Of all roof coverings, slate shingles are the most durable. If they are of good quality, they can last indefinitely (at least in excess of one hundred years). The slate roof over the Saxon Chapel at Stratford-on-Avon in England is over eleven hundred years old and, according to the Vermont Structural Slate Company, is in good condition. A slate roof, however, does not remain maintenance-free, even though the slates are of good quality. I have seen very few slate roofs that did not have some cracked, loose, chipped, or missing slates, a condition that requires some repair. These repairs are considered minor roof maintenance and should be anticipated on a periodic basis. Slate roofs are often patched with asphalt cement, which has a tendency to dry and crack and requires periodic application. When inspecting this type of roof from the exterior, look for cracked, loose, chipped, or missing shingles. If you find any, make a note on your worksheet.
Repairs to a slate roof, even minor ones, can be somewhat costly. Several roofers have told me that their fee reflects additional work above and beyond the required repair because they always anticipate accidental cracking of some of the slates during the repair. One difficulty you should be aware of is that when replacing a slate shingle, the roofer might not be able to match the color of the new slate to the existing weathered shingles.
Sometimes a poor-quality slate, ribbon slate, is used as a roof covering. (See FIG. 2-7.) The ribbons within the individual shingles are softer than the normal slate and will cause the shingles to crack along the ribbon. Often, cracking occurs along the ribbon after only ten years. Repairs to these shingles must then be made as needed.
When inspecting the roof, flaking slates may also be noted. Surface flaking is of no concern, since the shingles are at least 3⁄16 inch thick and are basically impervious to water. However, if any of the shingles are deteriorating as a result of excessive splitting and flaking (a condition brought about by winter freeze-thaw cycles), they should be replaced.
If the house you are inspecting is in the northern part of the country where snow might accumulate, look for snow guards on the lower portion of the roof. In particular, they should be located above doorways, sidewalks, or other areas where people will pass or gather. Snow guards are needed to prevent sliding masses of snow and ice from falling off the roof and damaging the gutters. It might interest you to know that the slate roofs on the buildings of the Harriman estate in New York had 35,000 copper-wire snow guards.
Clay tiles are available in many patterns. The most common are Spanish and Mission. These tiles are made by shaping moist clay in molds and firing the various shapes. They are hard, durable, and fireproof. However, they are also brittle and can be easily damaged by falling tree limbs or climbing on the roof to make repairs. As with slate shingles, repair or replacement of individual tiles is more difficult and costly than that of asphalt shingles. Also, matching new tiles to the weathered tiles is usually a problem. When inspecting this type of roof, look for loose, broken, chipped, cracked, or missing tiles. If any of these conditions is found, it should be noted on the worksheet, as repairs are needed. Tiles can also deteriorate as a result of freeze-thaw cycles. You might find some cracked areas that have been sealed with asphalt cement. This condition is usually an indication of past problems. Since asphalt cement does dry and crack, periodic reapplication should be anticipated.
Check the joint (valley) between two sloping sections of the roof. (See FIG. 2-8.) If it’s filled with asphalt cement, it’s an indication of a problem condition that has been temporarily corrected. The flashing in that joint should be replaced. As with slate roofs, repairs, even minor ones, can be somewhat costly. This item should be noted on your worksheet.