March 2020

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An Inspector CallsMAKE WIND LOADING A BREEZEIn An Inspector Calls, our regular monthly column, Total Contractor has teamed upwith the experts at BMI UK & Ireland to help you avoid the common pitfalls that canoften cost you both time and money and achieve roofing success.After storms Dennis and Ciara wreakedhavoc on the UK, the Inspector talkswind loadings this month and why theyare a crucial factor when designing a roof soit will not be damaged.Wind speeds and loading vary all over the UK butthere are particular circumstances that you need tobear in mind when designing a roof or, moreparticularly, deciding how to fix it in place.Most tile failures on a pitched roof occur due to thelack of a fixing specification or a failure to properlyfollow the specification provided. BS 5534, thestandard for slating and tiling on pitched roofs,strongly urges the use of a manufacturer’s fixingspecification for your roof.LocationWhen putting together a specification the mainfactor we consider is location – where the buildingis and what surrounds it. For instance, is thebuilding next to open terrain, such as the sea or aplain? Wind speed over flat surfaces is a lot fasterthan when it’s been disrupted by contours.Similarly, wind gains speed as it moves uphill: thesteeper the slope, the greater the wind uplift. So,buildings at the crest of a hill will need differentfixing specifications to those at the bottom. As arule of thumb we add hills into our calculationswhen a slope is more than 10% and when thebuilding is more than halfway up.Altitude is another important aspect of locationbecause every metre above sea level affects thecalculation of the fixing specification. Each townand city has its own unique altitude and windspeed, which is detailed in BS 5534, though care isrequired when dealing with areas of varying heights,where we then revert to contour lines on theOrdnance Survey maps.Heartbreak ridge: damage from wind uplift can be extensive.The same is true of the built environment, whichcan act to slow down the wind speed in the sameway as contours. That is why you need to take carewhen working on the edge of towns when theymight border a wide, open space for some time.Even on a new development, where a house mayeventually be surrounded by similar buildings, youmust take this into account as the roof will bevulnerable to wind uplift while those buildings arebeing built.Smaller buildings can provide some shelter to ataller building, when we would need to look at the‘effective’ height. This is when a tall building isdown wind, and the small building can give shelterto the first few storeys. But when the taller buildingis upwind, it causes turbulence in the air flow to thesmaller buildings in its wake and this can dragdown some of the fast-moving wind from higher up.These external factors may affect the roof design sothat the wind loadings are kept to a minimum – orthey can be made worse, and you will have tospecify the roof fixings to mitigate. The shape of thebuilding determines the wind uplift around it, asdoes the pitch and height.The basic rule is that mono pitch roofs have thestrongest wind uplift, whatever their pitch but,generally, the lower the roof pitch, the more winduplift there is acting upon it. Steeper roof pitchesgive some shelter to their leeside (negative) whilelower pitches give less protection to their leeside, aswell as exposing their leading edge. On complexroofs, services such as chimneys and dormers maycause 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 inmaintaining the integrity of a building’s roof but areas visible as its foundations.Research shows that a roof fitted without underlayis five times more likely to sustain damage than aroof with underlay. This is because the underlaykeeps the internal and external pressures separate.Internally, uplift forces act upon the roof underlaywhenever air can move within the building to theunderside of the roof covering. Instead of this upliftload affecting the underside of the roof, its fullimpact is taken by the underlay.Fixing specs: No one size fits allEvery tile or fitting we put onto our roof must befixed, using either a nail, a screw or a speciallydesigned clip. Research shows that the most securefixing is a combination of both nail and clip, whichhas a resistance of more than 200 Newtons/m², ormore than seven times secure than a single nail ortwice as strong as a fully clipped. Just which fixingspecification is needed for the roof in question canbe answered by your tile manufacturer, as they willall supply one, normally free-of-charge. It isimportant you get one for each job, and each tiletype because the specification may vary betweenmanufacturers. You must not assume that a ‘onesize fits all’ fixing specification will be sufficientbecause, as we have seen, every specification istailored to the needs of each site and location.Contact BMI National Training Centre01285 863545www.bmigroup.com/uk@_Redland / @Icopal_UK44 TC MARCH 2020
GEMINI ®THE ROOFING TILETHAT HAS CAUSEDA REVOLUTIONPROJECT SHOWN IN IMAGE:Broadbridge Park Care HomeCLIENT:Halebourne Care GroupCONTRACTORS:Lawrence Baker and BracknellRoofing ContractorsPRODUCT:40,000 Gemini ® roof tiles in Ember BlendGemini is a true innovation in roof tile design. With its ‘doublecambered’ design, this interlocking concrete tile gives theappearance of traditional plain tiles when laid.Because of Gemini’s design, fewer tiles are required to coverthe same area as a conventional double lapped plain tile. Thisinnovative product assists in reducing the build cost, and helpsto reduce the environmental impact by reducing the amount ofquarried aggregate used and haulage movement per laid m².Call 01525 244900 or visitwww.forticrete.co.uk to find out more.www.forticrete.co.uk
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March 2020

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