A plain tile's dimensions are 265mm by 165mm whilst the dimensions of a large format tile are greater.
Roof pitch is generally determined at an early stage and is influenced by the need to provide usable space within the roof and the consideration of cost. Pitches of 42.5 degrees or 47.5 degrees are considered to look better than 45 degrees, an angle which Sir Edwin Lutyens described as "the ugly angle". The basic roof structure may be formed with pre-fabricated trussed rafters which must be designed to support normal superimposed loads together with the weight per square metre of tiling. The effectiveness of water-shedding is a function of roof pitch and the amount by which one course of tiles overlaps another. Experience and tests show the standard clay plain tile fixed to battens at 100mm centre (4") provides a totally waterproof pitch at 35 degrees (and upwards). Large format tiles may be laid at lower pitches and interlocking large format tiles are designed for use down to 22.5 degrees.
On sites exposed to high winds and heavy rainfall, additional security can be provided by increasing the roof pitch or lap of plain tiles or both, depending on the degree of exposure. Traditionally, plain tiles have been laid broken bonded and double-lapped to ensure water is kept out and to accommodate the changes in temperature. It has been recommended that plain tiles are laid with a slight gap not exceeding 3mm.
To assist in all stages of design, manufacturers recommend installation specifications which comply with British and European Standards, thus ensuring quality assurance. Detailed guidelines are available on all aspects of use - for both roofing and vertical tiling. Measurements are outlined and guidance given on underlay, eaves, hip, verges, ridges and main roofing. In addition to the strict guidelines on fixing, cutting and thermal insulation, other areas covered under the tough compliance rules include the tolerance levels of the clay tile in terms of its strength and the effects of fire, frost, chemicals, sunlight and high and low temperatures. Technical representatives employed by manufacturers are able to assist with the specification and detailing of roof products. Services include: Architectural assistance, CAD, drafting and estimating, calculation of fixing requirements, condensation, ventilation and U-values.
For detailed technical enquiries associated with the use of clay roof tiles please contact the manufacturer of the tiles in question (see CRTC members).