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Roof Underlays

Is it better to use an air permeable membrane or vapour permeable and additional ventilation?

The use of these 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 might be argued that air permeable membranes are more effective in allowing the dissipation of moisture, but their use needs to be considered in the tile fixing specification as there will be increased wind loading on the tiles.

If there is any doubt about the permeability of the ceiling construction or air openness of the outer roof covering, then ventilation openings should be provided.

How long can I leave your membrane range open to the elements and UV prior to tiling and why?

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.

I’m roofing in a sheltered urban location. Do I need to do anything with the underlay laps?

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.

What are the benefits of a taped membrane as opposed to a restraining batten?

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 necessary and the wind load and zone performance is often improved. Separate lap tap tapes can be more difficult and time consuming to use. They have to be applied carefully to the underlay that must be clean, dry and free from all dust and debris. For this reason, underlays with an integral lap tape are recommended for speed, ease of use and reliability.

Which works better an air permeable membrane without ventilation or an air tight membrane with ventilation?

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 for the application. This might change with the publication of a revised BS 5250 later this year.

Is the NHBC guideline for ventilation in chapter 7.2 adequate with only high-level ventilation with a breather membrane?

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.

Do I need extra ventilation if I use a breathable underlay?

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.

If the house is registered under NHBC and high-level ventilation is required, can this be achieved by using your CON6+ ridge and hip system?

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

What is breathable roofing underlay, and can I use it?

Breathable roofing underlay, 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 on the strength and weight of the underlay, there may be limitations on where it can be used in different wind zones. The product data and packaging should identify these limitations.

Why should I use a breather membrane on a boarded roof?

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 as cost effective as non-permeable or High Resistance (HR) types.

In sheet forms of sarking, the vapour permeability is not considered as the roof cannot ‘breathe’ through the boards. In planked applications, providing that there are sufficient and regular gaps between the planks, then the permeability of the underlay is still useful. Leaving these gaps is generally not recommended where slates or tiles are being nailed directly to the sarking boards because of missing the board or splitting the board edge with the nail.

I’m using a vapour permeable underlay but using mechanical fixings with the ridge and hip tiles in conjunction with mortar because it is a conservation area. Do I need any other ventilation products?

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 to a 3mm continuous opening equivalent for a well-sealed or ‘continuous’ ceiling. This is not work usually carried out by the roofing contractor.

Has the underlay got to be out of the gutter?

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.


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