Guidelines for Roof sections falling below a 35° Pitch

On some developments design limitations can arise on comparatively small areas of the roof which restrict the available roof pitch to below 35° which is the minimum pitch recommended for all plain tiles in BS EN 5534.  In these situations, it is necessary to use a secondary roof system laid below the tiles to provide assured weather protection and enhanced ventilation.

Below are some alternatives supplied directly by the respective manufacturers and the information is provided only as a reference, please be aware that Dreadnought Tiles do not supply or provide any warranty on the information or products described.

Onduline

The Onduline system manufactured by Onduline Building Products Ltd allows plain double lap tiles to be laid to roof pitches as low as 22.5°, and is covered by British Board of Agrement certification, details are available from the Onduline website www.onduline.net.

A leaflet is also available to download as a PDF, click here to download.

Tyvek

General Statement : Tyvek® Roofing Membranes, Low pitch applications.

This information relates to the application of a Tyvek® vapour permeable underlay where proposed roof constructions incorporate pitches that are lower than that of the tiles/slates recommended minimum pitch.

Generally, our standard roof underlay Tyvek® Supro should be laid at a pitch suitable for the slate or tile that is being used. However, constructional constraints dictate instances where a roof pitch can only be built below the recommended minimum.

Where doubt exists as to the roof system’s water shedding ability there are some extra precautions that can be taken. These normally relate to a roof tile or slate that is laid at an unsuitably low pitch:-

1.         The tile/slate is on the minimum - Increase the Tyvek® lap from 150mm to 250mm.
2.         The tile/slate is just below the minimum - Seal the Tyvek® laps with adhesive tape.
3.         The tile/slate is well below the minimum - Install the material in two layers with staggered laps.

Rather than recommending a ‘blanket’ solution for all situations we prefer to address each individual case, considering factors such as roof shape, building height, geographical location (rain index), exposure rating, etc. Other influential factors to consider are laps and/or detailing; valleys, rooflights, etc. However, in most cases we will accept a pitch no less than 60% of the minimum with option 3 above - and it is this option (double-felting) that is the most widely accepted solution.

We can confirm therefore that when installing Tyvek® Supro in two layers as above our standard 30 year warranty will apply.

Tyvek® membranes have a high water resistance with test results indicating a 2m water head. Moisture sitting on the membrane would therefore present no water ingress risk. Adhesive tape can be used to improve weather-tightness at penetrations such as soil stacks and rooflights, etc.

For further advice on the use of Tyvek® membranes please contact DuPont™ Tyvek® Technical Support on 01275 879770. www.tyvekhome.com

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