Fixing the Undertile Membrane

The undertile membrane in all cases should be fixed between rafters and battens (except at the lower edge of a bottom course of tiles where it overlaps the tilting batten and/or fascia board into the gutter), and must overlap horizontally and vertically by at least 150mm at all joints.

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Eaves overhang

Determine the specified eaves overhang and cut the rafters/trusses accordingly.

Tilting batten

A tilting batten (or fascia board) must be used at the bottom end of the rafters, rising above the batten line to ensure that the first course of tiles will be on the same plane as the following courses. The average tilting dimension is plus-minus 14mm higher than the battening being used.

Valley undertile membrane

If the roof has valleys, start by fixing a strip of undertile membrane at least 600mm wide, centred on the valley's full length, overlapping the ridge on the top and carrying it well into the gutter at the bottom. Secure the undertile membrane on the edges with clout nails.

Eaves undertile membrane

Lay the first horizontal strip over the rafters starting from the eaves, ensuring that it will carry over the fascia board. Secure this first strip to the rafters with clout nails in the upper half only, leaving the lower half free for draping over the tilting batten and well into the gutter. Care should be taken to ensure that the undertile membrane does not form any troughs where water may be trapped. To achieve this the undertile membrane must be taut or supported, if necessary, behind
the fascia board/tilting batten.

If the roof is to have open soffits, it is good practice to install a thin covering (fibre-reinforced cement or other weather-resistant sheet) on top of the rafters for the extent of the eaves or verges overhang
before proceeding.

Positioning the bottom and top battens

Fix the batten, which is to carry the first course of tiles on top of the undertile membrane.

The distance of this batten from the fascia board should allow sufficient overhang of the tiles over the fascia board/tilting batten, enabling rainwater to discharge efficiently into the gutter (normally 350mm from the outside of the fascia board to the top of the first batten.) Fix the apex batten temporarily, but accurately, at a distance of 25mm from the apex of the rafters, which is adequate for most pitches.

Calculating the batten gauge

Determine the pitch of the roof and the appropriate batten gauge. Measure the full rafter length and read off the spacings on the table in Inspecting the Roof Structure. Set the tilting batten and first batten, then proceed to batten at the centres shown in the table. Make sure that the battens run parallel to each other at all times.

Split apex

A split apex is a design feature. When calculating the batten gauge, the higher apex should always be used. A short course can occur at the lower level apex.

Roof undertile membrane and battening

Proceed with fixing undertile membrane horizontally with clout nails, observing the recommended overlaps. Batten up simultaneously to the apex ensuring that the batten joints are always located on a rafter. It is bad practice to join all the battens on the same rafter.

Valleys

At valleys, the horizontal strips of underlay shall overlap undertile membrane previously fitted. Determine the width of the valley flashing to be used. Secure the valley counter battens along both sides of the valley to the rafters securing the undertile membrane. Horizontal battens are now mitred and secured to the valley counter battens.

Hips

At hips, the undertile membrane may be cut close to the hip rafter, or may overlap one another on both sides of the hip rafter. A strip of 600mm wide undertile membrane is then placed over the full hip length, overlapping the ridge at the apex and carrying down into the gutter at the bottom. The hip counter battens are secured to the rafters as close to one another as possible along the length of the hip, securing the undertile membrane. Horizontal battens are now mitred and secured to the hip battens.