Design Specification

The design issues that need to be considered during the design stage of a roof can be divided into Prescriptive design specifications and Performance design specifications.

Prescriptive design specifications :

Rafter / pitch

Clay plain tiles conforming to the dimensional tolerances given in EN 1304 can be laid on rafterpitches down to a minimum of 35°. Plain tiles which, for aesthetic reasons, do not comply with the dimensional tolerances given in EN 1304 must be laid at pitches not less than 40°.

Head and sidelap

The headlap specification in BS 5534 is 65 mm minimum and the maximum gauge of battens should be 100 mm. Gauges of less than 88 mm are not recommended.

The sidelap should be not less than one third the width of the tile, typically 55 mm.

Performance design specifications:

Wind load

On the lee side of a building the wind can create a suction on the tiles and the vacuum effect can be significantly higher adjacent to the perimeters. The methods for calculating the wind uplift load are given in BS 5534 and BS 6399 and an outline of the calculation method, and examples, are given in appendix B.

The minimum fixing specification for plain clay tiles is to fix every fifth row but in all cases uplift calculations should always be completed to ensure that the specification meets the wind load requirements.

Where the calculation indicates that the tiles are to be mechanically fixed and the minimum fixing specification of two smooth nails does not meet the calculated wind uplift resistance, alternative fixing methods must be used, e.g. improved nails (ring shank), screws, clips and proprietary fixings.

Traditionally peg tiles are not nailed; rather, they are once pegged to allow them to be aligned in horizontal coursing.

Note: The Building Research Establishment has published a guide that describes the effect of aircraft vortices on roofs and gives recommendations for the fixing of tiles in areas that are on the flight path of aircraft takingoff and landing.

Control of Condensation

The method of ventilation should be established prior to the assembly of the roof covering. The position of the roof insulation will affect the method of ventilation and the illustrations describe examples where the insulation is at ceiling height (cold roofs) and at rafters (warm roofs). The ventilation methods must also take into consideration the type of underlay that will be used, e.g., bituminous felts, vapour permeable underlays.

The method of assessment given in BS 5250 should be used and where the risk of condensation is identified appropriate ventilation should be provided and / or a vapour control layer should be incorporated within the structure.

Rain and snow resistance

The lap arrangement in BS 5534 for plain clay tiles provides an excellent rain and snow protection system.

Tile durability

Tiles that meet the stringent requirements of BS EN 1304 have demonstrated that they have the necessary durability for the UK environment.

Thermal capacity

The thermal insulation contribution of clay tiles and batten cavity (0.17R) can be ignored. The product properties are specifically related to water impermeability, durability and aesthetics and it is the role of the insulation and other products to provide the necessary thermal performance for the building.

Fire resistance

Clay tiles are deemed-to-satisfy the UK building regulations with respect to external fire performance. The European external fire test will not apply to clay tiles because they are incombustible and have a reaction to fire rating better than class A2.

Note: The resistance to the spread of fire through the soffit into roof is a requirement for multiple occupancy buildings (Building Regulations Section 9 Concealed Spaces and B4 External Fire Spread).

Insect and bird resistance

The correct design and installation of a plain clay tile roof will ensure that the ingress of insects and birds to the loft or wall structure is prevented.