Victorian Slate Roofs in London: A Guide for Owners and Buyers

A large share of London's housing was built between about 1840 and 1914, and most of those houses were roofed in natural Welsh slate. Many of these roofs are now well over a hundred years old. Some are still sound, some have been re-covered once or twice, and some are living on borrowed time. This guide explains how these roofs were built, what tends to go wrong, and what we look for when we survey one.

How a Victorian Slate Roof Is Built

Slates nailed to timber battens, usually Welsh slate on the original roofs, sometimes replaced with Spanish or other imported slate, or with man made slate or concrete tiles
Battens fixed across the rafters, often with no underlay at all on an original roof
Torching (lime mortar applied to the underside of the slates) on some roofs, to reduce wind blown rain and draughts
Lead or mortar at the junctions: valleys, abutments, chimneys and parapet walls
Party walls rising through the roof between terraced houses, which affects how water and fire move between roofs

Nail Fatigue: The Most Common Failure

Natural slate can last a very long time, but the iron nails that hold it often do not. Over many decades the nails corrode, and slates begin to slip or drop. Signs include:

Slipped slates, or slates held up with metal clips (tingles)
Gaps in the courses, or broken slates in the gutter
Patches of replacement slates that do not match the rest of the roof
Daylight visible through the roof from inside the loft

A few slipped slates can be refixed. When slipping is widespread across a slope, the fixings have usually reached the end of their life, and the slope will generally need stripping and re-covering rather than piecemeal repair.

Other Common Defects

Delaminating or flaking slates, where poorer quality slate splits in layers and becomes porous
Heavy replacement coverings, such as concrete tiles laid on a roof structure designed for lighter slate, which can cause rafters to sag or spread
Coated or sprayed roofs, where slates have been sealed with a foam or bitumen treatment, which can trap moisture and makes the slates hard to reuse
Failed mortar fillets at abutments and chimneys, which should usually be replaced with lead flashings
Worn ridge and hip tiles, with loose bedding mortar
Condensation in lofts where insulation has been added without adequate ventilation

Repair or Re-cover?

This is the question most owners and buyers want answered. Repair is reasonable when defects are isolated and the slates and fixings are generally sound. Re-covering is usually the right decision when slipping is widespread, the slates themselves are failing, or the roof has been patched so many times that further repair is not economical. In a terrace, the condition of the neighbouring roofs and shared details such as party wall junctions also matters.

What We Look For in a Roof Survey

The type, condition and likely remaining life of the slates and their fixings
Ridges, hips, verges, valleys, chimneys and all leadwork
Gutters, parapets and rainwater goods
The roof structure, underlay, insulation and ventilation from inside the loft where accessible
Signs of past or current leaks, and their likely cause

We use a drone and a 10 metre pole mounted camera, so we can usually inspect the whole of a London roof closely without scaffolding, including slopes hidden behind parapets and rear roofs that cannot be seen from the street.

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Prowle & Co LLP are residential building and roof surveyors covering every London borough and the Home Counties. Prowle & Co LLP is regulated by RICS. Our advice is fully independent. Call 07731 939956 or email lee@prowleco.co.uk with the property address, or read more about our roof surveys.