When it comes to premium roofing, slate stands unmatched in beauty and durability. However, to maximize its lifespan—often exceeding 100 years—the underlayment system beneath the slate must be flawless. The double-layer underlayment application is the gold standard for slate roofs, providing redundant protection against water intrusion.

First Layer: The Base Sheet. The initial layer, typically a self-adhering rubberized asphalt membrane (ice and water shield), is installed directly over the roof deck. This flexible, waterproof membrane is applied horizontally along the eaves and valleys, extending at least 3 feet past the interior wall line. This base layer seals nail penetrations from the slate and prevents ice damming issues in colder climates. Ensure the membrane overlaps by 3-6 inches at the seams and is rolled firmly to bond with the deck.

Second Layer: The Overlay. The second layer uses a traditional roofing felt (ASTM D226 Type II, 30-pound felt) or a synthetic underlayment. This layer is applied horizontally, lapping over the lowest edge to create a "staggered" seam pattern. The double layer works like a fish scale: if water gets past the outer slate and the first underlayment, the second felt layer acts as a backup. Overlap all seams by at least 4 inches, and staple or nail the felt with corrosion-resistant fasteners, placing nails only in the overlapping zones to avoid exposed holes.

Critical Installation Steps. Begin at the drip edge: install the first layer of membrane flush with the eaves, and the second layer overlapping the drip edge by 1-2 inches. For valleys, apply a 36-inch-wide strip of membrane on each side, then the felt on top. Never use only single-layer felt under slate; the double layer is necessary to handle the high volume of rainwater and snow melt that slate roofs shed. The felt layers also allow ventilation, helping the roof deck breathe and preventing rot.

Pro Tips for Longevity. Lay the felt horizontally and avoid vertical joints over gaps. Use copper or stainless steel nails or staples for the felt, as slate’s long lifespan outlasts standard fasteners. Always check local building codes, as some jurisdictions require specific fire-resistant underlayments. A correctly applied double-layer underlayment ensures your slate roof remains leak-free for generations, protecting the investment and the home below.