Butterfly Roofs, Valley Gutters and Parapet Walls in London

Walk down almost any Victorian street in Islington, Camden, Kensington, Lambeth or Southwark and you will see flat looking fronts with no roof visible at all. Behind the front parapet there is usually a butterfly roof, also called a London roof or V roof. These roofs are a big part of London's character, and they are also one of the most common sources of leaks we investigate.

What Is a Butterfly Roof?

A butterfly roof is made of two slopes that fall inwards from the party walls to a central valley gutter, which runs from the front of the house to the back. Rainwater runs down both slopes into the valley, along to the rear, and out through a hopper or outlet in the back parapet wall. From the street, the roof is hidden behind a parapet, which is why problems often go unnoticed for years.

Why Butterfly Roofs Leak

Valley gutter failure. The central valley is usually lined with lead or, on repaired roofs, felt or a liquid coating. Splits, failed joints, inadequate falls and patch repairs all let water into the house, often well away from where it appears inside.
Blocked outlets and hoppers. Leaves, moss and debris collect where the valley discharges at the rear. When the outlet blocks, water backs up the valley and over the lining.
Parapet walls. Parapets are exposed on both faces and on top. Cracked copings, eroded brickwork and failed flashings let water soak down into the wall and the rooms below.
Party wall abutments. Where the slopes meet the party walls, the junction is often finished with a mortar fillet rather than lead. These crack and let water in.
Box and parapet gutters. Gutters set behind the front parapet have the same weaknesses as valley gutters: small falls, hidden joints and blockages.

Signs to Look For Inside

Staining along the line of the central valley on top floor ceilings
Damp patches high up on the front or rear walls, below the parapet
Staining at the junction with the party walls
Recently redecorated top floor ceilings, which may be covering past leaks

Why These Roofs Are Hard to Inspect

Butterfly roofs cannot be seen from the street, and there is often no loft hatch, or only a very small void, so they can be missed in a standard survey. Many basic surveys simply note that the roof could not be seen. For a top floor flat or a house with a butterfly roof, that is the part of the building most likely to cause you problems.

We inspect these roofs with a drone and a 10 metre pole mounted camera, which lets us see the valley, the outlets, the parapets and both slopes closely without scaffolding or access to the roof itself.

What We Report On

The condition of the valley gutter lining, its joints and falls
Outlets, hoppers and rainwater pipes
Parapet copings, brickwork and flashings
Party wall abutments
The slopes, slates and fixings
The likely cause of any staining or leaks found inside

Buying a Top Floor Flat?

If you are buying a top floor flat under a butterfly roof, the roof may be the freeholder's responsibility, but leaks will affect you first. A roof survey before you exchange tells you what condition it is in and what work may be coming, which is useful when you look at the service charge and any planned major works. See our Flat Buyers Survey for a full Level 3 survey of a flat.

More London Roof Guides

Arrange a Roof Survey in London

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.