roofing material and its attachment requirements.

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