Can an Older Roof Be Upgraded With a New Slate Covering?

Most articles about changing a roof covering are written about going from slate to tile, usually to save money. This one is about the other direction, which is a different proposition in almost every respect — and, technically, a much easier one.

On Stretford's late Victorian housing it is also frequently a restoration rather than a change, because a good many of those roofs were slate to begin with.

Why This Is the Easy Direction

Weight

Natural slate is lighter per square metre than concrete tile. Going from concrete tile to slate therefore asks less of the roof structure than it is already carrying — which means the structural question that dominates the reverse change simply does not arise.

Fibre cement slate is lighter still, which makes it a genuine option where a structure is marginal for any reason.

Pitch

Slate performs at lower pitches than most tiles. So a roof that currently carries tile will, in the great majority of cases, comfortably carry slate. Again, the constraint that blocks the reverse change is not in play.

Detailing

Slate is a small-unit covering, so it can be cut and dressed around hips, valleys, dormers and bays in ways large interlocking tiles cannot. On a shape with a lot of detail — an inter-war hipped semi, or a Victorian terrace with a bay and an outrigger — slate sits better.

What You Actually Gain

• Appearance. On late Victorian and inter-war housing, slate almost always looks better than concrete tile — the units are smaller and finer, so the roof reads as a quiet plane rather than a coarse one.
• Longevity. Good natural slate has a very long service life, generally longer than concrete tile.
• Colour stability. Slate does not fade. Concrete tiles are coloured with a surface pigment that weathers away over decades.
• Reduced load. A lighter covering on a structure built for something heavier is a small permanent gain.
• Value on the right property. On a period house in a street of slate roofs, a concrete tile roof is a visible outlier and buyers notice.

The Honest Downsides

It would be easy to write this as a straightforward upgrade. It is not quite that.

• Cost. Natural slate is the highest material cost of the common coverings, and slating is slower than laying interlocking tile — more units per square metre, each individually fixed. Expect a meaningful step up from a like-for-like tiled re-roof.
• Time. The same reason. A slate roof takes longer on site.
• Nail fatigue, eventually. Slate outlives its fixings, so a slate roof will need stripping and relaying at some point even though the slates themselves are fine. That is a long way off, but it is the pattern.
• Matching a neighbour. On a semi or a terrace, changing your half to slate when the other half is tile is a very visible difference — arguably more so than the reverse.
• The planning question. A visible change of covering can materially affect external appearance, which is where the street-specific Article 4 position matters.

Which Slate

Note the last row carefully. A slate-effect concrete tile is not a lighter covering and is not slate — it is heavier than the material it imitates, and at close range the larger units and thick edges give it away. If the reason for changing is appearance, it defeats the purpose.

Within natural slate, the grading matters more than the country of origin. Producers declare performance under BS EN 12326-1 for water absorption, thermal cycling and sulphur dioxide resistance, and 1 is the best grade in each — so W1 T1 S1 is the best specification available. Ask which slate is being quoted, and for its declaration of performance.

The Planning Position, Which Is Street-Specific Here

A like-for-like renewal is not development at all under section 55 of the Town and Country Planning Act 1990, because it does not materially affect the external appearance of the building. A change of covering is different, because it may.

In Stretford there is no town-wide answer to give. Trafford Council makes Article 4 directions in two categories, one of which covers development affecting areas and buildings of historic interest — so whether roofing rights have been withdrawn on your street depends on which direction, if any, applies there.

Trafford Council can confirm the position for a specific address at no cost. Do that before ordering materials rather than after.

There is a reasonable point to make here, though: where a heritage constraint does apply, going to slate on a property that was originally slated is usually the change a conservation officer would welcome rather than resist. This is the direction that generally works with the grain.

What the Job Involves

The same strip and rebuild as any re-roof, with the covering being the part that changes:

• Strip and disposal of the existing tiles, battens and underlay
• Structural inspection, with an agreed rate for defective timber
• New breathable underlay
• New treated battens at the gauge the slate size and pitch require — a different gauge from the tiles that came off
• Slate laid and nailed to specification, with a fixing pattern set by the exposure
• New lead to chimneys, abutments and valleys
• Hips and ridge finished appropriately — on a hipped roof this is a real decision
• Eaves and ridge ventilation
• Insulation upgraded to meet regulation 23 of the Building Regulations 2010 — 0.16 W/(m²K) for a roof in an existing dwelling under Approved Document L Volume 1
• Scaffolding, and a footway licence if required
• Building control notification or self-certification, with the certificate issued

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