Underlays

Overview

The new Code of Practice incorporates a new standardised test to access the wind uplift performance of underlays. This will mean that some lightweight underlay may not be suitable for use.

Contractors should use a roofing underlay that has been tested to the new standard and clearly identified as being suitable for the site location and batten gauge in question.

The use of underlays that are clearly labelled can easily be checked by NHBC Inspectors or Clerks of Works on site. Alternatively, the manufacturer should be contacted and evidence provided.

For more details see the sections below.

Why the change?

The primary purpose of a roofing underlay is to reduce the wind load acting on the roof tiles by taking a significant proportion of the wind load itself. To do this successfully, it must not stretch to the point where it can touch the underside of the roof tiles when subjected to wind pressure.

If a roofing underlay does stretch, or "balloon", so that it touches the back of the roof tiles, then it ceases to perform this primary function with the full wind load transferred onto the roof tiles.

The tile manufacturers' fixing specifications do not allow for this additional load and roof tiles can be blown off the roof even if they are nailed and clipped.

A related problem concerns the bursting open of unsealed underlay laps when subjected to wind pressure which can also cause the removal of roof tiles. Both the old and new Code of Practice recommend an additional timber batten be installed over the horizontal lap between courses of underlay to prevent the risk of this happening. While this is recommended practice many roofing contractors do not like installing these additional battens and they can cause a trip hazard.

The European product standard for underlays does not address this issue. So even if an underlay is CE marked it does not mean it necessarily complies with the new Code of Practice.

Previously, third party certification bodies, such as the BBA, used different tests to assess the wind uplift resistance of roofing underlays. The differing test results caused confusion about which underlays performed better than others.

The new Code of Practice introduces a single test for the measurement of the wind uplift resistance of underlays, which all testing bodies will follow to allow for greater transparency.

A new labelling system has also been introduced indicating where an underlay can be used for different batten gauges.

Be Warned:

Under these new standards many current popular brands of underlay will no longer be fit for installation on UK roofs. Before you buy, ensure that your preferred manufacturer has the necessary checks and testing in place.

What to do

Spirtech 400 2S

Redland’s innovative new Spirtech 400 2S is an underlay with a difference. It has two integrated glue strips that seal together once the underlay is installed.

This unique system passes the new BS 5534 wind uplift test for use anywhere in the UK. This means you don’t have to worry about the batten spacing when using it. The technology also eliminates the need for additional timber battens over the horizontal laps, otherwise required by other underlays on the market, improving safety on the roof while saving material costs.

It is clearly labelled for easy checking, as recommended in the new Code of Practice.

The underlay’s unique combination of horizontal sealed laps (achieved via integrated double glue strips) and low vapour resistance ensures that the underlay fully resists the wind load whilst not compromising its ability to adequately control the risk of roof space condensation.

Key Benefits:

  • Eliminates the need for additional battens over the horizontal underlay lap (required by other underlays)
  • Easy to glue laps whether the underlay is laid draped or fully supported.

See an example of the new easy-to-understand labelling that will be present on the packaging of every roll of Redland’s new Spirtech 400 2S underlay. It shows the underlay can be used everywhere in the UK without restriction irrespective of the slate/tile batten gauges.

Videos

Spirtech 400 2S

Spirtech 400 2S Installation