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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.

Only except in very exceptional circumstances should pitches of less than 30° be used with plain clay tiles, please refer to our sales team to discuss any specific queries you may have.

 

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.

 

It has been accepted for quite some time for Tyvek® to be installed in two layers with staggered laps to accommodate roof systems that are built below the minimum pitch for the tile or slate. This originated in the 90’s as a solution for projects where a building or design constraint existed and has been widely accepted by local authorities throughout the UK. Understandably the ‘double felt’ method has been implemented widely within the industry to overcome low pitch problems.

 

The ‘double felt’ solution has proven to be particularly useful for small roof areas which would be expected to discharge only a limited amount of rainfall. Typical examples would be single-storey lean-to extensions and it is these types of roofs that we have been aiming at particularly. This method should therefore be avoided on main roof systems, especially ones of two storey height, as they invariably incorporate details with even lower pitch such as a hip, a valley or a large tilt fillet at the eaves.

 

To ensure good practice we have over the last several years ‘fine tuned’ our recommendations and have stated a preference for us to view proposals beforehand. This doesn’t always happen and unfortunately from time to time we encounter roof systems that are outside our scope of acceptance.

 

Wherever possible, our standard pitched roof underlay Tyvek® Supro should be laid at a pitch suitable for the slate or tile that is being used. However, we will accept the ‘double felt’ method using the following rules as a general guide:

 

Tile/slate min pitch      Double felt allowance

14° - 20°                                     -2°

20° -30°                                      -3°

>30°                                           -4°

 

Following this system will ensure that the absolute minimum pitch allowed with two layers of Tyvek® Supro is 12° (for a tile which has a minimum pitch of 14°). Please note that our recommendations for metal clad industrial roofs are unaffected.

 

A Tyvek® ‘double-felt’ solution is restricted to roof systems of single storey height incorporating no valleys or hip details. Rafter length should not exceed 9m for areas of normal exposure to driving rain and 6m for areas of high exposure. Additional factors to be considered are topography and detailing such as eaves sprockets and rooflight flashings. These factors all govern the risk of water penetration onto the roof underlay. In all cases the underlay must be laid to a fall with no ponding of water on the underlay under any circumstances.

 

Written confirmation must be obtained from DuPont™ Tyvek® for each case.

 

Tyvek® Supro's standard 30 year warranty is applicable to the double felting method, provided it is installed in accordance with these guidelines.

 

Please note: Tyvek®'s 30 year warranty is still applicable to any projects accepted prior to this amended policy (05.12.08) that have had written acceptance based on our previous recommendations.

 

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

Nick Williams - Technical Support Manager

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