Restoration Guide: Slate Roof

Editor's Note: This is article 11 of 13 in the Roofs Chapter of the Old House Web's Home Restoration Guide. This guide was developed and edited for old homes from original material in the U.S. Housing and Urban Development (HUD) Rehab Guide.

Slate Basics

Slate came to this country from England and has been used as a top-end roofing material for more than two centuries. Slate use slowed during the Great Depression with the drop in housing starts and the introduction of asphalt composition shingles. Today, slate is about 5 percent of the new residential roofing market and about 3 percent of the re-roofing market.

Slate, mostly from the marine deposits of clay and sand, is millions of years old. In the United States, it typically is quarried in Virginia, Pennsylvania, New York and Vermont; it also is found in Newfoundland, Canada.

The origin of the slate matters. Material from Pennsylvania has a service life of about sixty years; New York and Vermont slate will last more than 125 years, while Virginia Buckingham slate can exceed 175 years. Hardness is the determining factor. Slate can discolor and stain, a problem caused by two chemicals that can be present in slate--calcium carbonate and iron oxide--as affected by acid rain.

In recent years, renewed interest in slate for high-end homes and for old-house renovations has led to production from long-dormant quarries and the importation of tiles from Spain, England, Newfoundland and also from low-labor-cost countries such as Brazil, China and India. As with slate from different regions of the United States, the quality of slate can vary markedly from the different quarries around the world. It is wise to order your slate from a reputable distributor in the U.S. to assure you are getting quality material.

Slate is graded for hardness, with S1 being the hardest. It also is graded on its color-fastness: "weathering" means it might change color as it is exposed to the elements, and "non-weathering," means it does not significantly change color.

Methods and Materials

1. Repairing Slate Roofs

   Slate is a material with longevity, commonly lasting seventy-five to one hundred years, and it is possible that other elements of the roof will fail before the slate does. But if you have a really old house, it is also possible that the time is here for some close attention--the slate, flashings and fasteners all could have problems.

   Preliminary assessment of the roof can be done from the ground with binoculars, noting:

   • chipped or cracked slate
   • slipped tiles, especially along the ridge, in each horizontal row, in valleys or where the roof changes direction
   • the state of the flashing and fasteners
   • old repairs that may not have been done correctly

   Other inspections you should make in connection with your decision whether it's time to repair a slate roof include:

   • the old house's gutters and leaders (preferably on a rainy day)
   • the attic, checking for signs of water--stains, rot or other indications of structural distress

   If you have limited damage, you probably can get by with spot repairs. And if your old house's roof is less than fifty years old, you probably can find matching slate, either salvaged or new.

   Slate repair generally is a job for a professional with slate experience. The material is easily broken, and it can be very slippery.

   Beware of an inexperienced roofer who may recommend replacing an entire roof when it still could be repaired. If most of the slate still is good, but other problems exist (such as flashing or underlayment), it is possible the slate could be removed, repairs made, and the salvaged slate re-installed.

   Slate repair work can be difficult to estimate, with further damage becoming evident as the repair progresses. And it can be difficult to find matching tiles.

   As always, don't push your luck with a badly damaged roof. Postponing a replacement could lead to extensive water damage and a major home renovation down the line.

2. New Slate Roofing

   Slate colors range from gray, blue-gray, black, green, deep purple, red and variations of those shades, depending on the origin of the slate. It can be ordered up to 2" thick for high-end houses, though the common thickness is 1/4" to 3/8". It is installed by hand, using copper or stainless-steel nails.

   Because slate is very heavy, it usually requires trusses or rafters on 16" centers (standard spacing is 24"), and sheathing of 3/4" minimum.

   It can be installed on solid sheathing, spaced sheathing or on solid sheathing with battens. The standard layout has each tile overlapped by two other layers. Some economy layouts will have only one layer of overlap, with a layer of roofing felt also overlapping. However, this is not wise, as underlayment will deteriorate long before the slate needs replacement.

   There are two new products designed especially for slate roofs: a ridge vent called "Top Slate," from Peterson Aluminum and a mounting/ventilation system called "Fastrack for Slate" from Slate International, Inc.

   While slate is very expensive, it is cost effective over its extended lifetime. Spot repairs are relatively simple by an experienced professional, if matching slate can be found. The material is fireproof, and won't rot, mold or mildew.

   If you are considering a new slate roof, you need to make sure your structure is strong enough to take the load, or be prepared for some further home renovation. You also need to be aware of quality differences in slate (fading, discoloration, de-lamination and softness), especially of imported products.