Roof Underlay

Yes. The underlays are not designed to resist UV or water saturation over long periods as damage can occur.  It is advisable to use an underlay support tray or a strip of BS 747 type 5U bituminous underlay at the eaves.

The use of this type underlay allows the level of ventilation provided to be reduced, not omitted. The continuous 10mm equivalent opening at the eaves in conventional cold roof construction can be reduced to a continuous 7mm equivalent opening for a normal ceiling, or down

An underlay is required under slates and tiles on boarded roofs, whether the sarking boards are in sheet or plank form to carry any moisture from condensation or wind driven rain down to the eaves to drain safely away. Vapour permeable underlays are now just

Breathable roofing underlays, more correctly referred to as a vapour permeable or Low Resistance (LR) underlay, is designed for use under slate and tile roofs and can reduce the requirement for roof ventilation in order to prevent interstitial condensation and consequential damage and decay. Depending

Yes, the Danelaw CON6+ and CLAY6+ ventilated dry fix ridge and hip kits provide both the ventilation at the ridge and mechanical fixing of the ridge and hip tiles required under NHBC Technical Standards and British Standards requirements

No, a breathable or more correctly a vapour permeable or Low Resistance underlay generally allows the amount of ventilation to be reduced, but this does depend on other factors, particularly the air-openness of the outer roof covering.

Yes, clause 7.2.15 allows the use of high level or ridge ventilation only where a vapour permeable or Low Resistance underlay is used in conjunction with an outer roof covering that is classified as ‘air open’. This is usually limited to single lap interlocking tiles.

An airtight, moisture vapour permeable membranes in conjunction with ventilation openings is a method recognised in British Standards and one of proven reliability. Unventilated roofs, including air permeable membrane only use are outside the scope of the Standards and each case needs to be designed

The use of a retraining batten on the underlay laps means the use of an extra batten, or the lap length increasing to coincide with a planned batten position reducing the coverage rate of the underlay. Using a taped lap system, none of this is

Yes, in all situations the horizontal laps should either be restrained by a batten located on the lap, or the two layers should be effectively taped together.

The membranes are not designed for long term UV stability as this is not required in service. The Danelaw range have a generous three-month exposure limit.

The use of these roofing underlays/membranes and requirements for ventilation depend on other factors, often outside the control of the roofer, so these must be considered. If the roof covering is not sufficiently ‘air open’, then ventilation openings will need to be provided anyway. It