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Roofs 10° roof pitch and above with a low resistance underlay installed. Insulation at ceiling level.

Roofs 10° roof pitch and above with a low resistance underlay installed. Insulation at ceiling level.

Requirements of BS:5250: Control of Condensation in Buildings

In dwelling size roofs

i) Eaves ventilation equivalent to 3mm continuous opening at eaves, if the ceiling is 'well sealed' as defined by clause 8.4.1.2 of BS 5250 or eaves ventilation equivalent to 7mm continuous opening at eaves, if the ceiling is not well sealed (likely in re-roof situations).

Note: If there is expected to be a significant moisture load from construction water in a new build construction, additional 5mm minimum ventilation should be provided at high level.

OR

Alternatively, high level ventilation equal to a continuous opening of 5mm at either side of ridge for both well sealed or not well sealed ceilings.

 

In larger than dwelling size roofs

i) Eaves ventilation equivalent to 5mm continuous opening at eaves, if the ceiling is 'well sealed' as defined by clause 8.4.1.2 of BS 5250 or eaves ventilation equivalent to 10mm continuous opening at eaves, if the ceiling is not well sealed (likely in re-roof situations or  suspended ceiling situations).

ii) High level ventilation equal to a continuous opening of 5mm at either side of ridge is required for both well sealed or not well sealed ceilings.

Important Note

Where vapour permeable underlays (type LR) are used to contribute to the control of condensation, they do so by allowing water vapour to escape through the material by diffusion.

It is important that this water vapour can escape from the batten space (the area between the underlay and the roof tile) out to the atmosphere. Specifiers and designers can achieve this in two ways.

1. Install ventilation openings to the batten space that are equivalent to a continuous opening of 25mm at eaves and 5mm at the ridge and 25mm counterbattens.

2. Install a roof covering that is deemed as sufficiently air open (as defined in Annexe L of BS 5534: 2003) and, as a consequence, will allow sufficient air movement in the batten space without the need for a counterbatten cavity or additional ventilation other than the ventilation requirement shown above for dwelling size roofs and larger than dwelling size roofs.

At the design stage, it is important to consider how construction details that are robust can be achieved over the lifetime of the building, and the following issues should be accounted for:

BS: 5250 states that it is important to ensure that air paths remain consistent and unobstructed during the life of the building.

The BRE’s Good Building Guide 37 reminds specifiers and designers that the airtightness of tiling systems including those that are initially air open may alter with time.

It is important to note that with most traditional tile systems, the air permeability of the tile changes, depending whether the roof covering is wet or dry.

It is also important to note that most roof coverings are designed to posses a life span of in excess of 30 years and, during this design life, the air permeability is likely to be compromised by the build up of dirt silting up the tile laps and the accumulation of moss or lichen.

Decra Solution:

50mm x 25mm counterbattens (finished size).

i) Decra Eave Mesh, providing 25mm ventilation, used in conjunction with Decra Warm Roof Eaves Closure Flashing.

ii) Decra Universal Ridge/Hip Ventilator (all Decra tile profiles) or DecraVent D Ridge system (Decra Classic and Noxite Tile only) providing continuous ridge ventilation.

Alternatively: tile vents at appropriate centres to provide the equivalent of a 5mm continuous opening

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