星空传媒

An answer to their prayers...

It was when they ran out of buckets to catch the drips that the congregation of St Mary鈥檚 Cathedral in Edinburgh realised they were in real trouble. Having shielded the Victorian structure for over a century, the slate roof was failing fast. The Aberfoyle slates, quarried in the Highlands and notoriously soft, had worn away around the single nail that held each one down. And as the slates lost their grip on the nails, they slid down the roof slopes and landed in the gutters, particularly during Edinburgh鈥檚 windy winter. 鈥淭he cathedral was littered with containers to catch the drips,鈥� says architect Dave Willis. 鈥淭hey were even using dustbins.鈥� Prohibitive costs
The original idea was for the cathedral鈥檚 workshop to replace the roof, slope by slope, over 25 years with a harder, better-quality slate. But a few years into the programme, it became clear that the roof was descending rapidly into the guttering. Even worse, the cost of piecemeal replacement was punitive. 鈥淪late is expensive, but the big cost was scaffolding. On a yearly basis, that鈥檚 no joke,鈥� says Willis. A new programme, with 90% of the 拢800,000 cost funded by the Lottery and Heritage Scotland, shrank the 20-odd years still left on the project to 18 months, slashing the cost of scaffolding. With more than one roofer needed to complete the project in the timescale, the cathedral appointed a specialist contractor, HL Bolton, who had up to eight people on the job. Slate expectations
The project started in 2001, with the scaffolding erected for each roof slope in turn. HL Bolton stripped off the Aberfoyle slates and replaced them with Westmorland Greens from Burlington Slate鈥檚 Elterwater quarry in the Lake District, which closely matched the original Aberfoyle in texture and colour. 鈥淚t鈥檚 a quality slate, brittle but very hard,鈥� says Willis. 鈥淲e wanted a quarry that would still be producing in 20 years鈥� time.鈥� To cover the 3000 sq m of the cathedral鈥檚 10 different roof areas, Burlington supplied upwards of 25,000 slates with a maximum size of 18in by 9in, fixed by copper nails to a sarking deck and laid in diminishing courses, with smaller slates towards the apex of the roofs. The project finished last year. Enquiry number 200

Blue blood, green slate

Blue blood runs all the way through Burlington Slate, which, along with Alfred McAlpine Slate, dominates slate production in the UK. In 1843, William Cavendish, Earl of Burlington and seventh Duke of Devonshire, set up the company which today is owned by his great-great-grandson, Lord Cavendish, who lives at Holker Hall in the Lake District. And with 160 quarrymen on his books, he鈥檚 a sizable local employer. The company operates six slate quarries, including Elterwater, which produces Westmorland Green. Burlington quarries slate by sawing through the rock with diamond wires and blasting it free. The resulting clog is then moved to the quarry workshop where craftsman repeatedly split, or rive, the block along its natural cleavages. Its edges are then chamfered by hand because it gives a more perfect finish.

Rolling out the barrel vaults

Stylish steel decking costs less and looks great
A single-ply membrane doesn鈥檛 always mean a drab expanse of flat industrial roofing. On its Hathersage Park site in Derbyshire, Hallamshire Construction is building a complex of flats and offices. The crowning glory of each of the four multistorey blocks is a series of barrel vaults constructed from steel decking and insulation topped by a membrane. Style matters
Barrel vaults typically consist of a metal sheet with an insulating quilt slung underneath. Indeed, the original plan for the design-and-build Hathersage project was to roof the barrels with stainless steel. But the material is expensive, as is the cost of curving it to create the barrels, so Hallamshire asked Ashgate whether a cheaper, single-ply system could work instead. The problem wasn鈥檛 the membrane, which will, after all, fit virtually any shape. It was getting the insulation below it to curve smoothly and aesthetically enough to please the planners. Ashgate MD Andy Fell had three possible insulation solutions: boards of rockwool, polystyrene or polyurethane. But Rockwool to a depth of 110mm was needed to achieve the necessary U value, adding thickness and weight to the roof. Fell also says that Rockwool boards are easily damaged. Much the same was true of polystrene board. Fell chose polyurethane. And although EcoTherm鈥檚 standard 75mm-thick polyurethane boards wouldn鈥檛 curve to the profile either, the company agreed to supply two thinner boards that would, and in half-size, easier-to-lay sheets. It offered maximum U value for minimum thickness, and was robust enough for the roofers to stand on. Fell decided to put the EcoTherm on top of Corus PMF steel decking, delivered to site crimped. 鈥淚 was a bit too scared to risk spanning the full width of each barrel with a single sheet,鈥� admits Fell, 鈥渟o we ran pairs of sheets down the length of each barrel, overlapping them at the top.鈥� Top technique
You can see his point. It only takes one purlin to be slightly out of alignment for a pre-cut metal sheet to fall short of the edge of a barrel or overshoot it, leaving the installer with a time-consuming bodge job. Fell therefore divided the barrels in half at their highest point, cladding each side with a single sheet that overlapped at the top. What keeps the rain off is FDT鈥檚 Rhenofol composite membrane (a reinforced PVC sandwich). It comes in a shade of grey to ensure there is no dazzling distraction for motorists higher up on the slopes of the valley. The final touch is a set of 35mm-high PVC ridges hot-air welded to the membrane to give the impression of a standing seam. 鈥淚t鈥檚 not off the shelf. I don鈥檛 know anything else like it,鈥� says Fell. 鈥淚t proves our technical abilities, but it really appeals because it looks so great. It鈥檚 one for the website.鈥� Enquiry number 201 (Corus)
Enquiry number 202 (EcoTherm)
Enquiry number 203 (FDT)

Barrel o鈥� laughs?

There are four separate blocks at Hathersage Park: one with four barrels, another with 16, and two with eight each. Although the barrels within each block are identical, each block鈥檚 barrels are different. As a result, the barrels can have a radius from 7.75m to 8.3m, arc about 8m from one valley to another and are between 10 and 13 metres long.

Shear destruction for derelict hangars

scissor-style hydraulic demolition vehicle (literally) takes the roof off!
At Weeton barracks near Blackpool, the glory days when Spitfires scrambled to bring down German bombers attacking nearby Liverpool and Manchester have long gone. But the ex-aerodrome saw one last action last year when demolition company J Freeley cut the roofs off two long-empty aircraft hangars with an enormous pair of scissors. The hulking hangars were dangerous as well as redundant. They weren鈥檛 built to last, and over the years the cement cladding cracked, the gutters dropped off and the door seals gave way. But with the roof and walls made of asbestos cement, demolition was difficult. Cutting no corners
Blowing the buildings up would have sent asbestos dust flying over hill and dale, while the height and fragility of the roofs made taking the sheets off manually too risky and too expensive. Which is why J Freeley attacked the hangars with a giant pair of scissors 鈥� in reality, a hydraulically operated pair of shears. 鈥淚t was like watching Edward Scissorhands come to life on a tracked vehicle,鈥� jokes Peter Holmes of Atkins Defence Asset Management, who oversaw the job. As the machine alternated between the trusses on either side of the hangar to cut through the 8in by 4in steel joists where they met the vertical beams, the asbestos cement roofing collapsed slowly into the well of the building, whose still-standing walls confined the debris to the enclosed area of the hangar. 鈥淭he biggest job was clearing up all the rubbish,鈥� says Holmes. 鈥淔ortunately, the asbestos came apart from the steel when the roof hit the floor.鈥� J Freeley then loaded the asbestos into sealed skips for disposal while the shearing machine cut the steel up into six-feet lengths for scrap. After dealing with the roof, J Freeley did the same with the walls, cutting the verticals near the floor and pushing the cement sheets over into what was left of the building. Enquiry number 204

Hip replacement speeds up reroofing

Lafarge's 鈥楻apid hip and ridge鈥� system slashes time on council job
Two to three hours per house. That鈥檚 what Ray Lewis reckons he saved by using a dry-fix ridge and hip system on a 12-month contract reroofing 400 council houses in Stoke-on-Trent. Nail rot, torn felt and broken battens meant Lewis, the MD of Lewis Brothers, had to completely strip the roofs 鈥� tiles, felt, battens and all. Instead of mixing up a sand and cement mortar as a fixing bed for the roof ridges and hips, Lewis used Lafarge Roofing鈥檚 Rapid Hip and Ridge products. Roofers simply fix a dry roll to the batten that crowns the ridge or hip, and then screw down and seal the tiles to it. Lafarge says the system is 33% quicker to fix than mortaring the tiles into position. 鈥淵ou can have it up in 20 minutes,鈥� says Lewis. 鈥淚t鈥檚 the simplicity of it. You just batten up the hip, roll out the fabric, sit the tray on top and fix the ridge and hip tiles down.鈥� Time is money
Faster installation is essential to reducing your total cost, though, as mortar-bedded materials cost less. 鈥淯sing Rapid Ridge will add approximately 拢20 to the material cost of a roof with a 10-metre ridge line,鈥� says Lafarge Roofing product manager Stuart Pocock, who claims that the resulting productivity gain will bring substantial savings. 鈥淎nd there鈥檚 no need to return to the roof to put right washed-out or defective mortar bedding.鈥� Lafarge supplies all the required fixings in the same pack, so you can carry all you need to fix the ridge and hip onto the roof in one trip. Enquiry number 205