Slate Roofing Installation Guide

Before You Begin: Planning Your Installation

A successful slate roofing installation begins long before the first tile is laid. Thorough planning — encompassing project scope definition, permit acquisition, contractor selection, material ordering, and site staging — is the foundation upon which a quality installation is built. Rushing or skipping any of these preparatory steps is one of the most common causes of project delays, cost overruns, and ultimately, installation quality problems that may not become apparent until years later.

The first step in any slate roofing project is a thorough assessment of the existing structure. For re-roofing projects, this means a complete inspection of the existing roof deck, rafters, and attic structure to identify any deterioration, inadequate structural capacity, or ventilation deficiencies that must be addressed before new slate is installed. For new construction, the structural engineer must verify that the framing is designed to accommodate the weight of slate — typically 700-1,400 pounds per square depending on thickness — which is 4-8 times heavier than asphalt shingles. Failure to address structural deficiencies before installation is a serious and expensive mistake.

Permit requirements for slate roofing vary by jurisdiction but typically include a building permit and, in historic districts, a certificate of appropriateness from the local historic preservation commission. Obtaining permits before work begins is not merely a legal requirement — it ensures that the installation will be inspected by a qualified building official and that any deficiencies will be identified and corrected. Some jurisdictions also require that roofing contractors be licensed and insured; verifying these credentials before signing a contract is essential.

Contractor selection is arguably the most important decision in a slate roofing project. Slate installation is a specialized craft that requires years of training and experience; a contractor who is skilled at asphalt shingle installation may have little or no experience with slate. When evaluating contractors, ask for references from completed slate projects, request to visit those projects in person, and verify that the contractor has experience with the specific type of slate and installation system you are specifying. Membership in the National Slate Association or similar professional organizations is a positive indicator of commitment to the craft.

Material ordering should be completed well in advance of the installation start date. Premium Vermont slate is a natural product that is quarried and processed to order; lead times of 4-8 weeks are typical, and longer lead times may apply for custom sizes or large quantities. Order a minimum of 10% overage to account for breakage during installation, cuts at hips and valleys, and future repairs. Keeping a supply of matching slate on hand for future repairs is one of the most valuable things a building owner can do to protect their investment — matching slate from the same quarry lot years later can be extremely difficult.

Roof Deck Requirements

The roof deck is the foundation of the entire slate installation, and its condition and construction have a direct impact on the longevity and performance of the finished roof. Slate is an unforgiving material — it will faithfully reflect any irregularities, deflections, or deficiencies in the deck below. A deck that is sound, flat, and properly ventilated is an absolute prerequisite for a quality slate installation.

Minimum roof pitch for slate installation is 4:12 (4 inches of rise for every 12 inches of horizontal run). Below this pitch, the standard 3-inch headlap is insufficient to prevent water infiltration during wind-driven rain events, and special installation techniques with increased headlap are required. Pitches of 6:12 and above are ideal for slate roofing, providing excellent drainage and allowing the use of standard headlap dimensions. Very steep pitches (above 20:12) require special fastening techniques to prevent slate from sliding down the roof under its own weight.

Deck materials for slate installation are typically either solid board sheathing (1×6 or 1×8 boards) or plywood/OSB panels. Solid board sheathing, the traditional choice, provides excellent nail-holding capacity and allows for natural ventilation between boards. If solid boards are used, they should be a minimum of 3/4 inch thick, free of large knots, and installed with a 1/4-inch gap between boards to allow for seasonal movement. Plywood or OSB sheathing is acceptable if it is a minimum of 5/8 inch thick (3/4 inch preferred) and is exterior-grade. OSB is generally less preferred than plywood for slate applications due to its lower resistance to moisture and its tendency to swell at edges if exposed to water.

Structural requirements for slate roofing go beyond the deck itself. Rafters must be sized to carry the additional dead load of slate — typically 10-15 psf for standard 3/16-inch slate — in addition to live loads (snow, wind, maintenance workers). In many older buildings, the existing rafter sizing was designed for lighter roofing materials, and structural reinforcement may be required before slate can be installed. A structural engineer should review the framing whenever there is any doubt about its adequacy. The cost of structural reinforcement is almost always justified by the dramatically longer service life of slate compared to lighter alternatives.

Roof ventilation is a critical but often overlooked aspect of deck preparation. Inadequate ventilation leads to moisture accumulation in the attic, which can cause deck deterioration, ice dam formation, and premature failure of the underlayment. The standard recommendation is 1 square foot of net free ventilation area for every 150 square feet of attic floor area, divided equally between intake (soffit) and exhaust (ridge or gable) vents. Before installing new slate, verify that the ventilation system meets current standards and correct any deficiencies.

Underlayment & Moisture Barriers

The underlayment system beneath a slate roof serves as the secondary line of defense against water infiltration — the first line being the slate itself. While a properly installed slate roof should shed virtually all water before it reaches the underlayment, the underlayment must be capable of handling the occasional water that penetrates through broken slates, open joints, or flashing failures. Selecting and installing the right underlayment system is therefore a critical component of a long-lasting slate installation.

Traditional felt underlayment — specifically 30-pound asphalt-saturated felt (ASTM D226 Type II) — has been used under slate roofs for over a century and remains a reliable choice. Felt is vapor-permeable, allowing moisture that accumulates in the attic to escape through the deck, and it is compatible with the long service life of slate. For premium installations, two layers of 30-pound felt are recommended, with the second layer offset from the first to eliminate aligned seams. Avoid 15-pound felt under slate — it is too thin and fragile to provide adequate protection during the installation process and will not last as long as the slate above it.

Synthetic underlayments have become increasingly popular as alternatives to felt. High-quality synthetic underlayments offer superior tear resistance (important during installation), better UV resistance (important if the underlayment will be exposed for an extended period during installation), and lighter weight. However, not all synthetic underlayments are appropriate for slate applications — some are vapor-impermeable, which can trap moisture in the deck assembly. When specifying synthetic underlayment for slate, always verify that it is vapor-permeable and rated for use under heavy roofing materials with long service lives.

Ice and water shield — a self-adhering, rubberized asphalt membrane — is required at the eaves and in valleys in cold climates where ice dams are a risk. Ice and water shield should be installed at the eave from the drip edge up to a point at least 24 inches inside the exterior wall line (or to the ridge on very low-slope sections). In valleys, ice and water shield should extend a minimum of 18 inches on each side of the valley centerline. Use only ice and water shield products rated for use under slate — some products are not compatible with the long-term heat exposure that can occur under a dark slate roof.

Proper lapping of underlayment is essential for weather resistance. Horizontal laps should be a minimum of 4 inches (6 inches preferred), with upper courses lapping over lower courses. End laps should be a minimum of 12 inches. At hips and ridges, the underlayment should be lapped over the peak and secured on both sides. All laps should be oriented so that water running down the roof will flow over, not under, the lap. Staples or cap nails should be used to secure the underlayment, with fasteners spaced no more than 12 inches apart along laps and 18 inches in the field.

Selecting the Right Fasteners

Fastener selection is one of the most consequential decisions in a slate roofing installation. The fasteners must outlast the slate itself — a slate roof installed today should still be performing in 150 years, and the fasteners must remain sound throughout that entire service life. Using inferior fasteners in a slate installation is a false economy that will result in premature roof failure, requiring expensive repairs or complete re-roofing long before the slate itself has reached the end of its useful life.

Copper nails are the gold standard for slate roofing and have been used successfully for centuries. Copper is highly resistant to corrosion in the atmospheric conditions encountered on a roof, and copper nails installed in the 19th century are routinely found in sound condition when historic slate roofs are repaired or replaced. Copper nails for slate should be a minimum of 10-gauge (0.135" diameter) with a large, flat head (at least 3/8" diameter) to prevent pull-through. Nail length should be sufficient to penetrate the deck a minimum of 3/4 inch — for 3/4-inch plywood decking, this means a minimum 1-1/2-inch nail; for 1-inch board sheathing, a minimum 1-3/4-inch nail.

Stainless steel nails (Type 316) are an acceptable alternative to copper and are preferred in coastal environments where salt air accelerates corrosion of even copper fasteners. Type 316 stainless steel contains molybdenum, which provides superior resistance to chloride-induced corrosion. Type 304 stainless steel is less resistant to salt air and should not be specified for coastal applications. Stainless steel nails are typically less expensive than copper and are available in a wider range of sizes, making them a practical choice for many applications.

Galvanized nails should never be used for slate roofing. While hot-dipped galvanized nails are acceptable for many roofing applications, the zinc coating on galvanized nails corrodes within 20-30 years in the conditions encountered on a roof, leaving the underlying steel nail vulnerable to rapid corrosion. When galvanized nails fail, the slate tiles they secure become loose and can slide off the roof — a safety hazard as well as a maintenance problem. The cost difference between galvanized and copper or stainless nails is trivial compared to the cost of premature roof failure.

Pre-drilling nail holes in slate is required for all but the softest slates. Attempting to drive nails through slate without pre-drilling will crack or split the tile. Nail holes should be drilled with a carbide-tipped bit sized to provide a snug fit around the nail shank — typically 3/16" for 10-gauge nails. Holes should be located approximately 1 inch from the top edge of the tile and 1 inch from each side edge. The nail should be driven so that the head is flush with the tile surface — not countersunk (which weakens the tile) and not proud (which will interfere with the overlapping course).

Laying the First Course: Starter Slates

The starter course is the foundation of the entire slate installation, and its correct execution is critical to the performance of every course above it. The starter course serves two functions: it provides the necessary headlap for the second course of slate, and it creates the proper drip edge at the eave to direct water away from the fascia and into the gutter. Getting the starter course right requires careful attention to alignment, overhang, and the double-coursing technique that is unique to slate installation.

Before laying the starter course, verify that the drip edge is properly installed. Metal drip edge (copper or galvanized steel, minimum 26-gauge) should be installed at the eave before the underlayment, with the underlayment lapping over the top of the drip edge. At rakes (the sloped edges of the roof), the drip edge is installed over the underlayment. The drip edge should extend a minimum of 1/4 inch beyond the fascia to direct water into the gutter and prevent it from running back under the roofing.

The starter course consists of two layers of slate at the eave. The first layer (the true starter) is installed upside-down — with the thick end (tail) at the eave and the thin end (head) pointing up the slope. This inverted installation creates a slight upward tilt at the eave that helps direct water away from the fascia. The starter slate should overhang the drip edge by 1 to 1-1/2 inches. The second layer of the starter course is installed right-side-up on top of the first, with its head aligned with the head of the starter slate below. This double layer at the eave ensures that the second course of slate has the correct headlap.

Horizontal alignment of the starter course is established using a chalk line snapped parallel to the eave. This line should be positioned so that the tails of the starter slates will overhang the drip edge by the correct amount. Vertical alignment (the spacing of slate courses up the slope) is established using a course layout rod or by measuring and marking the deck at regular intervals equal to the slate exposure. Taking the time to establish accurate layout lines before beginning installation will prevent the cumulative alignment errors that can make a slate roof look amateurish.

At hips and valleys, the starter course requires special treatment. At open valleys, the starter slates on each side of the valley are cut to follow the valley line, with a minimum 4-inch clearance from the valley centerline to allow for water flow. At hips, the starter slates are cut at the appropriate angle to follow the hip line. These cuts should be made with a slate cutter or angle grinder with a diamond blade — never with a circular saw, which will create excessive dust and may crack the slate. All cut edges at valleys and hips should be sealed with roofing cement to prevent water infiltration at the cut face.

The Main Field: Laying Slate

With the starter course in place, the installation of the main field of slate can begin. This is the most labor-intensive phase of the installation and the one that most directly determines the visual quality and weather resistance of the finished roof. The key principles governing field installation — headlap, sidelap, and joint breaking — must be consistently maintained throughout the installation to achieve a roof that is both beautiful and watertight.

Headlap is the most critical dimension in slate installation. Headlap is the distance by which the top edge of a slate tile is covered by the tile two courses above it — in other words, the amount of triple coverage at any point on the roof. The minimum headlap for slate roofing is 3 inches, and this minimum should be treated as an absolute requirement, not a guideline. Insufficient headlap is the single most common cause of premature slate roof failure; water driven by wind can travel several inches up the slope under the overlapping slate, and if the headlap is less than 3 inches, this water will reach the deck. On low-slope roofs (4:12 to 6:12), headlap should be increased to 4 inches or more.

Sidelap — the horizontal overlap between adjacent slates in the same course — must be a minimum of 1-1/2 inches. Sidelap prevents water from flowing through the vertical joints between slates. When laying slate, the joints in each course must be offset from the joints in the course below by at least 1-1/2 inches (and ideally 3 inches or more). This "breaking of joints" is what gives a slate roof its characteristic appearance and is essential for weather resistance. Joints that align vertically — even for just two consecutive courses — create a direct path for water infiltration.

Random-width slate — where tiles of varying widths are mixed throughout the installation — is the traditional and most visually appealing approach for residential roofing. Random-width installation requires more skill than uniform-width installation because the installer must constantly plan ahead to ensure that joints are properly broken. A common technique is to sort the slate by width before beginning installation and to alternate wide and narrow tiles in a pattern that naturally breaks joints. Uniform-width slate, where all tiles are the same width, is simpler to install but produces a more mechanical, less organic appearance.

Each slate tile should be inspected before installation. Tap each tile with a knuckle — a clear, ringing tone indicates sound slate; a dull thud indicates a crack or delamination. Cracked or delaminated tiles should be set aside and not installed. Check each tile for thickness consistency — tiles that are significantly thinner or thicker than the specified thickness should be sorted into separate piles and used in areas where their thickness variation will be least noticeable. Install slate with the grain running vertically (parallel to the slope) and the smooth face (the cleavage face) exposed.

Hips, Ridges & Valleys

Hips, ridges, and valleys are the most architecturally complex and weather-critical elements of a slate roof. These are the locations where two roof planes meet, creating either an external angle (hip or ridge) or an internal angle (valley). Each requires specialized techniques and materials to achieve a watertight, durable, and visually appropriate result. Errors at these locations are disproportionately likely to cause water infiltration, and they are also the most visible elements of the roof from the ground.

Mitered hips are the traditional treatment for hip intersections on high-quality slate roofs. In a mitered hip, the slate tiles on each side of the hip are cut at a 45-degree angle (or the appropriate angle for the specific hip geometry) and butted together at the hip line, with a copper flashing strip beneath the joint. Mitered hips require precise cutting and careful fitting, but they produce a clean, elegant appearance that is consistent with the finest historic slate roofs. An alternative approach — the saddle hip — uses specially shaped hip tiles that straddle the hip line, eliminating the need for precise mitering but producing a more prominent, rounded profile.

Ridge treatment options include the saddle ridge (the most common), the comb ridge, and the coxcomb ridge. The saddle ridge uses specially shaped ridge tiles that are installed straddling the ridge line, with each tile overlapping the one below in a shingle pattern. The comb ridge uses a row of slate tiles set on edge along the ridge, creating a distinctive serrated profile that is characteristic of many New England vernacular buildings. The coxcomb ridge is a more elaborate variation of the comb ridge, with alternating tiles of different sizes creating a decorative pattern. All ridge treatments must be bedded in a flexible roofing cement and secured with copper nails to prevent wind uplift.

Valley treatment is one of the most critical details in slate roofing, as valleys concentrate the water from two roof planes into a single channel. Open valleys — where the valley flashing is exposed between the slate on each side — are the most reliable approach for slate roofing and are strongly preferred by experienced slate roofers. Open valleys should be lined with a minimum 20-ounce copper sheet, extending at least 12 inches on each side of the valley centerline. The slate on each side of the valley should be cut to a straight line parallel to the valley centerline, with a minimum 4-inch clearance from the centerline to allow for water flow without backing up under the slate.

Closed valleys — where the slate from one side overlaps the valley centerline and covers the slate from the other side — are sometimes used for aesthetic reasons but are generally less reliable than open valleys. Woven valleys — where the slate from each side is alternately interlocked — are not recommended for slate roofing because the overlapping tiles prevent proper inspection and maintenance of the valley flashing. Whatever valley treatment is used, the flashing must be installed before the slate and must be of a material (copper or lead) that will outlast the slate above it.

Flashing: The Critical Details

Flashing is the system of metal components that seals the junctions between the slate and other building elements — walls, chimneys, skylights, pipes, and other roof penetrations. Properly designed and installed flashing is essential for a watertight roof; improperly installed flashing is the most common cause of roof leaks, even on roofs where the slate itself is in excellent condition. The investment in high-quality flashing materials and skilled installation is one of the most important decisions in a slate roofing project.

Step flashing is used at the junction of the roof and a vertical wall (such as a dormer wall or the side of a chimney). Step flashing consists of individual L-shaped pieces of metal, typically 4 inches wide by 8 inches long, installed one per course of slate. Each piece of step flashing is bent to a 90-degree angle, with one leg lying flat on the roof deck under the slate and the other leg turned up against the vertical wall. The pieces are installed in a shingle pattern, with each piece overlapping the one below by at least 2 inches. Step flashing must be installed as the slate is laid — it cannot be retrofitted after the slate is in place.

Counter flashing is installed over the step flashing, with its lower edge overlapping the top of the step flashing by at least 3 inches. Counter flashing is typically embedded in mortar joints in masonry walls or secured under siding on wood-framed walls. The counter flashing prevents water from running behind the step flashing and into the wall assembly. Counter flashing should be installed in sections no longer than 12 inches to allow for thermal movement without buckling or cracking the mortar joint.

Chimney flashing is one of the most complex and failure-prone details in roofing. A properly flashed chimney requires base flashing at the front (downslope) face, step flashing on the sides, a cricket (a small peaked structure) at the back (upslope) face to divert water around the chimney, and counter flashing over all of these elements. The cricket is particularly important for wide chimneys — any chimney wider than 30 inches should have a cricket to prevent water and debris from accumulating behind it. Chimney flashing should be inspected every 5-10 years and repointed as necessary to maintain the integrity of the mortar joints.

Common flashing failure points include: mortar joint deterioration (allowing counter flashing to pull away from the wall), inadequate overlap between step and counter flashing, use of incompatible metals (galvanic corrosion occurs when dissimilar metals are in contact), and improper sealing of penetrations. All flashing should be inspected as part of the final quality control process and any deficiencies corrected before the project is accepted. Flashing failures are almost always less expensive to correct during installation than after the building is occupied.

QWIK Slate® Installation

QWIK Slate® is Newmont Slate Company's patented hook-based installation system that represents a significant advancement in slate roofing technology. Unlike traditional nail-through installation, the QWIK Slate® system secures each slate tile using a specially designed stainless steel hook that engages the bottom edge of the tile, eliminating the need to drill nail holes through the slate. This fundamental difference in attachment method has profound implications for installation speed, tile integrity, and long-term performance.

The QWIK Slate® system consists of two components: a continuous aluminum hook strip that is nailed to the roof deck at each course line, and individual stainless steel hooks that clip onto the strip and engage the tail of each slate tile. The hook strips are installed horizontally across the roof at intervals equal to the slate exposure, using the same layout lines used for traditional installation. The hooks are then clipped onto the strip at the appropriate spacing for the slate width being used. Each slate tile is simply hooked onto the strip — no drilling, no nailing through the tile face.

The installation sequence for QWIK Slate® differs from traditional installation in several important ways. Because there are no nail holes to drill, installation proceeds significantly faster — experienced installers report productivity increases of 30-50% compared to traditional nail-through installation. The elimination of nail holes also eliminates the most common source of tile cracking during installation, reducing waste and improving the quality of the finished installation. The hook system also allows tiles to be installed in any weather condition without the risk of splitting that can occur when driving nails into cold, brittle slate.

One of the most significant advantages of the QWIK Slate® system is its repairability. In traditional nail-through installation, replacing a broken tile requires removing the overlapping tiles above it, extracting the broken tile and its nails, and re-nailing the replacement tile — a process that risks damaging adjacent tiles and is time-consuming and expensive. With QWIK Slate®, a broken tile can be unhooked and replaced in minutes without disturbing any adjacent tiles. This dramatically reduces the cost and disruption of routine maintenance and repairs over the life of the roof.

The QWIK Slate® system is compatible with all standard slate sizes and thicknesses and can be used on any roof pitch from 4:12 to vertical. It is particularly well-suited for steep-pitch applications, where the hook engagement provides superior resistance to wind uplift compared to nailed installation. The system has been tested to meet or exceed all applicable building code requirements for wind resistance and has been used successfully on landmark buildings, university campuses, and high-end residential projects throughout North America.

Final Inspection & Quality Control

A thorough final inspection is the last line of defense against installation deficiencies that could compromise the performance of the finished roof. The inspection should be conducted by the roofing contractor, the general contractor, and ideally by an independent slate roofing consultant before the project is accepted and final payment is made. Deficiencies identified during inspection are almost always less expensive to correct at this stage than after the building is occupied and the first rain event reveals a leak.

The inspection should begin with a visual survey of the entire roof from the ground, using binoculars if necessary to examine details that cannot be seen from the eave. Look for misaligned courses, inconsistent exposure, broken or cracked tiles, and any areas where the installation pattern appears irregular. Pay particular attention to hips, ridges, and valleys, where installation errors are most likely to cause water infiltration. Any tiles that are visibly cracked, chipped, or misaligned should be flagged for replacement.

A close-up inspection from the roof surface (or from a ladder at the eave) should verify headlap and sidelap dimensions at multiple locations across the roof. Spot-check headlap by measuring the distance from the top edge of a tile to the bottom edge of the tile two courses above it — this should be a minimum of 3 inches everywhere on the roof. Check sidelap by measuring the horizontal offset between joints in adjacent courses — this should be a minimum of 1-1/2 inches. Any location where these minimums are not met should be corrected before the inspection is complete.

Flashing inspection is particularly important, as flashing failures are the most common cause of roof leaks. Verify that all step flashing is properly installed and that counter flashing overlaps it by at least 3 inches. Check that chimney flashing is properly embedded in mortar joints and that the cricket (if present) is correctly formed and flashed. Inspect all pipe penetrations to verify that lead or copper pipe boots are properly installed and sealed. Check valley flashing to verify that it extends the correct distance on each side of the valley centerline and that the slate is cut to the correct clearance.

Documentation of the completed installation should include photographs of all critical details (flashing, valleys, hips, ridges), the ASTM test certificates for the slate used, the manufacturer's specifications for the underlayment and any proprietary installation components, and the contractor's warranty. This documentation should be provided to the building owner and retained in the building's maintenance records. For historic buildings, documentation of the installation should also be provided to the State Historic Preservation Office if the project received any historic tax credits or grants.

Installation Checklist

Use this checklist to verify that all critical installation requirements have been met before accepting the completed work. Each item should be verified by the contractor and independently confirmed by the owner's representative or inspector.