Fibre-cement slate is a manufactured roofing slate, pressed from cement reinforced with fibres and finished to look like natural slate. Other man-made slates use resin or recycled composites. They are sold as lighter, more uniform and less costly than the real thing, and for some roofs that is a fair description. For others, particularly older houses in and around Bath, it is not. The honest answer depends on the building, on how the roof is seen, and on whether anyone with a say over appearance will accept a change. This guide takes the trade-offs one at a time: what you gain, what you give up, the pitches the makers specify, the weight question that touches on Building Regulations, and the settings where we would steer you back to natural slate. We have no interest in selling you the dearest product. The aim is a roof that suits the house.
What man-made slate is, and what it is not
Natural slate is split from rock, and each piece differs in thickness, texture and tone. Man-made slate is made in a factory to a pattern, so the pieces are alike. That uniformity is the main selling point, and it is also what tells a trained eye from the street that the roof is not natural.
Fibre-cement is the common type in the UK. The slates are cut to standard sizes, such as 600 by 300 mm and 500 by 250 mm, in dark grey, blue-black and sometimes a weathered finish. Each slate has pre-formed holes or is drilled on site, and is held with nails, rivets or hooks according to the maker's instructions. Resin and composite products exist too, and they are usually even lighter, though the guidance and the performance differ by maker.
What it is not is a like-for-like replacement for natural slate on a heritage roof. It does not split in layers, it does not take on the changing colour of real slate as it weathers, and it follows a manufacturer's specification rather than a quarry tradition. Older fibre-cement slates exist on many roofs, so if you are replacing a covering that you suspect is one, identify it before anything is disturbed. HSE guidance treats carefully handled removal of asbestos cement roofing as non-licensed work, and breaking it up changes that. Our slate roofing page describes how we lay and repair the natural product.
The real advantages
Man-made slate has genuine strengths, and on the right roof they count.
What it costs to buy
Fibre-cement slates are generally cheaper to buy than natural Welsh slate. The cost guides we rely on give no separate installed rate for them, so we do not quote one, with the firm figure coming after the survey. But where a budget is tight and the roof is plain, the saving in material can be real.
Uniform size and flatness
Every slate is the same thickness and the same size, so laying is quicker and the courses come out even. There is little sorting and less waste than with reclaimed slate, and a quantity can be ordered with confidence.
Lighter weight
Man-made slates are usually lighter than natural slate of the same area, which can ease the load on an older roof structure. A check is still needed: a covering that is significantly lighter or heavier than the one it replaces needs consideration under the Building Regulations, and approval plus a structural look is needed for one that is 15 per cent or more heavier.
Convenience on awkward sites
On a modern house with no heritage constraints, such as an extension roof, a garage or an outbuilding, man-made slate can give the look of slate without paying for a stone product. On those roofs it is often a sensible choice, and when it is, our survey notes will say that clearly.
The real drawbacks
The drawbacks are less often explained at the point of sale, and they are the reason we hesitate on older buildings.
Appearance. A manufactured slate has an even surface, a clean edge and a uniform tone. Seen from the road, a whole slope of it can look flat and dark against the varied texture of a genuine slate roof next door. On a terrace where neighbours have natural slate, the difference is visible from the pavement.
Weathering. Natural slate ages through changes of tone that most people find attractive. Man-made products age differently. Surface colour can fade or streak, and moss and algae may settle on the textured surface in a way that looks untidy. The result depends on the product, so ask the maker what ageing to expect, and ask to see an older installation.
Handling. Fibre-cement slate can be brittle at the edges and corners, and cut pieces at hips and valleys need care. One cracked slate is a repair, and the replacement should come from the same range. Foot traffic can crack it, so plan how people will reach gutters and chimneys.
Matching later. A manufactured range can be revised or discontinued. If a storm damages a few slates in ten years, a matching slate may no longer be made, and the repair then becomes patchwork. Natural slate, by contrast, is always a close match to other natural slate of the same origin.
Re-sale perception. Buyers, surveyors and conservation officers all recognise the difference. A house with a natural slate roof may be read as the better product, whatever the sales literature says.
Pitch, fixing and the maker's rules
Man-made slate must be laid to the manufacturer's rules or the guarantee on the product is at risk, so the maker's instructions are the starting point, not a detail.
The minimum pitch is the first of those rules. For natural slate in general, NHBC puts the minimum at 20 degrees, subject to the headlap. Fibre-cement slate depends on the product and the lap. Marley Eternit's older fitting instructions for Rivendale give 23 degrees in moderate exposure and 26.5 degrees in severe exposure for 600 by 300 mm slates on a 110 mm lap, and say rivet-and-nail slates are generally suitable for pitches of 20 degrees and above. Cedral's current Rivendale 60 by 30 slates carry a minimum pitch of 25 degrees, according to the maker's Irish site. These are a few manufacturer figures, not a design rule for your roof, so check the maker's current data and BS 5534 for your exposure.
Fixing is the second rule. Many fibre-cement slates are secured by a nail through the head and a rivet or clip at the tail, because a slate that is only nailed at the head can lift in strong wind. Natural slate is usually nailed twice and does not need the extra tail fixing in the same way. The wrong fixing, or too few fixings, is a frequent cause of failure.
Battens and underlay follow the same logic. The batten gauge must match the slate size and lap, and the underlay should be one the maker accepts. Mixing a manufactured slate with a salvaged underlay or battens of unknown age is a false saving.
The same product can behave differently on a roof near an exposed edge or at height. Wind is the main test of any slate roof, and it is why exposure is part of every pitch table. Our page on slate fixing methods sets out how the main fixings work.
Where man-made slate suits, and where it does not
A simple way to decide is to ask what the roof is, who sees it and who decides.
Where it can suit
- A modern house, built in the late twentieth century or since, with no listing and no conservation constraint.
- A rear slope, extension or outbuilding hidden from the street.
- A roof being re-covered where natural slate is not what was there before, and the pitch meets the maker's minimum.
- A job where the structure is light and a heavier covering would trigger a structural check.
Where it is the wrong choice
- A listed building. Replacing natural slate with an artificial one is a change in character and will probably need listed building consent, with a good chance of refusal. At Lambridge Place in Bath, an artificial slate was criticised in Trust comments and natural Welsh slate was welcomed as the replacement, which shows the direction of opinion.
- A prominent roof in the conservation area. Check with B&NES planning before assuming it is acceptable. Our page on listed slate roofs explains how officers think about matching.
- A natural slate roof that needs a part repair. The new slates will not match, and reclaimed natural slate is the better route. See our guide to reclaimed slate.
- A roof with a shallow pitch. If the pitch is below the maker's minimum, the product cannot go on it.
When you are comparing the two on cost, remember that life matters. Manufactured slate is generally expected to last less long than good natural slate, which narrows the gap once the cost is spread over the years. No sourced lifespan figure is available for either, so none is given here.
Laying it well: weather, detail and the survey
A cheaper slate does not mean a cheaper standard of workmanship. The details that keep a roof dry are the same on any covering.
Weather matters at the time of laying. Fibre-cement slate is cut and fixed in the open, and cold, wet or windy days make the work slower and less precise. Many roofers prefer to avoid the worst weather, and our page on slating in winter covers what that means in practice, for natural and man-made slate alike.
The hidden parts matter more than the covering. Valleys, flashings and soakers around a chimney, the lead in a parapet gutter, and the ridge and verges all need to be sound before a new covering goes on. The ridge in particular needs a mechanical fixing, because the British Standard for slating, BS 5534:2014, treats mortar as no longer enough. A new slate over rotten battens or a worn-out valley lining is a roof that leaks in two years.
At the survey we go up, look at your roof and photograph it, check the pitch against the covering you have in mind, and tell you whether man-made slate suits the house. If it does not suit, the survey states that and prices the natural alternative alongside it, so the comparison is fair. The fixed price in writing sets out the slate, the fixings, the battens and the underlay. The workmanship carries a 10-year written guarantee.
Where Building Regulations apply because of the size of the job or the weight of the covering, our team handles the building control side. If you are weighing two quotes with different slate types, ask each roofer for the pitch limit, the fixings and the underlay in writing.