Roofing Design – More than Meets the Eye

ACCORDING TO experts at Cedral Roofing, contractors need to consider more than just Building Regulations when specifying and designing roofing.

On the face of it, getting the design of a pitched roof right shouldn’t be that difficult. Dictated initially by the shape of the building, the span between the structural elements and the functional use of the space below the roof, the list of considerations is relatively simple. However, there is a host of other environmental factors that need to be taken into account, according to Paul Newman, site services manager at Cedral Roofing.

After structure and function have been taken into account, the design of the roof should include a consideration of other factors such as weathering and aesthetics – two vital elements of the project that sometimes get overlooked. Below are the key topics that should be on any roofing contractor’s agenda before making the final specification.

Exposure

While it might not be relevant to every development, it’s still important to check out any potential exposure zones of the site. You can do this by referring to the map in BS 5534 or from BRE Report 262 and BS 8104. This splits the UK into 5 wind zones – each with a recommended roofing standard. The ‘Code of practice for assessing exposure of walls to wind-driven rain’ and ‘Thermal insulation: avoiding risks’ are especially relevant here. The map applies to buildings of up to 12 metres in height at the ridge.

Once you have identified the relevant wind zone, you can then calculate the wind suction loading either in accordance with BS EN 1991: Part 1 – 4 or use the design calculations on wind loads in BS 5534.

In cases where the roof slope exceeds six metres in length and / or the site is rated to be in a severe exposure category, guidance on the suitability of the slate should be confirmed by contacting the technical advisory team of your roofing products supplier.

Ridges and hips

All ridge and hip tiles must be fixed with at least one mechanical fixing, (this includes mortar bedded units) as required in BS 5534. Mechanical fixing is best achieved with a dry fixing method which can also provide ventilation at the ridge. If you’re using a mortar-bedded system, then a mechanical anchor is required either through the unit or in conjunction with a clip through the mortar joint.

Aircraft vortices

Roofs near airports can experience remarkably high local wind load forces due to air vortices created by aircraft when taking off and landing. In certain circumstances, this can be greater than the calculated wind loads to BS 5534 – so care is needed when working on projects near major airports or military bases.

If this is the case, it’s a good idea for designers to seek advice from the Airport Authority Planning Department when designing roof fixings in these locations. It’s also acknowledged as best practice to refer to the guidance contained in BRE Digest 467 ‘Slate and tile roofs: avoiding damage from aircraft wake vortices’.

Calculating the fixing specification

The procedures for calculating the wind loads and determining the fixing specification for tiles and slates in accordance with BS EN 1991-1-4 and BS 5534 are not for the faint hearted and are complex to undertake.

We would advise designers to obtain full fixing recommendations from their roofing products provider’s technical advisory team well before any final specifications are made. It’s also important to keep up to date with the latest British Standards in this area as every standard is regularly reviewed to ensure the content is valid and current.

Paul Newman BSc Civ.Eng. MIoR, is site services manager at Cedral Roofing.

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