Slate Roofs on London Victorian Terraces: Welsh Slate, Repair, and Replacement

IDENTIFYING ORIGINAL WELSH SLATE ON A LONDON VICTORIAN TERRACE: - COLOUR: DARK BLUE-GREY (PENRHYN SLATE — THE MOST WIDESPREAD ON LONDON VICTORIAN TERRACES), GREY-PURPLE (FFESTINIOG/FESTINIOG SLATE — SLIGHTLY THICKER AND ROUGHER TEXTURED), OR GREY-GREEN (LESS COMMON — SOME CUMBRIAN OR IRISH SLATE). PENRHYN BLUE-GREY IS THE MOST UNIFORM IN COLOUR AND THE SMOOTHEST IN TEXTURE. - THICKNESS: TYPICALLY 4-8MM (CENTRE GAUGE — MEASURED AT THE MIDDLE OF THE SLATE). TRADITIONAL RANDOM WIDTHS (400-500MM × 200-250MM MOST COMMON — SOMETIMES DESCRIBED AS "18 × 10" OR "20 × 10" IN IMPERIAL — INCHES LONG × INCHES WIDE). - GRAIN DIRECTION: NATURAL SLATE HAS A CLEAVAGE PLANE AND A GRAIN DIRECTION — IT SHOULD ALWAYS BE FIXED WITH THE GRAIN RUNNING VERTICALLY (I.E., ALONG THE LENGTH OF THE SLATE, HEAD TO TAIL). SLATE FIXED WITH THE GRAIN RUNNING HORIZONTALLY (ACROSS THE SLATE) WILL DELAMINATE FASTER. - FIX METHOD: SINGLE OR DOUBLE NAIL FIXING — NAILS DRIVEN THROUGH A HOLE PUNCHED (OR DRILLED) IN THE SLATE, APPROXIMATELY 40-50MM FROM THE HEAD END. ORIGINAL FIXING: COPPER CLOUT NAILS (HIGHLY CORROSION RESISTANT — CAN LAST 60-80 YEARS IN THE SLATE HOLE). LATER FIXING: GALVANISED STEEL NAILS (LESS DURABLE — 30-40 YEAR LIFE — THEN RUST AND FAIL, ALLOWING SLATES TO SLIP).

COMMON DEFECTS IN VICTORIAN TERRACE SLATE ROOFS: 1. SLIPPED SLATES: A SLATE HANGS LOOSE (VISIBLE GAP AT THE TOP EDGE) OR HAS FALLEN OFF — USUALLY DUE TO NAIL FAILURE (CORRODED NAIL). THE SLATE ITSELF IS OFTEN STILL SOUND — IT CAN BE RE-FIXED (IF ACCESSIBLE) WITH A SLATE RIPPER (TO REMOVE THE OLD NAIL STUB FROM THE BATTEN) AND A CLIP OR TIN TACK FIX. A SLIPPED SLATE THAT HAS EXPOSED THE UNDERFELT (SARKING MEMBRANE) SHOULD BE REPLACED WITHIN DAYS — EXPOSED UNDERLAY DEGRADES RAPIDLY AND WIND-DRIVEN RAIN WILL ENTER. 2. CRACKED SLATES: A SLATE BROKEN ACROSS THE MIDDLE (IMPACT DAMAGE — FALLING CHIMNEY POT, FALLING TILE, PERSON STEPPING ON THE SLATE) ALLOWS WATER IN THROUGH THE CRACK. A CRACKED SLATE MUST BE REPLACED — IT CANNOT BE REPAIRED. REPLACEMENT SLATE: A MATCHING RECLAIMED WELSH SLATE (SOURCED FROM ROOFING MERCHANTS — WALES SALVAGE, BURDENS RECLAIMED, NATIONAL SLATING — £2-6 PER SLATE DEPENDING ON SIZE AND CONDITION). 3. DELAMINATING SLATES (SPALLING): OVER TIME, EVEN GOOD WELSH SLATE CAN DELAMINATE — THE SURFACE LAYERS SEPARATE AND FLAKE OFF, REDUCING THE SLATE THICKNESS AND EVENTUALLY ALLOWING WATER THROUGH. DELAMINATION IS ACCELERATED BY AIR POLLUTION (SULPHUR DIOXIDE IN LONDON AIR — HISTORICALLY HIGH FROM COAL FIRES — ATTACKS THE CALCITE IN LESS PURE SLATES). DELAMINATING SLATES CANNOT BE REPAIRED — THEY MUST BE REPLACED. A DELAMINATING ROOF (30%+ OF SLATES AFFECTED) REQUIRES FULL RE-ROOFING. 4. FAILED SARKING (UNDERLAY): THE SARKING FELT BELOW THE SLATES (OR, IN OLDER PROPERTIES, NO SARKING AT ALL — BARE BATTENS) MAY HAVE FAILED. WHERE THE PROPERTY ORIGINALLY HAD NO UNDERLAY, WATER BLOWING UNDER THE SLATES DURING HEAVY WIND-DRIVEN RAIN WILL REACH THE ROOF STRUCTURE. A FULL RE-ROOF (STRIPPING ALL SLATES, FITTING NEW BREATHABLE SARKING MEMBRANE, RE-SLATING) IS THE ONLY COMPREHENSIVE SOLUTION.

IDENTIFYING PROBLEMATIC NON-WELSH SLATE: SPANISH SLATE (THE MOST WIDELY USED LOWER-COST REPLACEMENT IN THE UK — IMPORTED FROM GALICIA, SPAIN): DARKER, OFTEN GREENER-BLACK IN COLOUR; SMOOTHER SURFACE; THICKER (OFTEN 4-6MM STANDARD THICKNESS). SPANISH SLATE CAN BE HIGH QUALITY (CITATION SLATE — CUPA ROOFING GROUP, IBERIAN SLATES) OR POOR QUALITY (THIN, INCONSISTENT THICKNESS, HIGH CARBONATE CONTENT — WILL DELAMINATE WITHIN 20-30 YEARS). IN A CONSERVATION AREA, SPANISH SLATE IS GENERALLY NOT ACCEPTED AS A LIKE-FOR-LIKE REPLACEMENT FOR WELSH SLATE — CONFIRM WITH THE LOCAL PLANNING AUTHORITY.

SLATE REPAIR OPTIONS:

SINGLE SLATE REPLACEMENT (TRADE SLATING — RIPPER AND TIN TACK): THE STANDARD REPAIR FOR A SLIPPED OR CRACKED INDIVIDUAL SLATE: 1. INSERT A SLATE RIPPER (A HOOK-ENDED FLAT TOOL) UNDER THE DAMAGED SLATE AND HOOK AROUND THE NAIL — STRIKE THE RIPPER HANDLE WITH A MALLET TO SHEAR THE NAIL STUB FROM THE BATTEN (RIPPING THE NAIL). 2. REMOVE THE DAMAGED SLATE AND SLIDE IN THE REPLACEMENT SLATE (MATCHING SIZE — SAME GAUGE [BATTEN SPACING] AND SAME COVER PATTERN). 3. FIX THE REPLACEMENT SLATE WITH A STAINLESS STEEL TIN TACK (A SMALL CLIP NAILED TO THE BATTEN BETWEEN THE SLATES BELOW — THE TIN TACK CLIPS UP INTO THE TAIL OF THE REPLACEMENT SLATE). THIS IS A CLIP FIX — NOT THE IDEAL FIX (A NAILED SLATE IS MORE SECURE) BUT IT IS THE ONLY ACCESSIBLE FIX FOR A SLATE HIDDEN UNDER THE OVERLAPPING SLATES ABOVE.

FELT AND SEAL (NOT RECOMMENDED AS A LONG-TERM SOLUTION): APPLYING ROOF COATING (LIQUID ROOF PAINT — RUBBERSEAL, HYDROTHANE, OR BITUMEN PAINT) OVER A DETERIORATING SLATE ROOF TO PROLONG ITS LIFE. ROOF COATINGS DO NOT PERMANENTLY WATERPROOF A FAILING SLATE ROOF — THEY MAY SUPPRESS LEAKS FOR 3-5 YEARS. THEY ALSO MAKE FUTURE TILE-BY-TILE INSPECTION AND REPAIR MUCH MORE DIFFICULT. NOT RECOMMENDED EXCEPT AS A SHORT-TERM EMERGENCY MEASURE.

FULL SLATE RE-ROOF (STRIP AND RE-SLATE): WHERE A SIGNIFICANT PROPORTION OF SLATES ARE FAILING (> 30%), OR WHERE THE SARKING IS FAILED, THE CORRECT SOLUTION IS A FULL RE-ROOF: 1. STRIP ALL EXISTING SLATES (CAREFULLY — RECOVERABLE SLATES SHOULD BE SET ASIDE FOR REUSE OR RESALE) 2. INSPECT AND REPAIR ROOF STRUCTURE (RAFTERS, BATTENS) 3. FIT NEW BREATHABLE SARKING MEMBRANE (KLOBER PERMO AIR OR EQUIVALENT) 4. FIT NEW TREATED SOFTWOOD BATTENS AT THE CORRECT GAUGE FOR THE CHOSEN SLATE SIZE 5. RE-SLATE WITH: (A) RECLAIMED WELSH SLATE (BEST MATCH — BEST CONSERVATION AREA SOLUTION); (B) NEW WELSH SLATE (PENRHYN QUARRY, CELTIC SLATE — HIGHER COST THAN SPANISH BUT AUTHENTIC); (C) SPANISH SLATE (CUPA ROOFING GROUP — RIVEN OR NATURAL; LESS EXPENSIVE)

COSTS (SUPPLY AND FIT, LONDON 2025-2026): SINGLE SLATE REPLACEMENT (RECLAIMED WELSH — TIN TACK FIX, SUPPLY AND FIT): £80-200 PER SLATE (MAINLY LABOUR + SCAFFOLDING CALL-OUT) FULL SLATE RE-ROOF (RECLAIMED WELSH SLATE — PER M² OF ROOF, SUPPLY AND FIT): £100-180/M² FULL SLATE RE-ROOF (NEW WELSH SLATE — PENRHYN — PER M²): £120-220/M² FULL SLATE RE-ROOF (SPANISH SLATE — PER M²): £70-130/M² FULL ROOF STRIP AND BREATHABLE SARKING MEMBRANE (WITHOUT SLATING — PER M²): £20-35/M²