Can You Change Roof Tiles to Slate?

Does your roof pitch allow it?

Pitch is the first test. Slate is thin and laid with small overlaps, so on a shallow roof wind-driven rain can be pushed up between the courses. Every slate has a minimum pitch published by its supplier, and it rises with the exposure of the site and the size of the slate. Many 1960s and 1970s houses around Basingstoke were built with low-pitched concrete tile roofs, and some of those fall below what a natural slate allows. A larger slate can sometimes lower the minimum, but the maker's table, not our preference, sets the limit.

If the pitch is too low, there are two choices. Keep to a tile that is rated for the slope, or accept a different system with a membrane that does more of the work of keeping water out. We measure the pitch on the survey rather than guess it from the ground, because a degree or two matters.

Weight and the roof structure

Concrete tiles are heavy, so a change to slate normally lightens the roof. That helps the rafters, and it means the 15% rule rarely bites. The rule is that a covering weighing 15% or more over what it replaces needs building control approval, and the structure may need strengthening. Going the other way, from slate to concrete, is where it applies. Thick Welsh slates on a big roof are not as light as people expect, so weight is checked, not assumed. We ask the supplier for the weight per square metre of the slate on the quote and compare it with what comes off, which takes a few minutes and removes the guesswork. Where the figures are close, a structural opinion is cheaper than a surprise.

The bigger point is what lies beneath. The timbers have been carrying tiles for decades and may have sagged, rotted at the wall plate or been cut for old pipework. Slate is unforgiving of an uneven roof plane, since a dip shows in every course. Rafters and purlins are inspected on the strip, and any repairs are made before the new battens go on. Slate also sets the batten gauge, the spacing of the timber laths, so the battens are cut and laid to the slate's own measurements, not to whatever the old tiles needed. Our page on heavier roof coverings and structural checks sets out the sequence.

Natural, Spanish or man-made slate

Natural Welsh slate is the benchmark for life and appearance, and it is also the dearest. Spanish slate is cheaper and widely used, though quality varies between quarries, so we say which grade we are pricing. Fibre-cement and other man-made slates are cheaper again and lighter, and they suit a budget job on a plain house, though they weather differently and do not carry the same long record.

For rough scale, typical UK ranges reported by cost guides put a natural slate re-roof at about £120 to £180 per square metre all-in, against £90 to £130 per square metre for man-made slate. By origin, Welsh runs about £140 to £180 per square metre and Spanish about £100 to £130. Treat these as typical ranges, not a price list: your fixed quote follows the survey.

Planning and building control

Replacing a roof covering with a different material can change the look of a street. On a house in a conservation area, or one that is listed, the change may need consent, so check with your council's conservation officer before ordering anything. Where the work is above the thresholds, building control gets involved: re-covering that affects 25% or more of the building envelope and 50% or more of the roof calls for approval, and the insulation of the whole roof may have to be improved. Our page on building regulations for roof replacement goes through it.

Is it worth it?

Slate earns its place on period houses and on rooflines where it was there originally and was lost to a cheaper repair. On a modern estate house, a switch to slate is mainly a matter of taste and budget, and a good concrete or clay tile is a sound answer too. Our comparison on choosing materials for a new roof lays the options side by side.

On the survey we measure the pitch, look at the rafters from the loft and photograph what we find, so you can see whether slate suits the roof. If it does not, we tell you, and we price the alternatives. You get a fixed written quote, and the workmanship is guaranteed for ten years in a written document. The full job is covered on our roof replacement page.