Mansard Roof: Coating Proposal Versus Historic Restoration
An 1895 mansard roof has 70% original 16-inch by 10-inch slate and 30% asphalt shingles installed on the lower slope after a 1980s storm. The slate is 3/16 inch thick, and many slates are slipped or cracked at the dormer. Two copper valleys have been patched with roofing cement. The owner wants to “restore” the roof with a spray-on elastomeric coating and keep the asphalt shingles because they are not leaking. A contractor proposes to replace only the broken slates, seal the valleys with more cement, and coat the entire roof. The building is in a historic district, and the owner hopes to qualify for preservation tax credits. The roof has a 10/12 slope and is 4,000 square feet. Advise the owner on whether this is a repair, restoration, or replacement, and what sequence to follow.
1. Diagnose the likely causes of the dormer and valley leaks. Why is the proposed coating not a suitable repair?
Read the worked answer
The dormer leaks are likely from failed step flashing, cracked slate, or nail fatigue, not from the slate surface. Slipped slates at the dormer suggest fasteners or flashing have failed. The copper valleys have been patched with cement, which can trap water and prevent proper drainage. The asphalt shingles on the lower slope are incompatible with slate and may hide deteriorated underlayment or deck.
An elastomeric coating is not a suitable slate repair. It can trap moisture in the slate and underlayment, change the roof’s appearance, and fail at movement cracks. It does not restore broken flashing or valleys, and it may jeopardize historic-district approval and preservation tax credits. The roof needs diagnosis and repair of the water-shedding details, not a surface coating.
2. Develop a repair/restoration plan. What should be replaced, what can be salvaged, and how should the valleys be treated?
Read the worked answer
Treat this as a restoration, not a coating job. Remove the asphalt shingles and inspect the lower-slope deck; if the original slope was slate, restore it with matching 16 by 10 slate. Salvage sound original slate during removal and replace cracked or slipped slate with matching thickness and color. Use slate hooks where the existing nail holes cannot be reused, but do not rely on surface cement.
The copper valleys should be repaired by a qualified sheet-metal worker-soldering small holes or replacing damaged sections-not by adding more cement. Replace the dormer step flashing and any failed underlayment. If the deck is sound, retain it; if not, replace only the deteriorated sections. The owner should approve a mock-up showing the repaired valley, dormer flashing, and slate blend before full work begins.
3. What should the owner require from the contractor regarding historic compliance and phasing?
Read the worked answer
Require written approval from the local historic district commission before removing the asphalt shingles or altering the roof appearance. The contractor should submit slate samples, flashing details, and a mock-up; the coating should not be approved as a substitute for slate. The owner should also require documentation of salvage quantities and photographs for tax-credit purposes.
Phase the work to keep the building weathertight: complete one slope or roof section at a time, install temporary underlayment, and protect the dormer and valleys. Require a final inspection of flashing and slate before payment. The owner should reject any proposal that keeps the asphalt shingles solely because they are not currently leaking; they are not part of the historic slate roof and may conceal ongoing damage.