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1 Slate must be laid on a firm base that will support it. Timber floors present a problem in that timber and particle (chip) board expand during humid periods, and shrink during drier times. Timber floors can be prepared in several ways. Care should be taken with all the procedures however, bearing in mind that timber or particle (chip) board is not the best base material for slate.

2 A number of slate suppliers have specially developed membrane systems available for the installation of slate on a timber floor. The one used here is the hessian and bitumen method and will provide the necessary `slip’ joint between the rigid tiles and the timber.

3 Clean the floor thoroughly, and punch any protruding nails below the surface.

4 Tack the hessian to the floor and apply the bitumen material to the rate specified by the manufacturer. Ensure t is well worked through the hessian, and achieves good contact with the floor. The hessian acts as reinforcement in the bitumen.

5 Allow to dry overnight.

6 Now the tiling can be laid out. As slates are more difficult to trim than normal tiles, we have laid them against two walls that are at right angles (checked using the 3:4:5 rule). Dry-lay a row of tiles along the two walls. Ensure that the cuts required at the ends of the row are greater than 50 mm, as smaller cuts result in very brittle pieces of slate that are difficult to bed. The rest of the room can then be squared from these rows. Also remember that slates should not be butted up hard against each other, but should have a 6mm grouting gap to allow for any thermal movement in either the slate or the surface on which it is laid.

7 Slates are not uniform in thickness. It makes the job much easier if they are sorted into similar relative thicknesses before even mixing the mortar. If, for instance, four separate piles were made, each with tiles of a similar thickness, they can be used in rank of thickness, on mortar beds of constant thickness. If you use the thickest tiles first, the mortar bed can gradually be increased for the thinner tiles as the job progresses.

8 It may be that some of the slates are slightly bowed. If so, they should be laid with the bow up. If they are laid the other way up it is difficult to provide a strong enough edge-bearing for the tile, and with time it will tend to loosen.

9 If you are a first-time slate layer it is probably easier, though more expensive, to use a proprietary adhesive and to follow the instructions closely. Most adhesives are cement based with a fine aggregate, together with workability additives and bonding agents already included. These provide a reasonable working time, so that you have more time to carry out adjustments and take levels. All are water mixable and washable while wet.

10 Once you are happy with the design and layout, the adhesive can be mixed. Bear in mind that many of the proprietary brand mixtures require you to leave the mix standing for up to 15 minutes before use. Most mixes remain workable for around 2 hours in the summer and 3 hours in the winter. After this period of time the adhesive should be discarded.

11 Mix enough adhesive for about 1-2 sq.m at a time and finish this amount of tiling before mixing more. This will easily be worked in the time spans available. Spread the adhesive over the area where you intend to start the tiling, and notch it using a 12 mm notched trowel.

12 Place the tile in the right position and gently rock it into position. The slate should be manoeuvred from the centre, and the gentle back-and-forward rocking should force most of the air out from underneath the tile. The final positioning can be done using the wooden handle of a hammer, tapping the tile only near the centre. It helps if the other hand is placed on the tile, as this provides shock absorption so that the tile doesn’t just rock around its spot and actually loosen the bond.

13 Every now and again remove a tile that has been properly laid, just to monitor that the bedding is covering the whole of the back of the tile, and that the consistency of the mortar is correct, with minimal air holes.

14 When you are happy with one tile, proceed to the next.

15 Once about a metre square of tiles has been laid, it is time to clean off any excess mortar or adhesive, before it sets and stains the tile. This is best done with a damp sponge and frequent changes of water. The sponge should only be damp and must be used gently, otherwise the mortar may be pulled out of the joints, which will stain the slate further. Remove as much as possible with the sponge; the rest can be removed by a scouring pad at the end of a day’s work. The aim is not to have to take any remedial action such as acid washing at a later date, as acids can affect the colour of the slate.

16 The tiles should be left for at least 24 hours before they are grouted.

17 The grout is usually the same mix as was used for laying. Once again, make sure you clean the tiles thoroughly to prevent stubborn stains.

18 Once you have grouted the tiles, allow a further 24 hours to pass before walking on the tiled area.

19 Slates are normally sealed on interior surfaces. Premature sealing, or sealing while there is any moisture in the slate or in the mortar, may lead to a milky stain forming. This is a white soluble salt that can badly stain the tiles as it forms when water evaporates from the surface. All evaporation must have stopped before you begin sealing or the moisture and any salt will be trapped. At worst, the salt can effect a slow break-down in the entire slate finish.

20 Sealing is done with either an acrylic or polyurethane coating system. Acrylic is easier as it is water based. It is not advisable to use any products other than those specifically designed for use on slates. Our coloured slates rely in part on the colour of the weathering bands, and should be thoroughly brushed to remove any excess powder, which could cause the sealer to fail. Application can be carried out by sponge. Refer to the directions on the manufacturer’s container when recoating or for any other special requirements.

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