Solar Panels on Slate Roofs

Why slate is a different job

Slate is not a small tile. It is thin, dense, brittle stone laid double-lapped and nailed through pre-drilled holes, and it behaves nothing like a nibbed interlocking tile. It cannot be lifted and hung back on a batten the way a tile can, it cannot be relieved to sit over a hook, and it cracks rather than flexes when a fixing bears on it in the wrong place.

Everything about a slate installation follows from that. Slower work, more care, a different fixing method, higher breakage and a higher price.

Fixing methods

Two approaches are used, and the choice depends on slate size, batten gauge and the state of the roof.

Slate hooks. A purpose-made hook designed to work with the slate courses, taken back to bolt into the rafter, with the mounting bracket carried above the covering. Fewer slates disturbed, but it needs the right hook for the slate dimensions and a batten arrangement that will accommodate it.

Flashing plates. A slate is removed and replaced with a proprietary lead or metal plate that is dressed into the courses and carries the bracket. This gives a positive weathering detail at each fixing point and is often the more robust answer on irregular or reclaimed slate, at the cost of removing a slate at every fixing.

In both cases the bracket bolts into the rafter. Not the batten, not the boarding. Slate roofs on older properties frequently have irregular rafter centres, so the array is set out against measured positions from inside the loft.

Condition assessment

The survey question on a slate roof is not usually whether the array will fit. It is whether the roof has enough life left.

Nail sickness is the recurring finding on Victorian and Edwardian roofs. The slate itself is a durable material and often has decades left, but the iron or steel nails corrode long before it does. Slipped slates, a scatter of mismatched replacements, and slates that lift free with almost no resistance are all signs. A nail sick roof will need stripping and relaying, and an array fixed to it will have to come off and go back.

Fibre cement slate is a separate case. Later products are stable; earlier ones become brittle and can be fragile to walk on. Age and manufacture need establishing before anyone works on the roof.

Slate is also heavy. A slate roof carries a substantial dead load before an array is added, and where a structure is already deflected or has been altered, that needs a proper look.

Design and appearance

Slate roofs are frequently on the properties where appearance matters most — Victorian terraces, older detached houses, conservation areas, listed buildings. All-black panels with black frames read well against slate; silver framing on a dark slate roof does not.

In-roof mounting is worth serious consideration on slate. Sitting the array flush in the plane of the covering is what makes it acceptable on a visible elevation, and it is far easier and cheaper to detail during a re-cover than as a retrofit onto a sound roof.

Access, breakage and limitations

Scaffolding on slate properties is often more involved because they tend to be older, taller and in tighter streets. Slate roofs are also slippery and steep, so roof ladders and proper edge protection are non-negotiable rather than optional.

Breakage is expected. We allow for replacement slates and match as closely as the roof permits, but reclaimed slate varies in size, thickness and colour and a perfect match is not always possible.

The honest limitations: slate costs more, takes longer, breaks more, and on a roof approaching the end of its fixing life it is the wrong sequence of work entirely.