Why is the area around our skylight spongy?
– Syed, Thornhill

I have a sky light, 4 feet x 4 feet, on my roof. About one foot from the sky light on the roof slop, I have noticed that some shingles are pressed and uneven and seem spongy when I stand on them. It is in an area of about 3 feet X 3 feet. What could be the cause and do I need repairs to be done?

Craig’s response – “damaged plywood and/or improper installation could be the problem”

If your home was built within the last thirty years in Thornhill, I suspect that the roof substrate (the surface under your shingles) is plywood.

If the plywood sheeting feels spongy under-foot or you hear a crunching sound when walking over the area, it may indicate that sheeting replacement may be necessary. Back when your roof was constructed, builders typically used 3/8" plywood. 1/2" plywood as a replacement standard would be recommended.

You commented on the condition of the shingles surrounding your skylight. You should also check (via the attic) the "tunnel" of the skylight. Has it been framed correctly, a necessary step to ensure the structural integrity of the roof? Severe dips or bows in the adjoining rafters would be a clue that there is a problem. Check also for doubled up header and sill framing at the top and bottom of the tunnel. Supporting roof rafters should be doubled up in the area.

Because skylights can be a source of heat loss, check for proper insulation and vapour barrier in the tunnel. Migrant cool air meeting hot attic temperatures can, over time, contribute to the plywood breaking down. This condition is common for many roofs, with or without a skylight. .

If you would like a professional to perform the steps described above, I would recommend an inspection to provide a written condition report. The cost of the inspection is often refundable if any required work is awarded to Avenue Road Roofing at a later date.