Deck Evaluation for a Heavy Tile Re-Roof
A contractor is replacing 20-year-old asphalt shingles with 9.5-psf concrete tile on a 1,800-ft² roof. After tear-off, the deck is 1/2-in. CDX plywood with butt joints staggered, fasteners spaced 12 in. at panel edges and 24 in. in the field, and rafters at 24 in. o.c. The plywood has delamination at two valleys and a soft spot near the ridge; a few panels flex underfoot. The existing shingles were installed over a single layer of 15-lb felt. The homeowner wants to save money by installing a synthetic underlayment directly over the existing deck and using 1¼-in. roofing nails. The roof slope is 5/12, and the local code requires tile to be installed per the manufacturer's printed instructions. The contractor has not checked the roof structure for the added tile dead load.
1. Can the new concrete tile be installed over the existing 1/2-in. plywood deck as proposed? Explain what must be verified or corrected.
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Not as proposed. The tile dead load is about 9.5 psf, compared with roughly 2-3 psf for the shingles removed, so the structure sees a significant load increase. 1/2-in. CDX at 24-in. rafter spacing is likely to flex and may not meet the deflection limit for tile; the soft spot, delamination, and flexing panels are evidence of distress. The 1¼-in. nails are also suspect: if any sheathing is added, they may not penetrate the rafters the required amount. Correct approach: have a qualified person or engineer verify rafter size, span, and deflection for the tile load; replace delaminated and soft panels; re-nail or re-sheath to a thickness and span rating acceptable for tile. If an additional layer is used, fasten through to framing with length and spacing per code and manufacturer instructions. Do not cover a flexible deck with underlayment and tile.
2. What underlayment and fastening details should be specified, and what code or manufacturer issues remain?
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The code requires the manufacturer's printed instructions, so obtain the tile manufacturer's underlayment specification first. For a 5/12 steep-slope tile roof, common practice is a minimum 30-lb felt or equivalent synthetic underlayment, with laps, fasteners, and eave and valley treatment per the manufacturer. Use corrosion-resistant fasteners with caps and do not overdrive. If the manufacturer requires two layers or a high-temperature membrane, follow that requirement. Install ice-dam membrane at the eaves if the code requires it, and maintain attic ventilation. The remaining issue is structural: underlayment cannot fix deck deflection. Document the deck condition and the engineer's approval before proceeding. Also verify tile fastener length: nails must penetrate at least 3/4 in. into framing or through sheathing as the manufacturer requires.