Written by Andrew, Lead Roofer
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Emergency Roof Repairs 24/7
Slate vs tile roof compared for London homes: what each covering really weighs, the minimum pitch, when a swap needs building control, and how costs compare.
Slate and tile roofs suit different London homes: natural slate is the original covering on most Victorian terraces and lasts longer than tiles, while clay and concrete tiles suit 1930s semis and later houses and cost less to lay. The real differences between slate and tiles sit in four places most comparisons skip: weight, minimum roof pitch, building control and planning.
Choosing between slate and tiles is rarely about looks alone. Slate and roof tiles put different loads on the rafters, need different roof pitches, and swapping one covering for the other can bring in Building Regulations and, in a conservation area, planning permission. A wrong choice on a Victorian terrace can mean a roof that is too heavy for its timbers or a planning enforcement letter from the council.
This guide covers the types of slate and tile on London roofs, what each covering actually weighs per square metre, and the minimum pitch for each. The guide also explains which covering suits which London house, how to tell what is on your roof now, the 15% weight rule that decides when a swap needs building control, the conservation area and listed building rules, and how slate and tile costs compare.
The comparison splits “tile” into the three types found on London roofs, because clay plain tiles, concrete plain tiles and concrete interlocking tiles differ from each other as much as they differ from slate.
| Natural slate | Clay plain tile | Concrete plain tile | Concrete interlocking tile | |
|---|---|---|---|---|
| Material | Split natural stone | Kiln-fired clay | Concrete | Concrete |
| Laid weight per m² | About 36-49kg | About 74kg | About 74kg | About 44-54kg |
| Minimum pitch | 20° | 35° (40° handmade) | 35° | 30°, or lower for some designs |
| Relative material cost | Two to three times concrete | About 50% more than concrete | More than interlocking, because about 60 tiles cover each m² | Lowest |
| Typical London homes | Victorian and Edwardian terraces | 1920s and 1930s semis, some Edwardian villas | Interwar and later re-roofs | Post-war houses, later re-roofs |
| Expected life | 75-200 years depending on the slate | About 60 years (manufacturer figure) | About 60 years (manufacturer figure) | About 60 years (manufacturer figure) |
Natural slate is a metamorphic rock split by hand into thin sheets, then holed and nailed to battens in overlapping courses. Most slate on London roofs came from North Wales, carried by rail in the Victorian building boom, and Welsh slate is still the benchmark. Spanish slate is the most common choice for new natural slate roofing today, and Spanish slate quality ranges from excellent to poor, so the grade matters as much as the country.
Man-made slate is a separate product. Fibre cement and composite slates copy the look of natural slate at a lower price and a lower weight. Man-made slates are uniform in colour and shorter-lived, typically lasting 20 to 50 years against a century or more for good natural slate.
Clay plain tiles are small tiles of kiln-fired clay, usually 265 by 165mm with a slight curve, laid at around 60 tiles per square metre. Clay plain tiles weather to soft reds and browns and give a finely textured roof. Concrete plain tiles copy the same size and shape in concrete, at a lower price and with a more uniform colour.
Concrete interlocking tiles are large, around 420 by 330mm, so only about 10 are needed per square metre. Concrete interlocking tiles lock together at the sides, which lets each tile overlap the one below by a single course instead of two. Profiles range from the wavy Double Roman to flat, slate-effect designs that look like slate from the street.
Slates and plain tiles are double-lap coverings, meaning every point on the roof is covered by at least two thicknesses of slate or tile. Interlocking tiles are single-lap. The difference between double-lap and single-lap explains most of the gaps in weight, pitch and cost between clay and concrete tiled roofs and slate.
Slate is not the heavy option. A common claim online is that slate is heavy and needs a reinforced roof structure, but manufacturer data says otherwise: natural slate is the lightest of the traditional coverings, and plain tiles are the heaviest by a distance.
| Covering | Laid weight per m² | Basis |
|---|---|---|
| Natural slate, 500 x 250mm, 5.5mm thick | About 36-39kg | Calculated from Welsh Slate’s published weights |
| Natural slate, 500 x 250mm, 7mm thick | About 46-49kg | Calculated from Welsh Slate’s published weights |
| Concrete interlocking tile | 44-54kg | Marley technical data, Double Roman to Duo Modern |
| Concrete plain tile | 73.8kg | Marley environmental product declaration |
| Clay plain tile, handmade | 74kg | Marley technical data |
The slate figures come from Welsh Slate’s table of slate sizes and weights, which gives the weight of every 1,000 slates. Working that through at a normal 75 to 100mm headlap gives a laid weight of roughly 36 to 49kg per square metre, depending mostly on slate thickness.
Plain tiles weigh so much because plain tiles are small, thick and laid two layers deep, so a square metre of plain tiling carries around 60 tiles. A slate roof is also laid two layers deep, but a typical roofing slate is only 5 to 7mm thick, which keeps the total load down.
Concrete interlocking tiles sit in the middle, close to a heavy slate. The practical point for London homeowners is that Victorian roof timbers were cut to carry slate, and many have been carrying slate for over a century. Putting a heavier covering on those timbers is where trouble starts.
Minimum roof pitch is the shallowest slope a covering can be laid on and still keep rain out. Natural slate goes lower than plain tiles, which surprises many homeowners, because tiles are often said to suit shallower roofs.
| Covering | Minimum pitch | Source |
|---|---|---|
| Natural slate | 20° | BS 5534, as summarised by Marley |
| Clay or concrete plain tiles | 35° | Roof Tile Association |
| Handmade or non-standard plain tiles | 40° | Roof Tile Association |
| Interlocking tiles, standard rule | 30° | Roof Tile Association |
| Interlocking tiles, proprietary designs | 12.5° to 22.5° | Manufacturer data |
BS 5534, the British Standard for slating and tiling, sets 20° as the minimum pitch for natural slate. Slate at the shallow end of that range needs larger slates and a deeper headlap, so a slater will choose the slate size to suit the pitch, not the other way round.
Plain tiles need a much steeper roof. The Roof Tile Association’s design specification puts the minimum for clay or concrete plain tiles at 35°, rising to 40° for handmade tiles that do not meet the standard dimensional tolerances. Interlocking tiles can go down to 30° under the standard rule, and lower where the manufacturer has tested a specific design. Marley’s Duo Modern, for example, is rated to 22.5° at a 75mm headlap and 17.5° at a 100mm headlap.
Pitch matters in London because many Victorian terraces have shallow roof slopes, including many of the butterfly roofs hidden behind front parapets. Slate works on those slopes. Plain tiles do not, so a homeowner wanting to replace a shallow slate roof with tiles is usually limited to interlocking tiles, which changes how the roof looks.
The age of a London house is the best guide to its original roof covering. The English Housing Survey found that 41% of homes built before 1919 have natural slate or stone roofs, while clay tiles are the most common covering on homes built between 1919 and 1945, at 40%. Across all homes in England, concrete tiles are the most common covering at 58%, with clay at 15% and slate at 13%.
| London house type | Usual original covering | What to watch for |
|---|---|---|
| Victorian and Edwardian terraces | Natural Welsh slate | Shallow and butterfly roofs that rule out plain tiles |
| Edwardian and Arts and Crafts villas | Often clay plain tiles | Original roof may be clay even where neighbours have slate |
| 1920s and 1930s semis | Clay plain tiles | Steep pitches suit plain tiles; hips and bonnets are labour-heavy |
| Post-war houses and estates | Concrete interlocking tiles | Usually the simplest roofs to re-cover |
| Period homes re-roofed after 1950 | Concrete interlocking tiles over original slate timbers | Extra load on old rafters; candidates for returning to slate |
Matching the original covering is usually the right call on a period London house, because the roof structure, the pitch and the street were all designed around the original material. A Victorian terrace with one house re-roofed in concrete tiles stands out from the end of the road, and that break in the terrace line is exactly what conservation officers try to prevent.
Original does not always mean slate, though. Parts of north-west London, including streets in Swiss Cottage and parts of Hampstead, were built with clay tile roofs, and restoring a clay roof with slate would be just as wrong as the reverse.
Insurance forms, mortgage applications and survey reports all ask whether a roof is slate or tile, and the answer is usually visible from the pavement. The lower edge of the roof, at the gutter, and the sloping edge at the gable end show the thickness of each piece most clearly.
Natural slate looks thin from below, with a near knife edge and no profile at all. Slates are long rectangles, often 500mm or more, laid in straight courses, and natural slate shows slight variations in colour from blue-grey to purple-grey.
Clay plain tiles are small and slightly curved, so a plain-tiled roof looks finely textured, with many short courses running up the slope. Clay plain tiles are red, orange or brown, often weathered and mottled. Concrete plain tiles share the same size and shape but have a flatter, more uniform colour and a slightly sandy surface.
Concrete interlocking tiles are large and thick, with an edge that shows as a chunky line at the eaves. Interlocking tiles either have a visible wave or roll, or a flat face with a slate-effect finish. Slate-effect concrete tiles are the most common source of confusion, because slate-effect tiles read as slate from across the street. The giveaway is edge thickness: a concrete tile edge is several times thicker than a slate.
Man-made slates are thin like natural slate but almost perfectly uniform, with identical colour across the whole roof and very regular edges. Natural slate never looks that consistent.
The loft gives a second view. In an older roof without underlay, the backs of the slates or tiles are visible between the battens: slates are thin and held by nails, while plain tiles hang on the battens by small nibs. Nobody should climb onto a slate roof to check, because slate cracks under foot. A drone roof survey shows every slope in close detail without anyone setting foot on the roof.
Replacing a slate roof with concrete tiles is common on London streets, and the swap is often made without anyone checking the extra weight. The Planning Portal’s guidance on work to an existing roof says building control approval is needed if a new covering increases the weight of the roof covering by 15% or more.
Using typical laid weights, the 15% rule catches many slate-to-tile swaps:
| Swap | Approximate weight change | Over 15%? |
|---|---|---|
| Thin 5.5mm slate (about 38kg) to concrete interlocking (about 50kg) | About a third heavier | Yes |
| Thin 5.5mm slate (about 38kg) to plain tiles (about 74kg) | Almost double | Yes |
| Thick 7mm slate (about 48kg) to concrete interlocking (about 50kg) | About 4% heavier | No |
| Any tile to natural slate | Lighter or similar | No |
A full re-roof usually needs building control approval anyway, because the Planning Portal says an application is normally needed once half or more of the roof is affected. The weight rule matters because a heavier covering also means proving that the rafters, purlins and walls below can carry the extra load, which often means a structural engineer’s check and sometimes new or strengthened timbers.
The risk is real on a Victorian terrace because the timbers were sized for slate and can have been weakened by more than a century of small leaks. The Planning Portal warns that changing roof elements can cause movement, and movement in a roof shows up as cracks in the walls below. A heavier roof on tired timbers can also sag over time, which is visible as a dip along the ridge line.
Going the other way, from concrete tiles back to slate, usually reduces the load. Restoring slate to a period house that lost slate to concrete in the 1960s or 1970s is one of the more satisfying jobs on a London roof, and the structure normally welcomes the lighter covering.
Re-roofing a house does not usually need planning permission if the work does not materially change the outside appearance of the house. Swapping natural slate for concrete tiles on a slope facing the street is the kind of change that does alter appearance, and many London streets sit in conservation areas.
Listed buildings have the strictest rules. Historic England’s guidance on repairing or renewing the roof of an older home says listed building consent is needed to change the type of roof covering. The same guidance says slate sizes, the number of courses and the overlap should be kept when a historic slate roof is re-covered. In a conservation area, Historic England advises checking with the local authority because planning permission may be needed.
Article 4 directions remove permitted development rights in specific streets. Camden’s design guide for the Swiss Cottage conservation area is a good London example: since September 2010, re-roofing a street-facing slope in a material that differs from the original or existing material has needed planning permission there. The same guide notes that many roofs in the area were originally clay tiles, not slate, which is why checking the original covering matters before choosing a new one.
Bernard Andrews Roofing carries out heritage and conservation roofing across London, including roof work in Hampstead, where conservation area rules apply to many streets. Checking with the council before ordering materials avoids paying for a roof twice.
In material terms, concrete tiles are the cheapest roof covering, clay tiles cost roughly 50% more than concrete, and natural slate costs two to three times as much as concrete. Checkatrade’s roof replacement cost guide, updated in October 2026, puts a full roof replacement at roughly £120 to £275 per square metre depending on the covering and location, and notes that prices in London and the South East run higher than elsewhere.
Slate costs more for reasons beyond the material. Every slate is holed and nailed individually, slates of slightly different thickness are sorted on site so the courses lie flat, and hips, valleys and abutments take careful cutting and leadwork. Concrete interlocking tiles cover a square metre with about 10 tiles, so labour per square metre is far lower.
Plain tiles sit between the two on labour, because around 60 plain tiles go onto every square metre, and the hips and bonnets on a 1930s semi add slow, fiddly work. Scaffolding costs much the same whichever covering goes on, so the difference between slate and tiles narrows on a small roof where scaffolding is a large share of the bill.
Lifespan changes the sums. A good natural slate roof lasts far longer than a concrete tile roof, so the cost per year of a slate roof is often lower than the upfront price suggests, especially on a house the owner plans to keep.
Slate and tile roofs rarely fail because the slates or tiles wear out. Roofs fail because the nails corrode, the battens rot and the underlay breaks down. On an old slate roof, rusted iron nails let slates slip one by one, a problem roofers call nail sickness, while the slates themselves often have decades of life left.
Good Welsh slate can last well over a century, and our guide to how long slate roofs last covers the differences between Welsh, Spanish and imported slate in detail. Marley gives a typical life expectancy of 60 years for its roof tiles, and the fixings, battens and underlay under a tiled roof face the same ageing as those under slate.
The practical result is that a tired slate roof often does not need new slate. Stripping the roof, setting aside the sound slates, fitting new battens and underlay, and re-hanging the original slate with copper nails is often the best value re-roof on a Victorian house, topped up with reclaimed Welsh slate where needed.
Natural slate is lighter than most roof tiles. Manufacturer data puts a laid slate roof at roughly 36 to 49kg per square metre, against 44 to 54kg for concrete interlocking tiles and about 74kg for clay or concrete plain tiles. The claim that slate is heavy and needs extra structural support does not match the published weights.
The main disadvantage of a slate roof is cost, because natural slate costs two to three times as much as concrete tiles and takes longer to lay. Slate also cracks if walked on, cheap imported slate can fail early, and matching a few broken slates on an old roof needs a supply of the right slate, often reclaimed Welsh slate.
Tiling a roof is cheaper than slating. Concrete interlocking tiles are the cheapest option, clay tiles cost roughly 50% more than concrete, and natural slate costs two to three times as much as concrete, with higher labour costs because every slate is fixed individually.
A slate roof can sometimes be replaced with concrete tiles, but three checks come first. Concrete tiles are often 15% or more heavier than the slate, which brings in building control and a structural check. Plain tiles need a pitch of at least 35°, which many London slate roofs do not have. In a conservation area or on a listed building, changing the covering may need planning permission or listed building consent.
Concrete roof tiles are cheaper and quicker to lay than slate, and concrete tiles suit post-war houses well. Natural slate lasts much longer, weighs less than most tiles, and is the right covering for Victorian and Edwardian houses that were built for slate.
A slate roof can last 200 years with good Welsh slate and regular maintenance, though the nails, battens and underlay will need renewing several times over that period. Lower-grade imported slate lasts much less, which is why the quality of the slate matters as much as the choice of slate over tiles.
Fibre cement slates fitted before 2000 may contain asbestos. The Health and Safety Executive says buildings constructed after 2000 are unlikely to contain asbestos, and that asbestos-containing materials in good condition are best left in place and managed. Old fibre cement slates should not be drilled, cut or broken, and a sample should be tested before any removal work starts.
The right roof covering for a London house is usually the one the house was built with. Slate suits Victorian and Edwardian terraces because slate is light, works on shallow pitches and matches the street. Clay plain tiles suit steep 1920s and 1930s roofs. Concrete interlocking tiles suit post-war houses and tight budgets. Any swap between slate and tiles needs the weight, the pitch and the planning position checked before the scaffolding goes up.
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