Slate roofs and the depth a conservation unit sits at

Why it is easy

Slate lies flat, so the soakers sit tight against it with no roll to bridge. The geometry is simple and the flush detail is achievable without the contortions a pantile demands.

Slate also cuts cleanly with the right tool, so the slates closing against the unit can be dressed accurately rather than approximately.

Why it is unforgiving

The courses are dead regular and the lines are visible from a long way off. A unit set half a course out, or a few millimetres off parallel with the courses, is obvious from the ground in a way it never would be on an irregular clay roof.

The setting out therefore follows the slate courses, and the position is agreed with a tape on the roof rather than from the ceiling below.

Setting depth

The unit is set so the frame sits just proud of the slate surface, typically a few millimetres, so water is shed onto the slate rather than being held at the junction.

Too deep and water sits against the frame. Too shallow and it loses the flush appearance that is the point.

That depth is set by the battens and the counter battens, which means the depth is decided while the roof is open rather than adjusted afterwards.

Broken slates

Old slate is brittle at the nail holes and breakages are normal when lifting. Matching reclaimed slate for the repairs matters: new Welsh slate beside weathered Welsh slate is visibly different for years.

We check availability at survey rather than with the roof open.

Nail sickness

On very old slate roofs the fixings corrode before the slate fails. Where we find it, the honest position is that the roof needs attention beyond the rooflight, and we will say so.

Why slate is the covering conservation rooflights were made for

The original cast iron rooflights of the nineteenth century went overwhelmingly into slate roofs, and the design assumptions follow from that.

Slate is flat, thin and regular. It is laid to steep pitches, usually 40 degrees or more, because slate needs the pitch to shed water. And individual slates can be lifted and replaced without disturbing their neighbours.

All three of those characteristics make it the easiest covering to fit a low profile unit into, and they are why a conservation rooflight in slate looks right in a way it does not always in a heavily profiled tile.

What depth actually means here

The measurement a conservation officer cares about is how far the unit stands above the plane of the covering.

Because slate is thin, typically a few millimetres, and lies flat, the plane of the covering is close to the plane of the battens. A unit designed to sit low has very little to clear.

On a profiled interlocking tile, by contrast, the covering has real depth: the tile profile stands up from the batten line by a considerable amount, and a flashing has to accommodate that. A unit that sits low relative to the batten line still stands proud relative to the eye.

So the same conservation rooflight reads lower in slate than in profiled tile, which is one reason slate roofs give the best results with these units.

The section drawing, and what to measure from

Manufacturers publish a section showing the projection of the unit above the covering, and that is the document that decides an application.

Read it carefully. Some drawings show the projection above the batten line and some above the finished covering, and those are different numbers. On slate the difference is small; on a profiled tile it is substantial.

The figure an officer wants is the projection above the finished roof surface, because that is what is visible from the ground.

Include the drawing with any application as a matter of course. Its absence is the most common reason a submission sits unresolved while correspondence goes back and forth.

Recessed installation

Several manufacturers offer a recessed flashing arrangement that sets the unit lower into the roof plane than a standard kit does.

On slate this works particularly well, because there is no tile profile to accommodate and the flashing can sit tight against a flat surface.

It is worth asking about specifically on any designated building, and worth naming in a pre application conversation, because it can turn a marginal proposal into an acceptable one without moving to a more expensive specialist product.

The flashing detail in slate

A flat profile kit, matched to the window size.

Soakers interleave with the slate courses up each side, an apron sits at the sill, and a head flashing goes at the top with the slates above lapping over it.

The sequence is what makes it work: underlay collar first, dressed and lapped so water reaching the membrane passes around the opening; then the flashing; then the slates reinstated over the flashing rather than under it.

Because slate is flat, the flashing lies close and there is no channel or void for water to find under it. That is why slate installations, done properly, are among the most reliable in this trade.

Fixing replacement slates

Worth knowing because it looks like a shortcut and is not.

A slate in the middle of a slope cannot be nailed, because the head of the slate is under the courses above and you cannot reach it. A replacement is held with a slate hook or a tingle: a strip of metal fixed to the batten below, bent up over the tail of the slate.

A visible metal tingle is what a correct repair looks like. It is not a bodge, and a roofer who has nailed through the face of a slate to avoid one has created a hole in the covering.

What varies between slate roofs

Welsh slate is hard, thin, regular and extremely long lived. Century old slate is frequently still sound and can be lifted and reused.

Imported and reclaimed slate varies in thickness and quality. Some delaminates with age, and on those roofs a higher proportion breaks on removal.

Fibre cement slates look similar from the ground and behave differently: more brittle at the edges, and matching a weathered one is harder than people expect.

On any of them, an allowance for breakages is sensible on a roof of age, and the answer where an exact match cannot be found is to take sound slates from a hidden slope and put the replacements there.

What is underneath

On a Victorian or Edwardian slate roof that has never been stripped, frequently no underlay at all: slates on battens with the rafters visible from inside.

Modern weathering assumes a membrane to lap into, and where there is none the detail is built around the opening instead. Entirely normal on a period property, more time, and it should be priced rather than discovered on the day.

Some older slate roofs were torched: mortar applied to the underside of the slates between the battens, as a draught and weather barrier before underlay existed. Where that is present it complicates the work and it is worth flagging at survey.

The steep pitch advantage

Slate roofs are steep because slate needs the pitch, and that helps a rooflight in three ways.

Water sheds fast, so gravity does much of the weathering work and the flashing is doing less than it would on a shallow roof.

Headroom gain from a roof window is generous, so a modest unit produces a real improvement in a loft room.

And self cleaning glass works properly, because rain sheets off and carries deposits away rather than sitting and evaporating.

Where the constraint usually is

Not the covering, but the structure and the consent.

Slate roofs of this age are traditional cut roofs with rafters and a purlin, and the purlin is not something to cut. An opening goes above it or below it, which frequently decides the position before anything else does.

And slate is the covering of choice on a great many listed and conservation area properties, so on these roofs the consent question usually arrives before the technical one.

Reading the section drawing correctly

The document that decides a conservation application is the manufacturer section showing the projection of the unit above the roof.

Read which datum it uses. Some drawings show projection above the batten line and some above the finished covering, and on a profiled tile those are very different figures. On slate they are close, because slate is thin and flat, which is one reason slate installations photograph so well against the drawing.

The figure an officer cares about is projection above the finished roof surface, because that is what is seen from the ground.

Include the drawing with any submission as a matter of course. Its absence is the commonest reason an application sits in correspondence for weeks.

What the ground actually sees

Worth thinking about, because the analysis is usually done on a section and experienced in perspective.

A roof is rarely seen straight on. From a street or a lane, a slope is seen at an oblique angle, and at oblique angles a projecting unit casts a longer shadow and its upstand reads as a thicker line than the section suggests.

That is why the difference between a low profile unit and a standard one is more visible in reality than on paper, and it is the argument for the conservation product on any slope that is genuinely seen.

Recessed installation on slate

Worth naming specifically in a pre application conversation.

Several manufacturers offer a recessed flashing arrangement that sets the unit lower into the roof plane than a standard kit. On slate it works particularly well, because the covering is flat and the flashing sits tight with no profile to accommodate.

It can turn a marginal proposal into an acceptable one, and on an unlisted building in a conservation area it may allow a standard roof window where a conservation unit would otherwise be needed. That is a substantial saving.

Number and proportion

Two modest units frequently read better on a slate roof than one large one, because slate coursing is fine and small units sit within it comfortably.

Tall and narrow suits the material and suits the structure, since a unit between rafters cuts nothing.

Aligning units with each other, and with any existing openings or chimney, matters more on a slate roof than on a profiled tile, because the regular coursing makes any misalignment obvious.

The practical warning

Slate roofs of this age are traditional cut roofs with purlins, and the purlin position frequently decides where a unit can go before the visual analysis begins.

Establish that first. Designing a beautiful proposal that the structure will not accommodate wastes the pre application conversation.