The craft of slate roofing

Slate is the natural choice for a pitched roof. However, whilst it is important to have a good stone, it is equally important that the slate be installed correctly. Slate roofing is a craft, and as such is to be learned truly only over a period of years.

Only a craftsman, practiced and proven in this type of work should be used. Although the present day practices of slate roofing are in many ways similar to the old, new materials, different cost values and a quicker rate of working have brought many changes.

The following are some of the specifications that must be carefully followed to ensure the slates will be able to perform to their full potential.

Generally we do not recommend a slate roof to be laid below 18½° (4:12) pitch. As it is such a good looking roof, why not let people see it; keep the pitch up.

The roof pitch also influences the size of the slate selected for the job. The flatter the roof the broader the slate that should be used. This is because there is more capillary action, causing a larger ‘angle of creep’, with flatter roofs.

The most common size natural slate used is the 500 x 250mm (20″ x 10″) ‘COUNTESS’ slate. We do import other sizes as required, as sometimes roofs may need these sizes for detail work.

As can be expected with a craft that is centuries old, picturesque names for the various sizes have been adopted.

This is the most important consideration with any form of roofing. As with interlocking tiles, slates must NEVER be fixed on a roof up to 35 degrees with less than 75mm headlap.  For roofs over 60 degrees, which are classified as a wall, the headlap is 50mm.

Any slate roof, or proprietary fixing system for slate, with a headlap less than described above, should be immediately rejected. For more exposed positions or with flatter pitches this headlap may have to be increased.

Now this is how to work out the gauge of a slate: Slate length less the headlap divided by 2 e.g. (500 – 75) divide by 2 = 212mm gauge.

Therefore a Countess Slate is laid at: 212mm gauge.

Always use a softwood batten due to slate nailing considerations. For the following rafter centres use the sizes: 600mm – 42x35mm, and 1200mm – 70x35mm.

A nail with a clout like head must be used, in either galv or copper. They should be 20mm longer than two thicknesses of slate. Copper clout nails have a smooth shank, and I have seen them work their way out of the batten over a period of time, so ensure they have a ring-barbed shank.

Nail holes in the slates must always be punched, not drilled, as the punching from the back gives a countersink on the face of the slate for the nail head. Nail holes to be at 25mm in from the edge of the slate.

It is important to ensure that the nail is made of a material that will last the same time as the slate. The slates will start slipping out before their life is up. This condition is called ‘Nail Sickness’.

As an alternative, a stainless steel hook system may be used. This will save approximately 30% off the fixing costs. This system has been used for decades in Europe with great success. Some installers say that hooks are better as the slate is not weakened in the holing process of the ‘2 nail’ method.

Using stainless steel hooks also gets around the problem of slates that have not been nailed tight enough (so the slate above ends up with a hole punched through its face), or with the over nailing of the slates.

There are five tools that are specially needed for the slater. Obviously, in centuries past they were made by the local blacksmith. Their shape and style is still the same as back then.

Before any slates can be laid they must be ‘holed and graded’. The slates are punched, not drilled, at a distance equal to the holing gauge up from the tail, and between 20-25mm in from the edge. They are punched from the back or ‘bed’ of the slate, which produces a natural countersink for the nail head.

The tail is the thick end of the slate. Yes, there is a thick and a thin end, and an experienced slater can immediately pick the difference.

As the slater puts down the slate after holing, he puts them down in one of 3 or 4 piles according to their overall thickness.

When a roof is slated, the thick slates are fixed at the eaves, where most of the water is, grading up to thin slates at the ridge line.

Laying a thick slate next to a thin slate will cause a gap under the slate in the next course above. This slate can easily be cracked when walking over the roof. Gaps like these do not improve the appearance of the roof!

To assist in laying slates neatly, a series of chalk lines should be struck. These lines run up the roof(parallel to the rafters.

They can be struck every 1½ slates wide apart, or as some do, a pair of lines, ½ a slate apart, every 2 or 3 slates.

A slate roof not struck out is easily detected – look up the roof and see if all the perpendicular joints line up.

In the laying of any roofing material, workmanship is as essential as the proper selection of the material. The more enduring the material, the more important this factor becomes. Slate, the most lasting material known, should be laid by roofers of experience and training – craftsmen.

It is a mistake to assume that those without such experience are qualified to properly lay slate.

For example one of the most critical areas is nailing. The following diagrams will show why.

The first diagram also shows why slates should never be drilled – it is not possible to achieve a flat bed for the next slate as there is not a countersink.

Slates should be laid with a broken bond an secured with nails or hooks to softwood battens. Before fixing, perpendicular lines should be marked giving the correct bond and spacings.

Approximately 4-5mm should be left between the vertical joints of the slate, so that there is a free thoroughfare for water running down the roof.

Slating should be started at the eaves and fixed diagonally across and up the roof slope, thus ensuring each slate is fixed with 2 nails or a hook.

There must be a double course of slate at the eaves, formed by laying a course of short undereaves slates, over which the first course of full slates are laid. The overhang into the gutter or over the fascia should be 50mm.

The overhang of slate at the verge should be 40-50mm and either half slates or wide slates be used in alternative courses to maintain the bond.

Top course and under top course slates must be of such a length as to ensure that a correct lap is maintained.

The edges of the slates cut on the rake must overhang the squash fold leaving a minimum 100mm clear width of the metal valley showing in the centre. Secret valleys are not recommended because of the difficulty in keeping the drainage channel clear.

There are 4 principle methods used for hip and ridge cappings as follows:

• Slate with metal under-flashing
• Concrete or terracotta or a ‘purpose made’ slate capping.
• Metal roll top in copper, lead, colorbond or zincalume.
• Mitred hip.

In exposed conditions and for roof pitches below 30° mitred hips are not recommended. Extra wide slates are required for cutting so that sufficient width is provided at the head of the slates. Only a skilled craftsman should attempt a mitred hip.

Slates and Shingles Roofing Service is committed to providing a service whereby our clients are provided with a finished product that will perform to time proven expectations.

This is particularly important, as some of our products have an expected life expectancy of well in excess of 100 years, so therefore, to realise this, Slates and Shingles work to an ITP Checklist (Inspection Test Plan).

These Checklists are based on centuries old methods of installation, which have withstood the test of time, and with modern equipment and facilities, will provide our clients with the best possible value for their purchase.

In conjunction with our Quality Control system, we have a Safety Policy whereby all our employees work to Approved OH & S Standards.

This includes the provision by Slates and Shingles of all protective/safety clothing and equipment as required. This is important to ensure that our employees are easily able to complete works to our required QUALITY STANDARDS.

To the aim of maintaining both our QUALITY POLICY and our SAFETY POLICY, Slates and Shingles have an on-going commitment to identify any risk to our employees and the general public to establish safe work practices for the performance of work on site.