Walk down almost any Victorian or Edwardian street in London and the roofs follow a pattern. Welsh slate on the main roof. A butterfly roof hidden behind a front parapet. Shared chimney stacks on the party walls. A small roof over the rear addition. These houses were built well, but many of their roofs are now well over a century old.
This guide walks through the parts of a terrace roof that cause the most trouble, the signs you can spot without climbing, and what a roof survey looks at.
Welsh slate and the nails that hold it
Natural slate is a long-lasting material. Checkatrade puts its life at 100 years or more (Checkatrade slate guide). The weak point on an old slate roof is rarely the slate itself. It’s the nails. As they corrode, slates start to slip out of line. This is called nail fatigue.
One slipped slate after a storm is a small job. Slates slipping across several parts of the roof is a sign the fixings are failing more widely. Our guide to slipped slates and nail fatigue explains how to tell the difference.
Butterfly roofs and parapet gutters
Many London terraces have a butterfly roof. Two slopes fall towards a central valley gutter, which runs front to back and drains out at the rear. From the street you only see a flat parapet wall, so the roof is easy to forget.
The valley gutter and any gutters behind the parapet are often lined with lead. If leaves, silt or a slipped slate block them, water backs up and can overflow inside the house. Damp patches on top-floor ceilings near the middle of the house are a common sign. See butterfly roofs and valley gutters explained for more.
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Shared chimneys and party walls
Terraces often share chimney stacks along the party walls. Each stack has pots, flaunching (the mortar around the pots), pointing and flashings where it meets the roof. Weathered flaunching and cracked pointing let water into the stack. Loose flashings let it in at roof level.
Party-wall upstands between neighbouring roofs can also crack or lose their flashings. Because these parts are shared, repairs may need a conversation with the neighbours. A chimney survey records what we can see on your side.
Rear additions and later extensions
The back addition usually has its own small roof. It may be slate, a later tile, or a flat roof added in a past renovation. Where it meets the main wall there’s a flashing, and that junction is a frequent leak point. Flat-roofed extensions and dormers added in later decades bring their own faults, such as ponding and splits.
What a survey looks at on a terrace
A roof survey on a terraced house looks at the slopes, ridges, valley and parapet gutters, chimneys, flashings, rear roofs and the roof space where there’s safe access. We inspect what can be seen safely: from the ground, from windows, from the loft and, where applicable, by drone. The written report has photographs and recommendations, prioritised by urgency.
If you’re buying, a pre-purchase roof survey gives you specific points to raise with the seller. Our case studies include a Victorian terrace in Islington.
Quick answers
Are slate roofs on Victorian terraces usually original?
Some are. Many have been re-roofed at least once, sometimes with different slate or with concrete tiles. A survey notes what’s there now and its condition.
Can you see the valley gutter from the street?
Usually not. It sits behind the front parapet. It can sometimes be seen from a rear window, from the loft hatch or by drone where applicable.
Do I need my neighbour’s permission for a survey?
We only inspect what you can give us access to. Shared parts like stacks can often be seen from your side, but any work on them is a matter between neighbours.
Related services
More advice
- Butterfly Roofs and Valley Gutters Explained
- Slipped Slates and Nail Fatigue Explained
- Chimney Stack Defects: Flaunching, Pots, Leans
- Lead Flashing Problems on Older Roofs
- All advice
This guide is general information about roofs and roof surveys. It isn’t legal, insurance or structural engineering advice; for those, speak to a suitably qualified professional.