A guide to fixing and working with fibre cement slates

Guest blog from Ged Ferris, Marketing Manager at Cembrit

Cembrit fixes it

Fibre cement slates are a popular choice amongst roofing contractors, builders, architects and property developers as they are a great material to handle and install, as well as providing an attractive finish to any building.

The advances in the production of fibre cement slate now mean that a wider range of projects can benefit from a slate roof.

Fibre cement slates are lightweight and pre-holed and come in uniform size, thickness, and shape, eliminating the time needed to sort on site. They are easily cut with hand tools, so do not require dust suppression equipment, therefore resulting in the slates being cut on the scaffolding or roof itself.

Fibre cement slates lend themselves to more complicated roof structures, where a lot of detailed cutting is required. For example, hips, valleys and abutments for dormers, chimneys and rooflights.

Installation

The installation of fibre cement slates should begin with the underlay being fully supported behind the fascia to prevent sagging between the rafter feet. This is generally achieved with a tilting fillet.

The underlay should finish by hanging into the gutter, so that any moisture on the underlay will drain into the gutter. Eaves should not be sprocketed, as this will affect fitting of the disc rivet at the tail of the eaves course.

Three courses of slates are required at eaves. The first under eaves course is cut and drilled so that it can be head nailed to the first batten. This acts as a base to support the tail rivet for the first full slate course.

1. The first under eaves course is cut to the length of the batten gauge. Eave overhangs should be 50-55mm for 100mm gutters or to the centre line of the gutter, if larger diameter gutters are used.

   Locate the centre point of the eaves and centre the first under eaves slate here. The first full slate will be laid over the top of this under eaves slate. This will mean that the slates on both verges will be cut to the same width. Work towards both verges with remaining under eaves slates.

   The second under eaves course is cut from the same slate as the first under eaves course and its length will be the batten gauge plus the slate headlap. This second under eaves slate provides the double lap for the next but one full course of slates.

   It is installed to cover half the width of the first under eaves slate, allowing the shank of the tail rivet (which rests on the first under eaves slate) for the first full course of slates to pass between adjacent second under eaves slates. The second under eaves slate also oversails the fascia 50-55mm.

2. The first full course of slates is nailed to the second batten (this batten also supports the head of the second under eaves slate) and is arranged so that the tail rivet passing between the two second under eaves slates protrudes through the hole in the tail of the full slate.

   The first full course of slates also oversails the gutter 50-55mm. The tails of all three courses of slate align and overhang the gutter.

3. In order to provide the correct bond the verge slate on alternate courses should be a slate and a half width cut from a double slate. These verge slates require pre drilled holes for three nail and two rivet fixings in addition to an extra hole to allow the tail rivet for the course above to pass through the slate and a half.

   The next single verge slate will also require an additional hole for the tail rivet of the subsequent slate and a half. Tail rivets should always pass between two adjacent slates and through a hole in the tail of the slate they are holding. The protruding shank of the rivet is then bent down the slope.

Ventilation

The most common form of roof construction in the British Isles is commonly referred to as a cold roof. Here, insulation is horizontal over the top of ceiling beams. Eaves to eaves ventilation is required and is generally sufficient (except on long rafter lengths such as commercial buildings) without the need for intermittent or ridge ventilation.

Eaves to eaves ventilation can be provided either by soffit or over fascia ventilation. It works on external pressure differentials on different sides of the roof moving warm air out of the cold space.

Whilst designing the eaves detail, it is important to consider roof ventilation. All cold airspaces/voids need to be ventilated as it is in these voids where the danger of condensation is highest.

Condensation is caused when moisture bearing warm air from the occupied space within the building rises to come in contact with cold surfaces, which will generally be the underside of roof coverings or underlays, on the cold side of insulation.

This is a particular concern with tight fitting fibre cement slates, which do not allow significant leakage of warm moist air into the atmosphere.

A spacer is used in the eaves so that the insulation can be installed over and beyond the wall plate without blocking the air path from eaves to ridge.

Where there is any doubt ridge ventilation should be incorporated in the roof design. Continuous ridge ventilation will be the most effective solution.

Advice and guidance

Cembrit has developed a fibre cement slate guide to provide specifiers, craftsmen and trades people with the relevant information for the successful design and installation of pitched roofs with ever popular fibre cement slates.

The book demonstrates how Cembrits knowledge of all forms of double lap slating, combined with their knowledge of fibre cement technology, offers users peace of mind not available from other suppliers.

The guide highlights the wide range of slates and accessories the company offers.

The 80-page document, entitled A Guide to Double Lap Slating with Fibre Cement Slates provides information on colour and format availability, as well as a full explanation on design considerations, technical specification and installation.