Historic Church Slate Matching and Phased Restoration
A 1920s Vermont church has an 8,000-square-foot, 12/12 slate roof in a cold-climate historic district. The original slate is 20 inches by 12 inches, 3/16 to 1/4 inch thick, unfading green. An inspection finds about 15% broken slates, widespread nail fatigue, several tar-patched repairs, and ice dams at the north eaves. The congregation wants to phase the work over three years and save as much original material as possible. A supplier proposes new 20-inch by 12-inch, 1/4- to 3/8-inch-thick black slate graded ASTM C406 S1 from a different quarry. The contractor says the new slate can be mixed randomly with salvaged original slate and that thicker slate is always better. The owner asks whether the proposed slate is an acceptable match and how to plan the first phase.
1. Evaluate the proposed new slate against the existing roof. What characteristics should govern acceptance, and what would you require before approving a mock-up?
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ASTM C406 S1 is a service-life grade, not a visual or dimensional match. The existing roof is unfading green, 3/16 to 1/4 inch thick; the proposed slate is black and 1/4 to 3/8 inch thick. Color and weathering type will not blend, and the thicker slate will raise dead load and may not lie fair with thinner salvaged material. Do not accept “thicker is always better” without structural review.
Before approving a mock-up, require the supplier’s grade certificate, quarry source, color/weathering classification, thickness range, and dimensions. Compare absorption or other durability data only if the supplier provides tested values. Then require a field mock-up on a least-visible slope using sorted and blended slate, not random mixing. Inspect color range, thickness variation, laying pattern, and fastener compatibility. If the historic district requires color match, the black slate may be rejected outright; if not, it should be limited to concealed or less visible areas.
2. The contractor proposes random mixing. Propose a salvage and blending plan that keeps the historic appearance and supports a three-year phasing.
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Random mixing will produce a patchwork roof because the new black slate and thicker stock will not weather like the original green. Plan by quantity and visibility. At 20-inch slate with 3-inch headlap, exposure is 8.5 inches; one square needs about 141 slates, so 80 squares need roughly 11,300 slates. With 15% breakage, about 1,700 may be lost, leaving roughly 9,600 sound salvaged slates. That is enough to dominate the most visible elevations if protected.
Remove slate carefully, clean it, sort by thickness and color, and store on edge. Use salvaged original slate on street-facing and high-visibility slopes; use new slate on less visible slopes and for future repairs. Blend in controlled patches or at natural breaks, not piece-by-piece randomly. Phase from least visible to most visible, approve a mock-up for each phase, and hold a 3–5% reserve of matched slate.
3. What documentation should the owner require for the first phase?
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Require submittals before mobilization: the slate supplier’s ASTM C406 grade certificate, quarry and color/weathering data, thickness and dimension ranges, and samples. Require fastener specifications-copper or stainless-steel slate nails, not electrogalvanized-and underlayment and flashing product data. Require an approved mock-up and a written salvage inventory showing how many sound slates were recovered and where they will be used.
The owner should also require photographs of deck conditions, a change-order procedure for concealed rot or failed sheathing, and a workmanship warranty separate from the material warranty. Because this is a phased historic project, each phase should have its own approved mock-up, salvage tally, and final inspection. This prevents the contractor from substituting random new slate after the first phase is accepted.