What are shadow slates?

1A strip of Welsh slate under the joint

The Stone Roofing Association (SRA) explains the term carefully, and the definition is worth having in plain English. A shadow is a piece of metamorphic slate, the Welsh slate familiar from Victorian terraces, placed under the vertical joint between two stone-slates. Roofers also call it a shade.

Welsh slate is thin, flat and dense, and it splits consistently. Horsham stone is thick and uneven. Put together, the two materials do different jobs: the stone gives the roof its look and weight, and the shadow does the discreet sealing underneath it.

From the ground the shadows are rarely noticeable, because the stone above hides most of them. You see them when a slab slips, when part of a roof is opened for repair, or when you look closely along a joint.

Grey slate in a stone roof is therefore often a deliberate detail rather than a sign that someone has botched the covering.

2Why single lap roofs need them

In double lapping, one course covers the course two below it, so the overlap alone keeps water out. Single lapping, which the SRA calls unique to Horsham stone roofs, overlaps only the course immediately beneath. The vertical joints are left open, and the shadows are what close them.

The SRA notes that the resistance to water penetration relies on both the shadows, which have to be of adequate size, and the bedding mortar, which has to have adequate strength, flexibility and durability. If either is poor, the roof leaks.

The guide suggests a plausible history. As supplies of roofing stone fell short, slaters may have stretched the overlap to make the available slabs cover the roof. Shadows and mortar then did the job the missing overlap would have done. That makes them structural, not ornamental.

Our page on single lap Horsham stone covers the system itself.

3Shadows as a repair where stone is short

Horsham District Council's conservation area appraisal records the outcome of years of mending: some roofs now combine both laying systems, using shadow slates in places where stone is short. In plain terms, a roofer ran out of slab and used slate to complete the lap, which is how a roof that was never built single lap can still carry shadows.

That is a real part of these roofs' history, and it is not automatically a fault. A sound shadow doing its job is a legitimate repair.

The SRA, however, favours retaining or restoring double lap slating wherever it can be done, so a roof that has drifted into a muddle of both systems may be worth putting back in order when the time comes. That decision needs care, because the guide says changing system means reviewing the roof loading and structural stability, and a listed building needs consent.

Ask for the system to be identified on survey before anyone proposes a fix.

4How a shadow should be laid

The SRA gives practical detail. Top nailed shadows go under the stone, preferably on their own battens, and they should be wide enough to give at least 150 mm of side lap each side of the slab in the lower courses and 100 mm in the upper.

Shadow battens are typically 38 x 15 mm, and the guide says they must be thin enough that the shadows do not push up the stone-slates. Nails for the shadows can be thinner than those for the stone, at about 3.0 mm.

Mortar goes on as a bed beneath the shadow and the stone, held back from the edges, so that water can drain and the battens stay dry. One photograph in the guide shows shadows 300 mm wide.

Mortar choice matters as much as the slate, which is why the detail sits closely with our answer on mortar on stone slates. A shadow bedded badly or left on a loose batten is a weak point in the roof.

5Spotting trouble and what a repair involves

Signs worth reporting are a cracked or slipped shadow, a stone that rocks, grey slate showing where it should be hidden, and a stain on the loft timbers directly below a joint. Moss that has built up along the joints can hide trouble too, so any cleaning needs to be gentle.

A repair usually starts with lifting the affected slabs, checking the battens, replacing the damaged shadow and re-bedding the stone. How many stones are disturbed, how high the roof is and how much scaffolding it needs all shape the price.

The cause comes first. You then see photographs of your own roof and receive a written quote giving one total. For the fixings that hold both slab and shadow, read how stone slates are fixed, and for the wider service see Horsham stone roofing.