Questions about Slate as a Material

Questions about Slate Roofing

Slate is mined all over the world, from the U.S. to Brazil to China to India and throughout Europe as well as many places in Africa. Even Australia has recently started to quarry slate. Most of these areas potentially have good quality slate. The trick is getting it produced. The question is whether or not the operators know how to quarry and finish the slate to high quality roofing slate. It often takes digging very deep down past the soft "top rock" to get to good stone, and then producing the product is a skill in itself.

Ribbon slates are Pennsylvania black slates that have small bands running through them. These ribbons contain a high amount of carbon that deteriorates much faster than the rest of the slate. This results in the slate becoming separated at the ribbon, allowing either the slates to fall out or water to penetrate the roof. Even so, ribbon slate roofs have still been known to last 50 to even 100 years if the roof is steep and well shaded. Once the ribbons become soft, the roof is virtually unrepairable and must be replaced.

It is very simple. S-1 means the slate will last more than 75 years. S-2 slate is meant to last 45 to 75 years and S-3 slates are meant to last 45 years or less. However, slates of all three grades generally do last longer than their required minimums.

Gray, green, purple, black, and red. Sometimes there is a mottling effect, mixing a couple of these colors together, the most common being gray-black, gray-green, and green-purple.

Yes. Weathering or semi-weathering green and gray slates can develop brownish and pinkish tints, due to iron in the slate. At the time of new installation, the percentage that will turn color is unpredictable. It can be as little as 5 to 10 percent or nearly 100%. The only way to predict for certain what a roof will look like (if one wishes to have a weathering slate) is to install a reclaimed slate which has already undergone the weathering process.

Unfading slates also change colors to some degree. Some can turn a deeper shade and some can become slightly washed out looking.

Weathering slates will change colors almost immediately after installation and may continue to change for years, while unfading slates change colors more gradually and subtly.

A simple, straightforward slate roof starts out at a little under $15 per square foot and can run more than $40 per square foot depending on the type of slate used, the design of the roof, the height of the building, the pitch of the roof, and so forth. Most slate roofs run between $15 and $30 per square foot, installed.

Aesthetics: Slate roofs come in a wide variety of sizes, colors, and thicknesses. One can create many different looks to enhance just about any architectural style.

Ecological considerations: Slate is a natural product that comes out of the ground and can safely go back into the ground. Petrochemical roofing made of asphalt and fiberglass typically lasts twenty years. According to statistics, construction and demolition debris make up 28% (by weight and volume) of the refuse being placed in landfills. That is more than the waste from styrofoam, fast-food packaging, disposable diapers, and all plastic packaging combined! Approximately 20% of all construction debris is roofing. Though not the only reason, environmental concern is certainly a good reason for installing slate.

Effective protection: A slate roof is the best chance you have for getting a leak-free roof. Most leaks in roofs occur in flat areas and areas with protrusions. Since slate roofs are installed by craftsmen who are thinking in service life of decades or even centuries, these details are usually installed with great care. As a result, the quality of leak protection is much higher. Thus slate roofs leak much less often than other kinds of roofing -- especially on houses with complicated roof lines.

Slate roofs are one of the most fireproof roofs.

Slate roofs are much more wind resistant than asphalt roofs. Slate roofs also do not rely on the underlying roofing felt to remain leak proof the way many clay and concrete tile roofs do.

Long-term Reliability: Slate has been the material of choice for a large percentage of the most important buildings built in the last five centuries. The method of installation has changed little over five centuries. New roofing products, such as many of the simulated slates, have come and gone in less time than asphalt roofs last. Some of these products have been sold with 50-year warranties, which became worthless when the businesses went under.

Standard thickness slate weighs approximately 800 to 1,000 pounds per square installed. (A square is a 10 foot by 10 foot area.) 3/8” slate weighs approximately 1,500 pounds per square. Every 1/8” of additional thickness increases the weight by approximately 500 pounds per square.

A good solid deck with 16” on center framing is preferred. Most structures are built to handle wind and snow loads that far exceed the weight of slate.

However, a weak structure that has too much flex to it will result in a poorly installed slate roof. Ideally, one would use a ¾” thick tongue and groove deck on 2” x 12” rafters 16” on center. However, this is an expensive deck, and would be a rare sight today. Otherwise, it would be best to have at least 5/8” plywood or ¾” plywood over 16” on center trusses built to handle slate.

We prefer not to put slate roofs on pitches less than 6/12. We will consider putting on slate roofs that are 4/12 and 5/12 if we double felt (30#) and run a 4” headlap on the slate.

Slates are cut using a slate cutter or the sharp edge of a slate hammer. One does not usually use a saw, as a saw will not provide the attractive beveled edge characteristic of roofing slates.

Flashing is the sheetmetal that is used at protrusions and roof transitions due to the fact that slate cannot be bent. It keeps a roof watertight in these areas. The preferred choices for flashing are copper, lead-coated copper, and terne-coated stainless steel. Aluminum should not be used, as it cannot be soldered.

No. When asphalt shingles are applied over slate, the nails are driven through the slates. This method leaves holes in the slate if the nails and shingles are removed. New slate must be applied to ensure a watertight roof.

No.

Yes. Even new slate roofs will shed a few slates. It is a good idea to visually inspect a slate roof at least once a year. The owner of the property can do this annual inspection. Any slates that are seen to be missing, sliding, or broken should be replaced. Every few years, it would be a good idea to have a more careful inspection from a slate roof expert.

A slate roof should not be coated with anything. Coatings will generally accelerate deterioration of slate.

Yes. Many contractors have inventories of reclaimed slate with which to repair old slate roofs. Old slate roofs often have weathered to a different color than new slate of the same type. Also, many old slates are thinner than the new slates that are being quarried today. That is why reclaimed slates are often the best choice for repairing an existing slate roof.

Most slate roofs are actually very durable and can withstand the necessary foot traffic of workmen. Hook ladders and roof planks can be used to distribute the weight of workmen, and they do help. However, a few broken slates can be expected and are easily repaired in the process of repairing the existing roof. If the work is done carefully by trained workmen, a slate roof will not be ruined by craftsmen replacing broken slates and damaged flashings.

Slate itself is fireproof. A slate roof is one of the most fire resistant roofs that exist. However, the wood deck under the slate is not fireproof. Often fires will jump from house to house as sparks hit adjacent roofs, igniting the surface if it is a flammable material. Many of the raging fires that engulf hundreds of houses every year would not occur if the houses were covered in slate.