A new pitch for roof designers
The revision BS 5534: Code of Practice for Slating and Tiling: Part 1: 1997: Design heralds a new approach to the design and specification of pitched roofs.
It marks the first substantial revision to the guidelines on pitched-roof design for almost 20 years, and is a significant step towards harmonisation with other European codes. Essentially, the new code addresses some issues which have been causing concern in the design, specification and maintenance of pitched roofs.
The damage caused to roofs by the two big storms of 1987 and 1990 highlighted inadequacies in the fixing of tiles and slates. Based on extensive research, the new code provides a more accurate calculation method for assessing wind loads on the roof covering and provides test methods for establishing the strength of tile fixings and the tensile bond of mortar used for bedding accessories such as ridge and hip tiles.
In addition, the new code acknowledges many new materials, reflecting the wider range of slating and tiling products currently available, and therefore accommodates the requirements of the Construction Products Directive of 1991.
The other major changes are amended design recommendations for rain and wind resistance, plus new guidance on durability, thermal insulation, the control of condensation, fire resistance, sound, environmental and health and safety issues. The main focus here is on changed materials, design recommendations and resistance to wind loads.
Roofing products In addition to traditional slates and tiles, the new code acknowledges a whole range of new materials, including bitumen shingles, fibre-cement, metal and other proprietary tiles, plus accessories in PVC-u and GRP.
Underlays Accepted primarily as a barrier to minimise wind loads generated on the underside of the roof covering, underlays have a role, recognised in the code preventing wind-driven snow or dust entering the roof space as well as in transporting any rain leakage into the roof gutter. To produce satisfactory resistance to wind uplift their performance criteria are: adequate strength, water resistance and durability for fully supported underlays; water resistance, tensile and nail- tear resistance and low extensibility for unsupported underlays.
Boards, sheathing and sarking Defined as being either tongued-andgrooved or square-edged, materials can include woodwool slabs, fibreboards and particleboard.
Battens and counterbattens In recognition of their important contribution to both the structural integrity of the roof as a whole and to the completed roof covering, major changes to the grading of battens and counterbattens have been introduced in an effort to improve quality. The upgrading of sizes of timber battens acknowledges problems stemming from undersizing when applying fixings such as clips, and the 'bounce' when nailing tiles and slates when rafters are at wide centres. Grading marks now appearing on timber will make it easier to check on use of the correct timber. The list of timber species suitable for battens now acknowledges the wider availability of home-grown timber and withdrawing certain species associated with splitting and other grading quality problems in the past. Batten sizes and structural calculations are given in the code.
Mechanical fixings The new code lists approved nail and clip materials, complying with either British Standards or manufacturer's specifications. Clips for slates and tiles must be tested for strength and durability and to ensure they do not affect the performance of tiles and slates while on the roof.
Accessories A range of new roof accessories and fixings have been included in the code, including rooflights, snow guards, safety hooks, lightning conductors and roof walkways as well as ventilation and dry-fix products, many of which reflect the new health-and-safety requirements for roof maintenance.
Flashings and junctions Traditional materials such as lead, copper, aluminium and zinc are included alongside proprietary flashing units.
Mortar It is important to be aware of the grades of mortar permissible for roof tiling. New mortar tests have been introduced. Mechanical fixing of ridge and hip tiles is required where the roofs span masonry supports and where differential movement is likely to occur.
Design recommendations The new code also highlights the importance of ensuring that pitched roofs are designed to provide resistance to rain and snow, as well as structural strength and stability, by offering adequate wind resistance.
As there is no generally agreed standard driving-rain test method for assessing pitched-roofing products, so the UK Map of Categories of Exposure to Driving Rain has been used as the basis for establishing pitch and lap recommendations for slates.
There is a new calculation method for double-lap slates to determine the angle of creep (to limit capillary action between slates and the water entering nail holes or passing over the heads of lower slates). This is clearly illustrated within the code and highlights the importance of establishing the correct position of nail holes. Guidance is also provided for proprietary products, which should be supported by independent testing or should have been used satisfactorily for a minimum of 15 years in the type of location of intended use.
Other areas include design guidance for lower pitches, such as sub-roofs. The code also addresses the design widths of valley troughs and the position of their weather checks and tilting fillets to cater for the volume and rate of rainfall for the area of the roof which the valley drains.
This is particularly important for large roofs where long valleys can result in a significant surcharge of water in deluge conditions.
The design recommendations include thermal insulation and the control of condensat ion in roof spaces, env ironmental and health-and-safety issues which have an important part to play in the overall design of pitched roofs.
Wind loads By far the most complex area of the revised code deals with wind loading.
New calculation methods allow a more accurate treatment of the forces acting on the roof tiles or slates and the parameters affecting the wind climate of the roof of the building in its topographical location.
They are also a lot more complicated.
Other changes to the code relate to mechanical fixings for boarding, sarking and flexible underlays, where calculations are provided to determine appropriate nailing recommendations for single and double-lap tiles and slates.
For assessing the design wind loads on the building structure, the new code refers designers to the recently introduced wind-loading code, BS 6399:
Loading for Buildings: Part 2: 1997: Code of Practice for Wind Loads. The effective wind speed and the dynamic pressure of the wind appropriate to the roof in its specific location (including altitude and other topographical conditions) can be calculated in accordance with this code's method.
The wind load is now taken as a uniformly distributed load over the exposed area of each tile or slate, instead of the previous methods which assumed that all the load was concentrated at the tail of the tile, leading to inaccuracies when calculating loads on products with thin front edges. The specifier can calculate the resistance which may be required to counteract uplift loads by way of fixings such as clips or nails.
Wind-tunnel studies have shown that in the majority of cases of wind damage to tiled roofs, it was the eaves, ridge and verges which suffered most damage. The new code concludes that the wind pressure distribution in these areas is determined more by the size of the building and by its roof pitch and shape than by the individual tile detail.
Use of BS 6399: Part 2 requires consideration of factors such as the building's height above sea level and distance from the coast and, in addition, the orientation of each and every building.
Complex roof shapes such as curved, mansard and hipped roofs can also be taken into account. Clearly, the wind loading design task is greatly increased.
However, the end result is a more realist ic loading model based on uniformly distributed external pressures over the different zones of the roof.
John Dodd is technical services manager of Marley Building Materials and was a member of the BS 5534: Part 1 drafting committee.
Assistance with roof design is available from Marley's Technical Advisory Service, tel 0990 626900