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 or concrete plain tiles conforming to the dimensional tolerances given in BS EN 1304 or BS EN 490 can be laid on rafter pitches down to a minimum of 35°. Proprietary tile designs that are close-fitting as laid may be used on roof pitches below 350 provided the manufacturer is able to produce evidence of satisfactory prolonged and extensive use of the product at the roof pitches, head-laps and side-laps for which they are intended to be used. Plain tiles which, for aesthetic reasons, do not comply with the dimensional tolerances given in BS EN 1304 must be laid at pitches not less than 40°.
Clay and concrete interlocking tiles conforming to the dimensional tolerances given in BS EN 1304 or BS EN 490 and without effective anti-capillary devices can be laid on rafter pitches down to a minimum of 300. Proprietary tile designs that have effective anti-capillary devices, head-lap and side-lock features, and are close-fitting, may be used at pitches below 300 provided that evidence is available from the manufacturer, of satisfactory prolonged and extensive use of the product (a minimum of 15 years) at the roof pitches, head-laps and side-laps for which they are intended to be used. Manufacturers should provide evidence based on an appropriate test method that can be directly correlated with the recommended conditions of use e.g. PD CEN/TR 15601 ‘ Hygrothermal performance of buildings – Resistance to wind-driven rain of roof coverings with discontinuously laid small elements – Test Methods’
Head and sidelap
For pitched roofs, the head-lap specification in BS 5534 recommends 65mm minimum and the maximum gauge of battens should be 100mm. Gauges of less than 88mm are not recommended. The sidelap should be not less than one third the width of the tile, typically 55mm.
For vertical walls, the head-lap specification in BS 5534 is 35mm minimum and the maximum batten gauge should be 115mm. The side-lap should be not less than one third the width of the tile, typically 55mm.
For pitched roofs, the head-lap specification in BS 5534 for variable gauge tiles without a headlock is 75mm minimum. The head-lap and side-lap recommendations of products that have a design feature on the top and underside surface of the tile and/or a side-lock, is determined by the design, and is subject to manufacturers recommendations.
For vertical walls, the head-lap is determined by design features on the top surface at the head of the tile and/or features on the under surface at the tail of the tile and subject to manufacturers recommendations. In BS 5534 it is recommended that for variable gauge products, the head-lap should be not less than 35mm below the nail hole/s, if present.
Performance design specifications:
On the leeward side of a building the wind can create a suction on the tiles and the uplift effect can be significantly higher adjacent to the perimeters. The methods for calculating the wind uplift load are given in BS 5534 and BS EN 1991-1-4.
The minimum fixing specification for plain tiles with nibs laid on roof pitches below 600 is to fix every fifth row with two nails in each tile. For roof pitches of 600 and above, including vertical, two nails should be used to fix each tile. At verges and abutments, and at each side of valleys and hips, the end tile in every course should be twice nailed where possible or mechanically fixed. At eaves and top edges, two courses of tiles (consisting of under-eaves courses and first full course at eaves, last full course and tops course at top edges) should be nailed or mechanically fixed.
Traditionally Plain tile peg tiles are not nailed; rather, they are once pegged to allow them to be aligned in horizontal coursing.
The minimum fixing specification for interlocking tiles is to mechanically fix each tile. For rafter pitches of 450 and over, each tile should be nailed with at least on nail, Additionally, for rafter pitches of 550 and over, including vertical, the tail of each tile should be mechanically fixed i.e clipped. At verges and abutments, and at each side of valleys and hips, the end tile in every course should be mechanically fixed. At eaves and top edges, one course of tiles should be mechanically fixed.
In all cases, wind uplift calculations in accordance with the recommendations of BS 5534 should always be completed to ensure that the specification meets the wind load requirements.
Depending on the outcome of the wind loading calculations, it may be necessary to use alternative fixing methods such as ring shanked nails, screws, clips, or proprietary fixings.
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 taking off and landing. [Digest 467 – ‘Slate and tile roofs: avoiding damage from aircraft wake vortices].
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 choice and method of ventilation used, and the illustrations describe examples where the insulation is positioned horizontally at ceiling height (cold roof) or at pitched rafter level (warm roof). The ventilation method adopted must also take into consideration the type of underlay that will be used, e.g., reinforced bituminous (HR), vapour permeable (LR) or air and vapour permeable (LR/AVP).
The method of assessment given in BS 5250 should be used and where the risk of condensation is identified appropriate ventilation to the roof spaces or voids should be provided and / or a vapour control layer should be incorporated within the ceiling/wall structure in order to prevent water vapour from reaching the cold side of the insulation.
Rain and snow resistance
The lap arrangement described in BS 5534 for plain, interlocking, clay and concrete tiles provides an excellent rain and snow protection system.
Tiles that meet the stringent requirements of BS EN 1304 or BS EN 490 have demonstrated that they have the necessary durability for the UK environment.
Although the thermal capacity of clay or concrete tiles can help to regulate the internal temperature of a building, the thermal resistance properties of clay and concrete tiles can be ignored as they do not in themselves provide a significant level of insulation. Roof tile properties are specifically related to water impermeability, durability and aesthetics. It is the role of the insulation and other thermally efficient products to provide the necessary thermal performance of the building structure.
The reaction to fire of a roofing or cladding product and its assembly as used in terms of flame spread and flame penetration should be determined by reference to the properties of the roofing or cladding element, its method of assembly, the effects of the sub-elements and the air permeability of the array as laid.
Both Clay and Concrete tiles manufactured to BS EN 1304 and BS EN 490 used on a conventional pitched roof with insulation, underlay, battens and tiles are deemed-to-satisfy the UK building regulations with respect to external fire performance and are designated Class AA in BS 476 – 3. The European external fire performance classification applicable to roof coverings made of clay or concrete determines that they are incombustible and have a reaction to fire rating of BROOF (t4) in BS EN 1187.
Note: The resistance to the spread of fire through the soffit into roof is a requirement for multiple occupancy buildings (Building Regulations 2010 Approved Document B Section 9 Concealed Spaces and B4 External Fire Spread).
Insect and bird resistance
The correct design and installation of a tiled roof will ensure that the ingress of insects and birds to the loft or wall structure is prevented.
Protected species, such as bats and swifts, that often nest in roof spaces, can be accommodated by the use of special roof fittings that provide an entry and exit to nest boxes located within the roof space. Some manufacturers offer such products for this purpose. Furthermore, when bats use the roof as a roost the choice of roofing underlay can be restricted as some types of underlay have been shown to cause harm to bats. The underlay manufacturer should be contacted for guidance.