Exposure and Fastener Errors on a Steep New England Roof
A contractor is installing a new slate roof on a 14/12 gable in northern New England. The specified slate is 24 inches long by 12 inches wide and 1/4 inch thick. The contractor proposes 11-inch exposure, 1-1/4-inch electrogalvanized roofing nails, and 3/8-inch plywood sheathing. The roof is 60 feet long with a 30-foot rafter run, and the eave is 28 feet above grade. The owner wants a 50-year warranty and wants the appearance to match neighboring 1930s slate roofs. The contractor argues that the 11-inch exposure saves material and that galvanized nails are standard. The building official has not yet inspected the deck, and the attic is unvented. The contractor also plans to use 30-pound felt with 6-inch side laps and no ice-dam membrane. Identify the installation issues and advise the owner before work proceeds.
1. Calculate the headlap implied by the proposed 11-inch exposure and compare it with the minimum for a 14/12 roof. What exposure should be used?
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Headlap is the slate length minus two exposures: 24 − (2 × 11) = 2 inches. On a 14/12 roof, the minimum headlap is 3 inches because the slope is well above 8/12. The maximum exposure for 3-inch headlap is (24 − 3) ÷ 2 = 10.5 inches. The proposed 11-inch exposure gives only 2 inches of headlap, reducing the water-shedding safety margin and increasing the chance that wind-driven rain will reach the underlayment.
The contractor should use 10.5-inch exposure or less. Saving material by increasing exposure is false economy on slate: leaks and premature replacement cost far more than the additional slate. The owner should require the headlap and exposure to be stated in the installation specification and checked in the field mock-up.
2. Evaluate the proposed fasteners and sheathing. What should be specified?
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Electrogalvanized nails are not an acceptable default for slate. In a cold, wet climate they can corrode and stain the roof; specify copper, stainless steel, or another approved corrosion-resistant slate nail. Nail length also needs review: a 1-1/4-inch nail through 1/4-inch slate and 3/8-inch sheathing leaves about 5/8 inch of penetration, less than the commonly required 3/4 inch. Use a longer nail, such as 1-1/2 inch, or as required by the deck and slate thickness.
The 3/8-inch plywood deck must be checked against the slate dead load and rafter spacing; many slate specifications require at least 1/2-inch sheathing or an engineer’s approval. The unvented attic and missing ice-dam membrane are related problems: specify ventilation and an ice-dam membrane at the eaves, plus a 30-pound underlayment with proper laps. The owner should not rely on the 50-year material warranty to cover workmanship errors.
3. What quality-control and warranty provisions should the owner require before work starts?
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Require a pre-installation conference and a field mock-up showing the approved slate, exposure, headlap, nail type and placement, underlayment laps, and eave/valley details. Require the contractor to submit the slate grade, fastener specifications, underlayment data, and flashing details. The building official should inspect the deck and sheathing fastening before slate is laid.
The owner should also require progress inspections at the underlayment, flashing, and mid-roof stages. The warranty should separate material coverage from workmanship coverage; a 50-year slate warranty does not protect against short nails, insufficient headlap, or missing ice-dam membrane. Finally, require a written correction procedure for any slate found with exposed nails, cracked corners, or improper exposure before final payment.