Seeking underlayment material for exterior wall shingles.
I own a home built in 1965. It has cedar shingles as the exterior wall surface. I will be rebuilding a section of one wall as part of a project to replace a rotted window.
The problem is that the original construction includes a material that I can’t find, and I’m not sure what could be used as a substitute. The material seems to be a fiberboard similar to Homasote, but it’s only about 5/16″ thick and seems to have somewhat coarser fibers than Homasote.
It is applied in wide strips underlaying each course of shingles, lapping atop the next-lower course, similar to the way an undercourse of shingles would be laid (except it’s used on every course rather than just the first). The strips have almost the same vertical dimension as the shingles, except the fiberboard is held about 1/2″ shy of the bottom of the shingle. The entire system (fiberboard and shingles) is applied outside the felt paper.
The overall effect is to produce a deeper profile for each course of shingles (they stand proud of the lower course by the thickness of the fiberboard). The material provides some cushioning of the shingles, presumably, and perhaps adds a tiny bit of insulation.
Another oddity is that the exposure between shingle courses is very large — about 13″ rather than the typical 6″ or less. If I were replacing the siding completely I’d do it differently, but in this case I’m just patching in a relatively small area around the rotted window and need to match the existing construction.
I have considered a few possibilities. Homasote would be the closest analog I’ve found, but it’s not available in less than 1/2″ thickness. I could use a stack of felt paper sheets to make up the proper thickness, which would be laborious but not too bad for such a small area. I could use 1/4″ closed-cell foam sheet material cut to the proper width, but it would be too thin to match exactly. If I used two layers of the foam, I might be able to compress it a bit as I nail up the shingles and get close to the right thickness that way; or perhaps I could use one layer of foam built up with felt paper.
I would appreciate any suggestions or possible sources of an exact replacement for my fiberboard. So far I haven’t had much luck with internet searches. Thanks for your help.
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Replies
You can use shingles in lieu of the fiber board. Don't use the combed shingles as they are spendy. Pick up a bundle of #1's and cross cut them at the desired thickness.
I also have a sneaking suspicion that the fiber board may have a tendency to promote rot at the base of the shingles as I have seen more issues with this system then a standard shingle application.
Debra has it covered. For deep exposures the underlayment was a cheap way to avoid a second layer of shingles which would be entirely covered. I have seen it done with 1/4" plywood strips too. You could do the same, but anything other than cedar will cause them to rot at an increased rate.
Thanks! I'll go with the low-grade shingles. I have some on hand, so it's a no-cost solution.