AboutRon Haynes Expertise I can answer questions related to all types of steep slope and low-slope commercial and residential roofing, including asphalt shingles, metal roofing, built-up roofing (tar and gravel), modified bitumen roofing, single ply roof systems, tile, and slate. Questions may relate to design, installation, problem resolution or repair of the roof system.
Experience I have over twenty years experience as a roof consultant dealing with various types of roof systems on commercial, industrial, residential and Government facilities. Experience includes new roof and reroof design, roof repair specification, roof construction, maintenance, failure investigations, performance analysis, inspections, moisture intrusion surveys and studies, and storm damage assessments. I am a registered professional engineer and a Registered Roof Consultant with the Roof Consultants Institute.
Organizations Roof Consultants Institute
Education/Credentials Bachelors Degree in Civil Engineering, Clemson University
Registered Professional Engineer
Registered Roof Consultant, Roof Consultants Institute Certified Infrared Thermographer
Awards and Honors Award Winner, Large Roof Replacement Design Document Competition at the 1999 Roof Consultants Institute International Convention. Dept of Navy Meritorious Civilian Service Award. Armed Forces Civilian Service Medal.
Answer Richard,
I'm guessing you are asking this question because you might have some bad decking underneath the existing shingles, either rot or deflection. Hopefully, this is also a shed or storage type of structure versus a residence.
In the general case, it would be best to remove the shingles and address any bad decking. Also this would typically be less expensive overall.
However, logically, yes you can put plywood over shingles as long as the structure will support the additional weight of the plywood and whatever you're going to put on top of it. If a residence, from a building code standpoint, it is an issue that is not really addressed (at least not directly) so there could be some debate with a code inspector. For that reason, if you are in an area that heavily enforces the building code you should check with a building department official first to potentially save yourself some headache.
As for attachment of the plywood over the shingles, it should be attached to the rafters instead of just nailing into the underlying sheathing under the shingles. In this case, you'd have to try to locate the rafters then maybe pop a chalk line on the existing roof surface along each rafter line. You should also maximize the possibility of staggering the joints in the new plywood with joints in any underlying existing sheathing. To do this, check the width and length dimensions at a starting point within an accessible area underneath (under shed roof or within attic if a residence) and start your first sheet installation with a different dimension but that still allows the plywood short edge to terminate atop a rafter. For example if the starting point you can view from underneath in a lower corner of the roof area has a 4x8 sheet, start with the new sheathing in that corner with a 2x4 sheet. Your first row of sheathing then would all be 2' wide. When you begin the second row you can go to 4' wide sheets and all the joints would then be offset and staggered.
