Roofing Repairs: Slates

The weather, and water in particular, has a habit of getting in where it is not wanted, more often than not through leaks in roof surfaces. Regular inspection of the roof is essential if you are to keep out the elements as an unwelcome visitor. Loose slates and tiles are not difficult to replace and flaws should be rectified quickly before damage has a chance to strike and spread.


Roof slates are laid, working upwards from the eaves. Each row, or course, overlaps the one below it, and as the vertical joints between slates are staggered in adjacent rows, any slate partly covers the two below it.

The design and slope of the roof and position of battens, to which slates are nailed, determines the size of the main slates. However, there are several sizes of slate used in slate roofing. Those on eaves are the same width as the main roof slate but are shorter. Another slate is half as wide again as the main slates; this is used at the end of alternate rows.

A narrow slate, or creasing or verge slate, is often used at the end of each course on a gable end. This type of slate is laid underneath main slates and tilts the roof edge upwards, to prevent rain from running off the roof edge and down the wall.

Since slates vary in size, shape and thickness, establish that replacements correspond with existing ones, otherwise the roof may let in water. If you cannot match a slate exactly in size, obtain the next largest size of the same thickness. The slates can then be cut to fit.

Slates can be cut to any size or shape. The tile is marked with a nail or a trowel and then laid over a board and chopped with that part of the trowel nearest the handle. Chop half way and then turn the slate round and chop from the other end.

To make holes in new slates, establish the method of fixing and position each slate, bevelled edge downwards, on a flat board and drill the holes with a brace and bit or use a hammer and nail. A good way of making sure that holes are correctly positioned is to lay an old slate over a new one and make the new holes through the holes in the old slate.

Slates are made in different colours so that you can match the roof. New slates can be bought singly or in bulk. Demolition sites are another source of supply, but check carefully that slates are in good condition. These cost only about two-thirds of the price of new slates.

One firm makes concrete tiles which resemble slates. Since these are a good deal heavier than slates you should check that the roof battens are equal to the load.

As slates age, they may flake and powder, particularly near nail holes. Hairline cracks, often difficult to see, develop along the grain. If slates have deteriorated at the edges, provided these are oversized, they can be cut down.

Slates are often referred to as bests, seconds and thirds. This has nothing to do with quality but refers to thickness and texture.

Take care when handling slates, since these are very brittle and break easily if roughly handled. Hold a slate along the longer side and tip it when lifting. Never try to lift slates from a pile, and if you are carrying several, wear canvas gloves and hold the slates on edge under an arm. Carried flat in a pile, they are likely to break or crack. Slates taken up to a roof should be stacked on edge, in a canvas or a plastic bag.

Before starting work, check the condition of the battens. If these are damaged or have deteriorated, they should be replaced. New battens should be nailed against the edges of the old ones with 50mm copper or zinc nails. Damaged timber should be cut out.

Main slates may be fixed by two edge nails or at the centre. The top and eaves courses of slates are always held along the top edge by two nails. In repairing a slate roof, always fix the new slates in the same way as the original ones.

For the first course above the eaves, short slates are used; these are covered completely by the second row. To stagger joins, nail slates either along their top edge or at centres. These are staggered in adjacent rows to make the roof watertight. For the courses at the ridge, short slates are used. These partly cover the course below, providing double thickness for weatherproofing.

Roof slates may crack or break or nails securing them may corrode. This can be caused by ageing of a house, causing movement in the roof structure, or through high winds, making the slates shear.

A tool for removing slates is called a ripper. These are not expensive to buy but can be hired. The ripper is slid under the slate directly above the one which has to be replaced. The head of the ripper is designed to hook the curved edges around the nails holding the broken slate. This ‘rips’ them away. It may take a little effort to remove the nails, but this must be done before the slate can be replaced.

Once the slate has been worked loose, a batten or slat will be visible and the new slate attaches to this. Slates are secured by 25mm galvanised nails. Before the nail is driven home, a length of lead 25mm x 230mm or a piece of thin copper wire, 230mm long, should be fixed to the nail. Drive the nail through the lead; wind copper wire round it before the nail is finally driven home. Line up the bottom edge of the slate with adjoining slates and bend up the end of the lead clip to secure it firmly in place.

To refit a section of slates, start work at the eaves by sliding a short slate under an adjacent, full-length slate, up to the batten centre line. As the first slate is partly covered, fix it with one nail. The others along the row are fixed with two nails. Carefully check that each slate is on the centre line, in order to allow the second row to be fixed to the same batten. Next, lift the slate in the row above and slide the first new full-length slate beneath it, making sure that the new slate aligns with the centre of the batten. Fix this with a single nail.

Lay remaining slates in the same row, so that these cover the vertical joins in the rows beneath. Carry on working up the roof and cover previous joins and securing nails with each fresh slate.

Since the nail holes are covered by the rows above, it may not be possible to nail the last few slates. Cut 25mm x 230mm lead clips to hold these slates. Fix the clips between nails securing adjacent slates. Again, fit these slates so that the bottom edges are flush with the remaining slates in the row and bend up the clips. Alternatively, you can make clips from thin copper wire.

Mortar between the ridge slates on top of the roof may fall out with age. Loose ridge slates should be lifted free and taken to the ground. Chip away old mortar from inside the slate with a trowel edge. Use care since you may crack the slate. With a club hammer and bolster, or cold chisel, remove old mortar from the roof. Apply a 1:3 cement and builder’s sand mix along the ridge, roughening the surface with a trowel.

Locate the cleaned ridge slate on the mortar and tap it gently until it lines up with the height of the adjacent slates. Trowel mortar into the joints on each side and along the bottom, then smooth. Take care not to dislodge the slate before it sets.

10. November 2011 by admin
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