In An Inspector Calls, our regular monthly column, Total Contractor has teamed up with the experts at BMI UK & Ireland to help you achieve roofing success and avoid the common pitfalls that can often cost you both time and money.
After storms Dennis and Ciara wreaked havoc on the UK, the Inspector talks wind loadings this month and why they are a crucial factor when designing a roof so it will not be damaged.
Wind speeds and loading vary all over the UK but there are particular circumstances that you need to bear in mind when designing a roof or, more particularly, deciding how to fix it in place.
Most tile failures on a pitched roof occur due to the lack of a fixing specification or a failure to properly follow the specification provided. BS 5534, the standard for slating and tiling on pitched roofs, strongly urges the use of a manufacturer’s fixing specification for your roof.
Location, location, location
When putting together a specification the main factor we consider is location – where the building is and what surrounds it. For instance, is the building next to open terrain, such as the sea or a plain? Wind speed over flat surfaces is a lot faster than when it’s been disrupted by contours. Similarly, wind gains speed as it moves uphill: the steeper the slope, the greater the wind uplift. So, buildings at the crest of a hill will need different fixing specifications to those at the bottom. As a rule of thumb we add hills into our calculations when a slope is more than 10% and when the building is more than halfway up.
Altitude is another important aspect of location because every metre above sea level affects the calculation of the fixing specification. Each town and city has its own unique altitude and wind speed, which is detailed in BS 5534, though care is required when dealing with areas of varying heights, where we then revert to contour lines on the Ordnance Survey maps.The same is true of the built environment, which can act to slow down the wind speed in the same way as contours. That is why you need to take care when working on the edge of towns when they might border a wide, open space for some time. Even on a new development, where a house may eventually be surrounded by similar buildings, you must take this into account as the roof will be vulnerable to wind uplift while those buildings are being built.
Smaller buildings can provide some shelter to a taller building, when we would need to look at the ‘effective’ height. This is when a tall building is down wind, and the small building can give shelter to the first few storeys. But when the taller building is upwind, it causes turbulence in the air flow to the smaller buildings in its wake and this can drag down some of the fast-moving wind from higher up.
These external factors may affect the roof design so that the wind loadings are kept to a minimum – or they can be made worse, and you will have to specify the roof fixings to mitigate. The shape of the building determines the wind uplift around it, as does the pitch and height.
The basic rule is that mono pitch roofs have the strongest wind uplift, whatever their pitch but, generally, the lower the roof pitch, the more wind uplift there is acting upon it. Steeper roof pitches give some shelter to their leeside (negative) while lower pitches give less protection to their leeside, as well as exposing their leading edge. On complex roofs, services such as chimneys and dormers may cause turbulent vortices at the perimeters.In common with a lot of construction components, the important elements of roofing are unseen. Fixings and underlay play an immense role in maintaining the integrity of a building’s roof but are as visible as its foundations.
Research shows that a roof fitted without underlay is five times more likely to sustain damage than a roof with underlay. This is because the underlay keeps the internal and external pressures separate. Internally, uplift forces act upon the roof underlay whenever air can move within the building to the underside of the roof covering. Instead of this uplift load affecting the underside of the roof, its full impact is taken by the underlay.
Fixing specs: No one size fits all
Every tile or fitting we put onto our roof must be fixed, using either a nail, a screw or a specially designed clip. Research shows that the most secure fixing is a combination of both nail and clip, which has a resistance of more than 200 Newtons/m², or more than seven times secure than a single nail or twice as strong as a fully clipped. Just which fixing specification is needed for the roof in question can be answered by your tile manufacturer, as they will all supply one, normally free-of-charge. It is important you get one for each job, and each tile type because the specification may vary between manufacturers. You must not assume that a ‘one size fits all’ fixing specification will be sufficient because, as we have seen, every specification is tailored to the needs of each site and location.