Sitework
Underslating membranes must be installed in accordance with relevant sections of BS 5534: 2003, BS 8000: Part 6: 1990 and Ruberoid fixing instructions. Underslating materials are designed as a secondary barrier to wind driven rain and snow; it should not be considered a primary waterproofing layer. Good roofing practice dictates that the primary waterproofing finish (e.g. tiles, slates etc.) be applied as soon as practically possible. Whilst providing a degree of protection, underslating membranes should not be considered as a totally weatherproof protection for occupied buildings or where internal fitting out is taking place. With new buildings or buildings that have undergone extensive renovation that have involved wet trades, the additional water within the building during this first heating season, may cause condensation to form in the roof space, which cannot be fully dispersed by the breather membrane. However, when the internal conditions stabilise, this temporary occurrence of condensation will not recur in a properly designed and constructed system. (Additional information available in BS5250. Section 14.1  Owners Manual and NHBC guidance notes).

Supporting structure
An underslating membrane when installed as a fully supported system is laid over the support and secured with counter-battens. Alternatively, where practical, the membrane can be installed over counterbattens and fixed at 200mm centres using corrosion-resistant straps or galvanised clout nails. Tiling battens are fixed to the counterbattens leaving an airspace between the roof sheet underlay and the tiles for drainage and ventilation.

Abutments
Abutment flashings should be wedged into a mortar joint 25mm deep and at least 150mm above the level of the slates or tiles. Underslating membranes should be turned up behind the flashing at least 100mm, and sealed where appropriate with Rubershield Jointing Tape to prevent rain and snow being blown into the roof-space, and to restrict unwanted air movement.

Lap sizes table - horizontal and vertical laps

Notes to Lap Sizes Table

• Reinforcing strips of 600mm minimum should be fixed at hips, ridges and valleys.
• It is recommended that where underslating is to be dressed into a half round external gutter, Ruberoid Eaves Protection Strip should be utilised in this area.
• Note - as with all breather membranes of this type, contact with solvents or wet timber preservatives can cause localised water penetration to occur prior to the main weatherproofing being installed.