Replacing broken or missing roof slates

Replacing broken or missing slates is an essential part of roof maintenance and timely repairs will prevent rainwater penetrating the timber and roofing felt below.

Slate is the most hardwearing of all roofing materials and provides a sound, waterproof protection against the weather for at least 50 years before it needs to be renewed. But over this time, individual slates may be shaken loose by high winds, vibration or movement of the roof structure. When this happens it is important to act at once and carry out repairs before rainwater finds its way into the roof timber below.

Roof layout

It is essential to understand how slate roofs are constructed and how the slates are fixed in position before attempting any repairs. Not all slates are the same size and many will have been shaped and cut according to their position on the roof.

Slates are laid from the eaves upwards, and each row—known as a course—is overlapped by the one above. The vertical joints of each course of slates are staggered—like brickwork—so each slate partially covers the two slates below it.

The slates are nailed to wooden battens which run horizontally across the roof at right angles to the rafters beneath. The battens are spaced according to the pitch—or slope—of the roof and the size of slate being used.

Most building regulations now require that new roofs are fitted with roofing felt laid horizontally across the roof between the battens and the rafters. This is to improve insulation and to help keep the roof watertight in the event of a slate breaking.

The slates on the first row at the eaves, and those on the last row below the ridge tiles, are shorter than those used on the rest of the roof. This preserves the bonding and overlapping arrangements of the slates, and so helps keep the roof watertight along these two particularly vulnerable areas.

Gable: At the gable end of the house each course of slates should come to a neat, well-tailored finish, level with the row of slates above and below. To do this, the gap left at the end of every second row is filled, either with a special wide tile-and-a-half slate or with a normal slate cut in half lengthways. If the roof is angled at the edge, the slates should be accurately cut to fit.

On the edge of many gable roofs there may be narrow clay tile known as a verge or creasing slate, which is laid under the slates at the end of each row. These tiles, and the bedding mortar holding the tile in place, give the edge of the roof a slight inward tilt and prevent rainwater running down the wall below. Ridge: Running along the ridge of the roof are a row of V-shaped ridge tiles usually made out of concrete or clay. They are laid in position across the apex of the roof straddling the two top courses of slates on each side and are bedded into place with mortar.

Hips: External angles formed by two roof surfaces meeting—called hips— are joined by shaping slates and then covering them with a row of hip or ridge tiles bedded in mortar. At the bottom end of the row of hips a hip iron is driven into the wood to help secure the whole structure. Valleys: Internal angles where two roofs meet—known as valleys—are formed in a similar way. Slates are shaped to form the correct angle and a thin sheet of galvanized zinc placed under them to carry away damaging rainwater.

Checking for damage

There are a number of ways in which you can assess the number and type of slates which are damaged and missing before starting work.

One of the simplest is to view the slates from a point far enough outside the house so that the whole roof is clearly visible. By looking up at the roof from all possible sides of the house it should be possible to get a good idea of the number of slates that are missing or badly cracked. If necessary use binoculars or a telescope, from a distance, to assess the damage. Check your findings by taking a closer look with the aid of a ladder erected up to the eaves. But remember when doing this that the ladder must be fitted with a stay to prevent damage to the guttering and that it should be anchored to the ground and fixed tightly and securely at the top.

Next closely examine the inside surface of the roof from the attic or loft for any signs of damage. Make a check on roofing felt if it is fitted, and look for rips or signs of general wear. If nearly all of the felt is badly perished there is little alternative but to renew it when the whole roof is being reslated. But if only small patches are affected, a number of slates should be removed from the outside and the felt renewed.

Where no roofing felt is fitted, small chinks of daylight showing through the roof covering may indicate where slates are missing or damaged. These should be traced to the outside and new slates inserted at the point where damage has taken place.

Look also for defective battening, caused by a badly leaking roof. If only a few of the battens are affected, they can be repaired by removing slates and replacing the rotten wood with fresh timber.

Damp on the ceilings of upstairs rooms is also another tell-tale sign of damaged or missing slates—although you should never let things get to this stage if you carry out regular checks and repairs on the roof. When trying to trace leaks back to their source do not imagine that you will always find the cause of the leak directly over the damp patch on the ceiling. Water penetrating the roof often runs down a rafter or batten until it reaches a knot in the timber and then drops to the ceiling below—so a more extensive search may have to be carried out.

Choosing replacement slates

Once you have assessed the full extent of the damage you should be able to calculate the exact quantity and type of new slates that need to be pur- chased. Replacements can be obtained from specialist slate suppliers or secondhand from demolition sites, but take great care that they match your existing ones.

Slates do not vary in colour and texture as much as tiles but to make sure that they are the same size and thickness as the originals take an old slate along when purchasing replacements. Attempting to repair a roof with slates thicker than those already used will tend to raise the adjacent slates and produce unsightly bumps which allow water to penetrate.

Slates obtained from demolition sites are perfectly adequate for roof repairs and are often easier to match since they are weathered on one side like the originals. However, they should be checked thoroughly to ensure that both nail holes are still sound and that the slates are not perished, or flaking too badly on either face before buying.

Tools and equipment

As well as a number of tools which you probably have already, a few pieces of specialized equipment are essential to carry out slate repairs successfully. These can usually be purchased at a fairly cheap price from most builders’ merchants. Slate ripper: This is a tool designed to slide under a defective slate and break it free from its fixing nails. It is made of thin steel and is about 750mm in length with a handle at one end and a curved hook at the other which has two cutting blades facing the handle. Keep the curved hook well lubricated with thin oil during use so that it can easily slide under the slates.

Pick hammer: This is a slater’s tool with double-sided heads mounted at one end of a central wooden shank—a pointed head for making holes in the slates, and a hammer head with a roughened striking surface for driving in special roofing nails.

The pick hammer is cheap to buy and can easily be hired but holes can also be made in slates using a hole punch or a galvanized nail. Drilling holes is not advisable as this tends to cause the slate to shatter. Slate cutter: Slates used to be cut to size either with a slater’s axe or by using the blade of a trowel to score the slate until it broke in two. Both of these methods are difficult to perfect without a great deal of practice and waste, and a far more exact cut can be obtained by using a slate cutter. This is similar in appearance to a metal worker’s shears except that the upper blade closes between two jaws mounted on the lower part of the cutter.

Trowel: Certain slates, particularly those around the ridge, hips and eaves need to be bedded into place with mortar. An old trowel, about 150mm long is useful for this purpose. Fixing strips: When replacing individual slates it is not possible to use the batten which held up the old slate to fix the new one in place since this is hidden by the row of slates above. Instead, the new slate needs to be held in place by a thin metal strip nailed into the next batten down and then wrapped around the bottom of the slate.

A number of materials are available to fix new slates but the best is stout copper wire and copper or galvanized nails. This is preferable to sheet lead or galvanized zinc which tend to break after a short period of time allowing the slate to slip. Copper wire also has the advantage that it cannot be seen from the ground and can more easily be fixed into small gaps on battens already occupied by other slates.

Fixing nails: Where a large area of damaged slates is to be replaced it will be possible to fix all but the upper row of slates with nails. Galvanized or sheradized nails should be used for this purpose since other types of fixings corrode—allowing the slates to become easily dislodged.

Safety on the roof

When working on the roof it is essential to use the correct equipment in order to work in safety and comfort. Ladders must be erected carefully and they should be secured firmly at the top and bottom. If a large number of slates need to be replaced it may be necessary to build a tower scaffold to allow easier and safer access to the roof.

Replacing defective slates

When individual slates are damaged the slate should first be separated from its fixing nails using the ripper and then a new slate inserted to be held in place by thick copper wire.

To remove a defective slate slide the end of the ripper under it and hook one of the cutting edges around a fixing nail. By pulling sharply down on the handle it should be possible to pull the nail free of the batten or to split it cleanly in two. Repeat the procedure with the other nail and then remove the slate by balancing it on the blade of the ripper and pulling it gently downwards.

If any nail proves difficult to break loose, locate the hook behind the nail and try to remove it by tapping the shank of the ripper smartly downwards with a club hammer. If the nail remains fixed even after this treatment never resort to excessive force to shift it or the ripper will be damaged. Instead, withdraw the tool, resharpen the cutting edge, and try to work the nail loose from a different angle.

Once the slate has been removed the vertical join between the two slates on the row beneath should be visible as well as the top half of the batten they are nailed to. To hold the new slate in place you have to fasten the copper wire around a nail fixed in the batten just above where the two slates divide.

To do this, cut a piece of wire about 150mm in length, loop one end of it around the shaft of the copper nail, then tighten the loop before driving the nail fully home with the pick hammer. Straighten out the wire so that it hangs downwards ready to receive the new slate.

Balance the replacement slate on the blade of the ripper and with the help of your other hand push it upwards into place until it is correctly aligned with the slates on either side. Using pliers bend the loose end of wire around the bottom edge of the slate and pull it upwards so that the slate is held tightly. Flatten the loose piece of wire against the slate to complete the fixing. Finally, remove the ripper and gently press down on the face of the new slate to bed it firmly into place.

Making holes

When a large number of slates are damaged and need to be replaced, it is possible to nail them into position on the same battens the damaged slates occupied except that the top row must be fixed with copper wire. Holes must be made in all of the new slates which are secured with nails.

When making holes in slate, select a flat surface and lay the slate face downwards. This ensures that a small piece of slate around each hole is broken away from the bottom side forming a dent which accommodates the head of the fixing nail.

Fixing holes can be made in slates either on each edge or at the top. To ensure that the holes are aligned accurately lay one of the old slates over the top of the new one arid use a nail to mark the positions of the holes. The pointed end of a slater’s hammer can then be brought sharply down on to each of the marks to form the holes. Continue in this way, holing all the slates you need to repair a particular area before carrying them on to the roof.

Cutting and shaping

A large number of roofing slates need to be trimmed and shaped to fit before fixing. Mark and cut the slates face downwards; again this leaves a flaked edge on the underside of the cut which helps to waterproof the slate and allow rain to run away once the slate is fixed the right way up.

To mark slates accurately before cutting, use an old slate as a guide, or form a template out of hardboard. Then run the point of a nail along the edge of the template to mark the slate clearly, before trimming it with the cutter. Any rough edges which still remain should be smoothed level by rubbing a piece of carborundum stone along the slate.

Ridge and hip slates

These are especially prone to damage and displacement as they occupy an exposed position on the roof. Although many were originally made from slate, they are now more commonly manufactured from concrete or clay in a range of colours to match existing slates and other roofing materials.

Ridge and hip slates are fixed in place on a bed of mortar and this needs to be loosened before the damaged slate can be removed and a new one put in its place. To do this, hold a bolster parallel to the roof and with the help of a hammer chip away at the mortar around the base of each slate until it comes loose and the slate can be carefully lifted clear. Then use the bolster to clean away loose mortar clinging to the slates around the apex.

To fix new slates, make up a 1:3 mix of mortar and use a trowel to spread it evenly and thickly in place.

While the mortar is still fairly wet lay the new slate in place and gently tap it level with the surrounding slates. Once the mortar has set, the joints between the slates should be pointed with mortar to prevent the intrusion of rainwater into the roofing timbers below.

Canadian roof shingles

Roofs of Canadian houses are usually covered with asphalt shingles. Unlike tiles or slates, these are flexible and so have to be laid on a smooth continuous surface, rather than on spaced battens. This smooth surface, called roof decking, is usually provided by covering the rafters of a house roof with sheets of plywood or particleboard; strips of lumber, laid side by side, may also be used.

The most usual type of asphalt shingle consists of a metre-long strip of asphalt-saturated and coated felt strip, with notches cut into it to form three ‘tabs’, which are coated in mineral granules of different colours. When the roof is covered, only the tabs are exposed, and these give the roof the appearance of separate slates or tiles. Shingles are available in bundles of 21, covering three square metres of roof.

For low-pitched roofs, shingles with a greater amount of overlap are available, and for areas of the country subject to high winds you can get shingles with special interlocking tabs. As with slates, the vertical joints in adjacent rows of shingles are staggered —you do this by using a half-width shingle at the edge of alternate courses. Extra protection against damp penetrating the roof decking is needed at the eaves. This is provided by ‘roll roofing’—an asphalt-impregnated felt material that is available in roll lengths of about 10m, and widths of about 1m. This is rolled out on the roof decking, parallel to the eaves and overlapping the roof edge by about 13mm, and is stuck down to the roof decking all around its edge with plastic roofing cement. This eaves protection should continue back up the roof to at least 300mm inside the inner face of the exterior wall, and preferably 600mm. Depending on the amount by which your roof overhangs the wall, you may need more than one strip. Overlap all joints by 50mm to 100mm, and seal them with roofing cement. Further protection can be provided around eaves and edges by fitting metal drip edging strip to the roof decking before the eaves protection.

Over the rest of the roof, an underlay -ment is sometimes used to provide additional protection.

Valley flashing is often built up from heavy-duty roll roofing. A strip about 500mm wide is first laid in the valley, nailed along its edges at 400mm intervals.. A second strip about 1m wide is then placed on top, mineral-side up. Use roofing cement to seal along the edges of this strip; and nail with just enough nails to hold the flashing in place until the shingles are laid. In some areas, such as Quebec, flashing is of 600mm wide galvanized metal, crimped in the centre. This is completely embedded in roofing cement, and nailed along the edges. Shingles are laid, as with other roofing materials, starting at the eaves. A ‘starter strip’ is first fixed; this could be a row of shingles placed with the tabs facing up the roof, or a strip of heavy-duty roll roofing. The starter strip should overlap the edge of the roof by about 13mm. Nail the first row of shingles directly on top of this starter strip, using 25mm large-headed nails, placed about 13mm above each notch, and about 40mm in from each end. Overlap each shingle in adjacent rows so that none of the material above the notch is showing. All the tabs should be stuck down onto the shingle below, by applying a 25mm dab of roofing cement underneath each tab before nailing in position. Some brands of shingle are self-adhesive; but even with these you should apply extra cement on mansard roofs, in areas with high winds or blowing dust, or when roofing in cool or cold weather. Shingles used on low-pitched roofs should be set in a bed of roofing cement.

Ridges are capped with single shingles, cut out of the strip by extending the notch. Each shingle is wrapped over the ridge, and nailed at one end. The next shingle to be laid then overlaps the previous one to cover and protect the nail-hole.

Repairing shingled roofs

Lifting or curled shingle tabs must be attended to before they allow water or snow to creep up underneath, and penetrate the roof decking. Stick them down with a dab of roof cement under the lifted area. As with all shingle repairs, this is best done in warm weather when the shingles are pliable: if you try to manipulate cold shingles, they may crack.

Badly torn or damaged shingles can also be stuck down again, covering the damage with a liberal application of roofing cement, but it is better if possible to remove and replace them. To remove a damaged shingle, carefully lift the tabs of the shingle above, to expose the damaged shingle’s nail heads. Remove the nails with a chisel, and pull the old shingle out. Slide a new one into place—cutting off the corners may help. Then nail it into place. Finally, stick down the tabs of the shingle above—lifting them is likely to have made them curl.

Shingles do not last for ever, and after some years, the roof will need recovering. If the whole roof is to be re-done, then there is no need to remove the old shingles; the new ones can be nailed straight on top.

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