When the exterior of a house is covered with shingles - wood or asbestos - theoretically there should be little or no need for repairs for many years, except perhaps for periodic application of paint or stain.

However, shingles do occasionally work loose or split, or they may be damaged when something heavy falls against the side of the house, or even by a carelessly thrown baseball or similar hard object. When any of these defects are noticed it is best to make repairs as soon as possible to keep water from entering behind the shingles, and to keep the damage from getting worse.

One of the first and most common signs of trouble is when nails pull out or loosen, thus permitting shingles to wrap (in the case of wood shingles), or to be blown off by the wind. Although driving loose nails back into place will suffice in some cases, as a rule nails that have worked loose once will do so again unless replaced by longer and thicker nails, or reinforced by driving in an extra nail a couple of inches away from the original one.

Aluminium, galvanized iron, or rustproof nails of the serrated or ring type should be used because they are less likely to work loose under normal conditions.

Although not always required with wood shingles, it is essential on asbestos shingles that a small pilot hole he drilled first to avoid splitting. This is also a good idea on old wood shingles that are quite dry, or when nails must be driven close to one of the outside edges. In all cases nails should be hammered only deep enough to bring the head down flush - driving deeper or hammering too hard near the end is likely to split the shingle.

If a shingle is cracked or split but both halves are still in place, an effective repair can be made by driving in extra nails to hold the two pieces in place after butting them tighly together.

It is a good idea to slide a sheet of waterproof roofing felt or building paper under the split before nailing the two halves in place, making sure that the nails are located where they will hold this in place (the bottom edge of the paper or felt should be trimmed off flush with the bottom of the shingle).

When part of a shingle is missing entirely or when the shingle is split in several places so that renailing is impractical, the only permanent repair is to replace that shingle. Unfortunately, lumber yards sell shingles by the bundle (100 square feet), but sometimes yards have broken bundles and will sell a few.

Otherwise it may be worthwhile to buy a bundle and save what you don't use immediately for future repairs, or when alterations are made. In some cases, it may be possible to buy a few loose shingles from a builder who has some left from a job.

Removing the damaged shingle and replacing it with a new one requires some care because shingles overlap and are nailed in such a way that the nails that go through the bottom of one shingle also go through the top of the one underneath.

This means that these nails must be cut off or otherwise removed first. After this the damaged shingle can be slid down and out without loosening the shingles above or below, and the new shingle can be slid in from underneath.

Although the overall procedure is basically the same when removing either a damaged wood shingle or asbestos shingle, there are some differences in technique due to variation in how each type is installed.

Asbestos shingles, which are much more brittle, are usually nailed directly on top of each other. Wood shingles may have an extra wood strip under the lower edge of each row of shingles, or the shingles may be installed in a double layer to give what is known as a shadow effect.

A damaged asbestos shingle can often be removed by first smashing it with a few sharp blows from a hammer. Because it is brittle it will split apart so the broken pieces can be slid out from underneath. After this has been done, the nails, which are generally along the bottom end only, can be pulled out with a claw hammer or lacking pliers. [TEXT OMITTED] the damaged one. Many prefer to remove the nails first to free the asbestos shingle without using a hammer.

There are three other methods for getting the nails out: drilling through the nail head (a trickly process that requires careful guiding of a drill bit); sliding a hacksaw blade under the shingle so that the nail can be cut off from underneath the shingle; using a cold chisel and a hammer to slice or chop off the head of the nail.

When the nail head is off, the shingle can be pried away from the wall to clear that part of the nail that still protrudes through the shingle, at which point the shingle can be pulled down and out.

Removing the nails from damaged wood shingles in order to pull the shingles free is done in one of two ways. The simplest, and often the quickest, method is to use a chisel as shown to remove enough wood around the nail head so you can grab the nail with locking pliers and pull it straight out.

An alternate method is to follow the same technique as described for asbestos shingles: slide a hacksaw blade under the shingle, then cut the nail off in back of the shingle. In those cases where there are two layers of shingles, or where there is an extra strip of wood under each row, the blade may have to be slid under a second time to cut it off there too.

After the nails have been cut off, the damaged shingle is removed and any remaining nail stubs pulled out or hammered in so they will not get in the way of installing the new shingle. Slide the new shingle, or shingles, up from underneath and line up the bottom edges to conform with those on each side. Then renail top and bottom in the same way the original shingles were nailed, following the precautions about drilling pilot holes first where necessary.

Because the color of the new shingles will seldom if ever match the old ones, painting or staining will almost always be required to make the repair less noticeable. If the paint or stain on the rest of the house is fairly new, just touching up the new shingles may be adequate. However, if the painto or stain is more than one or two years old, chances are you will have to do the whole wall, or at least a section of the wall, to make the repair inconspicuous.

The only trouble with this technique is that there is always the danger of cracking or breaking adjoining shingles.