Board Deck, Underlayment, and Chimney Leaks on a 12/12 Roof
An 1890s house has a 12/12 slate roof over 1-inch by 8-inch pine roof boards laid with gaps up to 3/4 inch. The rafters are 2 by 8 at 16 inches on center. The slate is 18 inches by 10 inches, and about 20% of the slates are cracked or missing. The chimney is 36 inches wide and passes through the roof; the existing step flashing is reused and heavily tarred. The contractor proposes to install 15-pound felt directly over the boards, relay the slate, reuse the step flashing, and seal the chimney joint with polyurethane caulk. The owner reports leaks at the chimney during wind-driven rain and ice dams at the north eave. The house is heated, and the attic has only a thin layer of insulation at the ceiling. Advise the owner on deck preparation, underlayment, and flashing before the slate is relayed.
1. Is the proposed deck preparation acceptable? What would you require before underlayment?
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The proposed 15-pound felt directly over boards with gaps up to 3/4 inch is not acceptable. Felt will sag into the gaps, rupture, and lose its continuity. The boards should be covered with a solid sheathing overlay, such as minimum 1/2-inch plywood or OSB fastened to the rafters, or the boards should be replaced. The deck must also be checked for the slate dead load; 2 by 8 rafters at 16 inches on center may be adequate, but the contractor should verify span and condition before adding weight.
Before underlayment, require inspection for rotten board ends, loose boards, and protruding nails. Address the ice-dam cause as well: thin ceiling insulation and poor attic ventilation allow warm air to melt snow at the ridge and refreeze at the eave. Deck repair alone will not stop ice dams.
2. Specify the underlayment system for this roof, including eaves and valleys.
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Use a 30-pound felt or an approved synthetic underlayment, not 15-pound felt alone. At the eaves, install a self-adhering ice-dam membrane extending at least 24 inches inside the exterior wall line; in a severe cold-climate application, follow the local code for additional coverage. Install ice-dam membrane in valleys and around the chimney as well. Lap felt according to the manufacturer’s instructions and keep it continuous over the deck overlay.
The underlayment is a secondary barrier, not a substitute for correct slate exposure and flashing. The owner should require the underlayment inspection before slate is relayed and should verify that no felt is torn at board gaps or fasteners.
3. Evaluate the chimney flashing proposal and describe the correct flashing approach.
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Reusing the tarred step flashing and relying on polyurethane caulk will not stop wind-driven rain. The existing flashing is at the end of its life, and caulk cannot replace proper metal flashing. Because the chimney is 36 inches wide-more than 30 inches-it needs a cricket on the uphill side to divert water. Correct flashing includes new step flashing interlaced with the slate courses, a base pan or back-pan flashing at the uphill side, counterflashing cut into the masonry, and a cricket with appropriate metal. Sealant may be used only as a supplementary detail, not as the primary water barrier.
The owner should require the mason and roofer to coordinate the counterflashing reglet and the slate installation. The chimney leak is likely caused by missing or failed counterflashing and tarred step flashing, not by the slate itself.