You’ve decided it’s time to replace your roof. After receiving a few estimates, some (or all) of the roofing contractors propose to tear off your existing roofing material and re-sheet with plywood.
If you’re wondering why the roofers can’t just install new shingles and roofing accessories over your existing roof deck, this article will guide you through one of the most common reasons: your roof deck may be shiplap.
If you’re replacing your cedar shakes roof, check out our discussion of re-sheeting over skip-sheathing.
Was your roof originally built with shiplap?
Roof decks on homes built before 1970 are often 1×8 fir pre-plywood shiplap. Until the 1940s, exterior grade plywood was not available. Since the boards used for shiplap were manufactured here in the Pacific Northwest, it was still cheaper than plywood up through the 1960s. Most Western Washington residential homes during this time were built with shiplap roof decks rather than plywood.
Why re-sheet over shiplap?
After the 1960s, the predominant residential roofing substrate became 1/2″ CDX plywood. Today, a shiplap roof is usually 60+ years old.
Over time, shiplap fir wood resin dissipates, causing the boards to shrink, crack, form knot holes, and become extremely brittle. Open spaces form between the once tightly-fitted shiplap boards. If the roofing underlayment is applied directly on top of the shiplap, it will sag into the spaces and eventually leak.
Older homes commonly have one or more layers of roofing over the original layer. Shiplap can only take so many nail penetrations before it splinters and develops pattern inconsistency. This, and the space left by diminished overall size over the years, creates more opportunities for the new nails to miss the wood completely, causing eventual leaks.
Installing your new roof
When installing new asphalt composition shingles, electro-galvanized roofing nails penetrate into the shiplap, cracking or splitting the boards and preventing a proper seal. Through expansion and contraction, these nails push upwards through the freshly installed shingles. The raised nail heads can cause migratory water seepage and mystery leaks.
While many of the roofing shingle courses will land on the 7-1/2″ increment, there can be entire courses of unsecured shingles that line up on cracks in the shiplap. Wind gusts can then blow these poorly fastened shingles off your roof.
Creating a solid substrate
Why take the risks of installing a 50 year lifetime roof just for it to leak in 5 – 10 years? If you want to avoid the headache of tearing up your new roof looking for a mysterious leak that “should not be happening”, it is in your best interest to apply solid sheathing over the existing shiplap for your roofing system to securely fasten to. The recommended materials are exterior grade 7/16″ OSB (oriented strand board), 3/8″ OSB, or 1/2″ CDX true plywood.
Don’t take shortcuts when protecting your biggest investment: your home!
You want to steer clear of roofing contractors that cut corners, and if you find one that says it’s okay to install a composition roof on top of shiplap, give them a no thank you! Your roof, and your peace of mind, will thank you in the future. At Chase Construction NW, Inc., we never cut corners! We care about our customers’ experiences now, 5, 10, 20 years down the line, and always recommend what is best for you and your roof. You need to protect your biggest investment: your home! And taking the cheapest route, isn’t always the best, so why take the risk?
Contact Us Today!
If you are looking for an honest, knowledgeable, and experienced roofing contractor, we are here for you! Contact us today for your free estimate! Call 253.445.8950, or fill out our quick estimate form! We look forward to working with you!