Why Do Older Slate Roofs Develop Leaks?

There Is Probably Nothing Under Your Slates

A modern roof has two barriers: the covering, and an underlay beneath it that catches whatever gets past and drains it to the gutter. On most Victorian and Edwardian roofs there is no second barrier at all.

The slates were laid directly on battens over open rafters. You can stand in the loft of a Birkenhead terrace and see the underside of the slates. That was normal practice and it worked, because the covering was laid with enough lap that water could not get past it.

It also means the covering has no margin. On a modern roof a small defect is caught by the underlay and shows up as nothing for years. On an unlined slate roof the same defect is a leak the first time it rains hard.

Torching, and Why It Crumbles

A lot of these roofs were torched - lime and hair mortar buttered onto the underside of the slates between the battens, from inside, to seal the gaps against wind-driven rain and snow.

Torching does not last forever. It dries, loses its key and drops off, and when it goes it goes as loose material that ends up on the loft floor. If you have found crumbly grey-brown mortar lying between the joists, that is what it is.

Once the torching has gone, the roof is relying entirely on the lap of the slates, and on an exposed elevation that is often not enough in driving rain. The roof did not change; the thing that was helping it stopped.

Torching cannot really be renewed in situ in a way that lasts. Where it has failed the honest options are living with it, improving the exposed details, or - when the slope is stripped for other reasons - putting a proper underlay in.

Headlap, and Water That Runs Uphill

Slates work by lap: each slate covers the head of the one two courses below it, and the amount of that cover is the headlap. Get it right and water cannot reach the nail holes. Get it wrong and it can.

Two things reduce headlap on an old roof. Slates creep downwards as their fixings age, taking a few millimetres off the lap each time. And previous repairs sometimes use slates that are slightly short, which looks fine and quietly reduces the cover.

The other mechanism is capillary action. Where two slates sit tightly together, water can be drawn sideways and even upwards along the joint - the gap is small enough for surface tension to beat gravity over a short distance. It is why headlap requirements go up on shallow pitches and exposed sites, and it is why an old roof on a shallow pitch leaks in weather that a steeper one shrugs off.

The Other Usual Suspects

On terraced Birkenhead stock the leak is very often not the slate field at all. It is the valley between back additions, the box gutter behind a parapet, the lead around a shared chimney stack, or the junction where an outrigger roof meets the main building.

All four carry more water than the slopes around them, all four are invisible from the street, and all four are older than anybody currently living in the house.

Who Deals With It

A great many Birkenhead properties are let or converted, and that changes what happens next rather than what is wrong.

If you rent, the roof is the landlord's responsibility rather than yours - the obligation to keep the structure and exterior in repair sits with the landlord under section 11 of the Landlord and Tenant Act 1985 and cannot be contracted out of in an ordinary short tenancy. Report it in writing and keep a copy.

If you own a flat in a converted house, the roof is usually the freeholder's responsibility, maintained through the service charge and apportioned by the lease. Check the lease rather than assuming, because conversions vary.

We are happy to work through agents and to provide a written assessment naming the defect, which is what somebody authorising the spend actually needs.