A variety of technical topics to help you install your roof correctly
What underlay should I use and do I require ventilation?
COLD ROOF CONSTRUCTION
Most people now use 'breathable' (Low resistance or LR) underlays when installing a roof, but there is a common misconception that all breathable underlays are equal. In fact there are two types (see below), although with both types a well sealed ceiling should be provided. If the ceiling can't be sealed then consider high level ventilation with either system.
1/ Vapour Permeable Underlays
These are often the cheaper of the two types and generally are dense enough to prevent water entering the roof from the outside, but sufficiently 'permeable' to allow moisture to exit the roof-space by diffusion. In fact there is an argument that this diffusion process alone cannot transfer all the moisture out of the building without some assistance, particularly in new constructions where there is often a lengthy drying-out period. The NHBC now recognise this fact and insist on additional ventilation at high level to aid the removal of water vapour.
2/ Air Open Underlays
Are generally more expensive and are sufficiently 'air-open' to allow diffusion without additional assistance in the form of ventilation. These can be installed in accordance with 3rd party accreditation without ventilation and are the only types of underlay that should be installed in this way - indeed the NHBC also recognises this fact. They can also be installed, if you wish, by following the guidelines in BS 5250 with the addition of high level ventilation. However, it should be noted that when specified with a close-fitting roof covering, such as fibre-cement slates, there is a risk of interstitial condensation forming between the underlay and the roof covering - to avoid this risk, batten cavity ventilation should be provided. There may also be a requirement to seal laps and joints.
All other underlays, including bituminous 1F Type 747 felt, are classed as high resistance (HR) and require the use of ventilation in accordance with BS 5250.
WARM ROOF CONSTRUCTION
Where the insulation is between and/or above the rafter, providing the ceiling is well sealed and a VCL is installed there is generally no requirement to ventilate.
PRESENCE OF BATS
If you have been asked by your local authority to allow roof space access for bats then you cannot generally use a breathable underlay, but will need to consider a bituminous 1F BS747 type felt. This is because the newer breathable underlays, which are normally polypropylene based and sometimes have a fleece liner, can cause bats to become entangled.
Fixing Specifications On-Line
Make sure your tiles are fixed to the roof correctly! Most of the major manufacturers now have on-line tools to help to ensure compliance. Please use the links below to access their tools.
The new Code of Practice, released in 2014, significantly increased the minimum fixing requirements for roof tiles.
For example ALL single lapped (interlocking) roof tiles on a roof now need to be mechanically fixed. In addition, tiles at the perimeters of a roof e.g. verges, eaves, hips, valleys, must now have a minimum of TWO fixings. The code also now recommends that ALL ridge and hip tiles are mechanically fixed, rather than be reliant on mortar bedding.
The links above will help you determine exactly what you require, or we will be pleased to help you with any queries on fixings that you may have.