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Ventilation and Condensation

Roofs incorporating trussed rafters should be designed to service class 1 & 2 as defined in BS 5268: Parts 2 & 3. Guidance on the prevention of
condensation in roofs is given in BS 5250.
Trussed rafters using metal fasteners should not be used where there is likely to be aggressive chemical pollution, unless special precautions are taken to ensure durability of the roof timbers and fasteners. Consideration should also be given to the possibility of the corrosion of fasteners in contact with some type of insulation materials.

Reasonable access to the roof space should be provided to allow periodic inspection of the timber and fasteners.

Thermal Insulation
In the majority of trussed rafter roofs, the insulation required to comply with the statutory regulations for thermal transmittance (U value), is provided by placing the insulating material between the ceiling tie members on top of the ceiling board. Placing insulation at this level results in a COLD ROOF SPACE.

Alternatively, the insulation may be fixed at rafter level, resulting in a WARM ROOF SPACE. A warm roof space is normally constructed where habitable rooms are to be provided within the roof, as in Attic or Room in the Roof construction.

It is essential that cold roof spaces are effectively ventilated to the outside to prevent condensation, which may form in the roof void.

In addition, to minimise ingress of water vapour into the roof space from rooms below, all joists and service entry holes in the ceiling construction should be sealed effectively.

The location and size of ventilation openings should be determined by the Building Designer, taking particular account of possible blockage by insulating materials, sound transmission, spread of fire and entry of birds, driving rain or snow. Openings should be located near ceiling level in the  eaves or external walls enclosing the roof space, or both, and should be equally distributed between at least two opposite sides of the roof. Additional ventilators may also be placed in the ridge.

The size and number of openings may be calculated, taking into account all the relevant factors, but disregarding any fortuitous ventilation through the roof covering, or they may be specified in accordance with the recommended minimum openings given on the following page. These are expressed as the minimum width of a continuous gap but, alternatively, a series of discrete openings of an equivalent total area may be specified, provided that the least dimension of any opening, gap or mesh is not less than 4mm.