1. General working practices
Do not walk on the slates during installation. Always work off roof ladders and roof scaffolds. This is to avoid slipping slates, i.e. slates that break or fall off after installation.
Chalk your slate courses. Install every course along a permanent chalk line on the surface of the roof. Do not chalk on the slates themselves. The lines mark the top edges of the slates. Measure where to put your line from the bottom of the roof to ensure accuracy and consistency. Don’t lay the slates without chalk lines as you can easily lose your headlap and ruin the roof.
2. Starter course
The starter course is one of the most important courses on a slate roof and you must install it correctly. First, nail a wooden or copper cant or shim under the bottom edge of the starter course. This will tilt the starter course so that it is at the same angle as all other slates. Then install the starter course with the slate face down and the back up. The starter course is the only course that is installed this way. All other slates are installed with the face up, exposing the bevelled edges.
Install the starter course and the first course of field slates at the same time. Do not run the starter course ahead of the first course unless your starter slates are the same width as the field slates and only have nail holes on the top edge. This way you will have correct side laps and ensured that no nail holes in the starter slates in the joint between the first course slates can create a leak.
3. Side laps
Make sure you create the correct side slaps, i.e. the lateral overlaps between the side butts of the starter slates and the first course of slates. The side laps should be a minimum of 8 cm.
4. Head lap
Every slate must overlap the two courses below and you have to ensure that you install the correct head lap. Head lap keeps the roof watertight. If you get it wrong your roof will leak. The head lap for 8:12 and steeper sloped roofs is 8 cm. If your roof has a low slope (4:12 or 6:12) the head lap needs to be 10cm.
Slate underlay should always be installed under the slates and never on top.
Slate roofs do not need an ice and water membrane. If you’re concerned about ice-damming, double the decking felt along the eaves. You can spread roof cement between the layers and increase the head lap along the eaves to 13cm.
7. Slate nailing
You have to get your nailing technique right or you will cause damage to the slates. Nails should only be driven to a depth where the nail head lies within the counter-sunk nail hole crater. Doing it correctly will ensure that the nail heads will not rub excessively against the overlying slates and wear a hole in them.
Do not over-nail or under-nail the slates. Over-nailing means the nail is driven too far and breaks through the slate, leaving it hanging on one nail or sliding out altogether. When the nail is sticking out of the nail hole this is known as under-nailing.
You have to install flashings with the correct overlap or they will leak. Uphill flashing should always lap on top of downhill flashing. Corner flashings, e.g. chimneys, must either be correctly folded or soldered to prevent leakage.
Folded flashings can be used on positive slopes whereas soldered flashings are required when the slope is so low that the direction of the water flow cannot be controlled by gravity alone. Soldered flashings must be installed with cleats or expansion joints to prevent strain on the solder joints. Never solder with an open flame.
9. Hips and ridges
Make sure the field slates meet tight at the roof hip and form a straight, weather tight, close-mitred junction or install ridge tiles laid to a true line with the edges and joints solidly bedded in mortar. Alternatively, you can use metal roll on the hips and ridges.
Installing a slate roof maybe slightly more difficult than installing other types of roofs. By following the advice above you greatly increase the chances of successfully installing a slate roof on your home.