Cedar shakes have always been a popular roofing material, especially in the Pacific Northwest. But in recent decades, the asphalt composite shingle has taken off as the main roofing style. Given the large numbers of homes with cedar shake roofs installed in the 1980’s and 90’s, many are coming to end of life now.
Most cedar shake roofs were installed over skip-sheathing, a poor substrate for asphalt composition shingles. When a cedar shake roof needs replacing, the shakes should be removed, and the skip-sheathing sheeted over with solid plywood.
What is skip-sheathing?
When you think of a modern plywood roof deck, you’re not imagining gaps between the sheets of plywood. The entire roof deck is relatively uniform, where you can step on any part of it without slipping through spaces.
Skip-sheeting, or spaced sheathing, is just as it sounds: sheathing with gaps between. Traditionally, skip-sheathing consists of 1 inch by 4 inch boards nailed to the rafters with gaps between the boards. Spacing between boards is optimal for wood shakes but cannot support asphalt composite shingles.
Why do wood shake roofs have skip-sheathing?
Most rainwater sheds off of a wood shake roof, but not all. The leftover moisture instead absorbs into the wood shakes. These saturated shakes must dry out in order to maintain their longterm function.
Air flow is the most significant way to design the proper drying of wood shakes. They need to dry uniformly, otherwise the shakes will curl, warp, and crack. Installing cedar shakes on top of plywood causes uneven drying, and substantially reduces their lifespans.
By installing skip-sheathing as the roof deck, the spaces between the boards allow the wood shakes to breathe so that they dry more evenly after being soaked in the rain, limiting warping. This does not apply in environments with wind driven rain and snow however. When installed correctly, wood shingles are rigid and allow the roof to be walked on. However, composition roofing, and some modern wood shakes need to be installed on a solid deck, not only for stability to walk on, but to prevent water penetration. Modern materials and synthetic roofing underlayments are breathable and have no need for spaces in the decking.
What material can I use to re-sheet the roof deck?
Some have said that you can put slats of new wood between the existing skip-sheathing to create a solid deck, but that creates a multitude of unforeseen problems. Depending on how old the skip-sheathing is and how many layers have already been applied to it, it has seen many nails, which means it will be riddled with holes, and will not make a good surface to nail new shingles down onto.
Also, when you put new wood between old wood, in time gaps will open up between the boards. When nailing down the shingles on a system like this, the nails have less of a chance to hit a solid surface, lowering the shingle’s wind resistance. For these reasons, solid sheets should be applied on top of the skip-sheathing to create a solid substrate for the roofing system. New roof sheeting should be either 1/2″ CDX plywood or 7/16″ OSB (oriented strand board). Both are readily available and come in 4×8 foot sheets. 3-inch nails should be used during application to ensure that it connects the new sheeting, skip-sheathing, and rafters together for a reliable hold. 3/8″ plywood can also be an option, though it is not an option if there is not underlying skip-sheathing or shiplap because it is not strong enough as a standalone sheathing.
Let Us Help!
Many handy homeowners have taken on their own roofing projects, some with great results! But some have had devastating results because they lacked the details that are crucial to a long lasting, leak free roof. We at Chase Construction NW, Inc. work hard to stay educated and trained on new trends and developments in the industry. Our crew is knowledgeable, experienced, and professional! We are licensed, bonded, insured and have been roofing right here in the Puget Sound since 2002! You can trust that you are getting a quality, long lasting roofing system! So if you have a Cedar Shake roof that is at the end of it’s life, or any other material, let us help! Contact us today for your free, no obligation estimate! Fill out our free estimate form or call us today at 253-445-8950. Thank you for considering us for your next project!
4 thoughts on “Why Re-sheet Over Skip Sheathing?”
Great article, explained skip sheathing in a layman language very clearly and effectively. I am sure homeowners visiting this website would be immensely benefited by this.
so if you had skip shearing can you take it off end me just sheet the roof with 7/16 sob board
You want to go over the top of the skip-sheathing.
What is the advantage of leaving the skip sheathing on versus tearing it off?