ROOFS A GUIDE TO THE REPAIR OF HISTORIC ROOFSOriginal slate Colour Availability <strong>of</strong> reclaimed slate Closest match in new slateIrish slateAhenny, Co. Kilkenny/ Blue/grey Limited amounts available Blue/black SpanishCo. TipperaryValdacal R5Intradima R83Benduff, Co. Cork Grey/black Few survive in reusable Cupa Quarry H5conditionCupa H83Villar Del ReyConvoy, Co. Donegal Grey/black Limited amounts <strong>of</strong> smaller Blue/grey Spanishsizes availablePiforsa R3Glaslacken, Co. Wexford Grey/green Limited amounts available USA Wea<strong>the</strong>ring Greenin small randomsKillaloe/Broadford, Grey/green Available both random USA Vermont GreenCo. Clare – Portroe, and sized Brazilian grey/greenCo. TipperaryValentia, Co. Kerry Dark hea<strong>the</strong>r Limited amounts available Valentia quarry is still open althoughonly producing flags, slates in smallsizes can be producedWelsh slatePenrhyn (Blue Bangor) Hea<strong>the</strong>r Available both random Penrhyn Quarry, Be<strong>the</strong>sdaand sizedCaernarfon Plum red Limited amounts available Canadian Hea<strong>the</strong>r, NewfoundlandDinorwic Dark hea<strong>the</strong>r Limited amounts available USA BangorsPenrhyn Welsh Hard GreysPorthmadog Blue/black Some available, mostly VilarchaosizedCampo Rande PRFerlosa PREnglish slateDelabole Light grey Limited amounts available Campo RandeSpanish InlusaWestmorland Olive green Limited amounts available Cumbria Quarries:Burling<strong>to</strong>n, Kirks<strong>to</strong>ne, etc.Scottish slateBallachulish Silver/grey None available in Ireland Piforsa Heavy 3O<strong>the</strong>r slateAmerican Bangor Dark hea<strong>the</strong>r Rare USA Bangors(Evergreen Slate Company)Norwegian quartz Silver green Few available Available new in small quantitiesTable <strong>of</strong> possible suitable sources for matching existing slates. Alternatives are given by quarry. The quarries receive <strong>the</strong> ENcertification (a European standard), which is reviewed on a yearly basis. As a general rule, any supply <strong>of</strong> slate which arrivesin unlabelled crates should be queried79
ROOFS A GUIDE TO THE REPAIR OF HISTORIC ROOFSRE-SLATING: SORTING THE SLATESHis<strong>to</strong>rically, slaters were responsible for sorting orgrading natural slates on <strong>the</strong> ground in<strong>to</strong> three or fourdifferent thicknesses, prior <strong>to</strong> fixing on <strong>the</strong> ro<strong>of</strong>. Thethickest grade and best quality slate was used forcourses at <strong>the</strong> eaves <strong>of</strong> <strong>the</strong> ro<strong>of</strong>, <strong>the</strong> level whichreceives <strong>the</strong> most water, progressing up <strong>the</strong> ro<strong>of</strong><strong>to</strong>wards <strong>the</strong> thinnest grade at <strong>the</strong> ridge. Where <strong>the</strong>ro<strong>of</strong> was a gauged ro<strong>of</strong>, <strong>the</strong> slate would be additionallysorted by length, <strong>the</strong> largest sizes for <strong>the</strong> eavesreducing <strong>to</strong>wards <strong>the</strong> ridge, and also graded withineach grouping for thickness. This grading allowedslates <strong>of</strong> a similar thickness <strong>to</strong> be laid on adjoiningcourses, preventing unsightly kicking up or gaps,which could lead eventually <strong>to</strong> slate loss.Slate produced using modern quarrying processesnow arrives on site in crates, in most cases pre-holedand pre-graded according <strong>to</strong> <strong>the</strong> quarry’s designation.Grading is generally by thickness, but may also takeaccount <strong>of</strong> colour, or <strong>of</strong> purity in terms <strong>of</strong> markings ortexture. The grading process in <strong>the</strong> fac<strong>to</strong>ry may havebeen perfunc<strong>to</strong>ry and <strong>the</strong> slater should generally notuse slates directly from <strong>the</strong> crates without somefur<strong>the</strong>r sorting on site. The grading <strong>of</strong> slate prior <strong>to</strong>fixing is <strong>of</strong> particular importance on ro<strong>of</strong>s <strong>of</strong> a slopeless than 30 degrees, as any slate which has a thickertail or end (<strong>the</strong> visible part <strong>of</strong> <strong>the</strong> slate) will be verynoticeable on <strong>the</strong> external ro<strong>of</strong> slope. The omission <strong>of</strong>site sorting prior <strong>to</strong> ro<strong>of</strong>ing will <strong>of</strong>ten result in a poorappearance <strong>of</strong> <strong>the</strong> finished ro<strong>of</strong>.GAUGED SLATE ROOFSSlates are priced according <strong>to</strong> <strong>the</strong> amount <strong>of</strong> labourassociated with <strong>the</strong>ir production, <strong>the</strong> most expensivebeing <strong>the</strong> thinnest and in <strong>the</strong> largest sizes. Gaugedro<strong>of</strong>s require a schedule for battening <strong>to</strong> be set out for<strong>the</strong> ro<strong>of</strong>ing contrac<strong>to</strong>r <strong>to</strong> follow. Diminishing coursesmay be in 2, 3 or 4 inch increments (50mm, 75mm or100mm respectively). Graduated or diminishing slatecourses cannot be laid where <strong>the</strong> slate is supplied withpre-drilled nail holes.An example <strong>of</strong> a gauged ro<strong>of</strong> <strong>of</strong> Ahenny slate, where<strong>the</strong> sizes <strong>of</strong> <strong>the</strong> slate courses gradually get smaller, ordiminish, <strong>to</strong>wards <strong>the</strong> ridge <strong>of</strong> <strong>the</strong> ro<strong>of</strong>HOLING OR RE-HOLING A SLATEIf holing is required <strong>the</strong>n it should be from <strong>the</strong> bedside through <strong>the</strong> thinnest part <strong>of</strong> <strong>the</strong> slate, leaving <strong>the</strong>thickest part <strong>of</strong> <strong>the</strong> slate <strong>to</strong>wards <strong>the</strong> tail (<strong>the</strong> exposedarea <strong>of</strong> slate on a ro<strong>of</strong> ). Slates were traditionallypunched through with a slater’s axe or machinedpunched <strong>to</strong> form a countersunk depression on <strong>the</strong>face <strong>of</strong> <strong>the</strong> slate. A large number <strong>of</strong> new slates,approximately 20 per cent, are now drilled at <strong>the</strong> time<strong>of</strong> manufacture ra<strong>the</strong>r than punched from <strong>the</strong> bedside. The countersinking detail was important so thatforged iron nails lay flat, but modern copper nails havea flat head so this does not present an issue.Large eaves slates helped <strong>to</strong> ensure that a goodwea<strong>the</strong>rpro<strong>of</strong> detail was achieved where <strong>the</strong>accumulation <strong>of</strong> rainwater was greatest. It is aparticular feature <strong>of</strong> his<strong>to</strong>ric slate ro<strong>of</strong>s and should beretained when undertaking repair worksAchieving a steady incremental decrease in course sizein a gauged ro<strong>of</strong> relies on <strong>the</strong> skill <strong>of</strong> <strong>the</strong> slater <strong>to</strong> beable <strong>to</strong> determine <strong>the</strong> weight and headlap, whichvaries with each course as work progresses up <strong>the</strong> ro<strong>of</strong>slope. In <strong>the</strong> covering <strong>of</strong> ro<strong>of</strong>s with varying degrees <strong>of</strong>headlap, a minimum headlap should be maintained,i.e. 4 inches (100mm) in most cases, or even up <strong>to</strong> 5 or6 inches (125–150mm) in lower-pitched ro<strong>of</strong>s in areas<strong>of</strong> more severe wea<strong>the</strong>r conditions.80
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