If you live in or are buying a Victorian or Edwardian terrace in London, there’s a good chance it has a butterfly roof. It’s also called a London roof or an inverted roof. From the street you can’t see it at all, just a flat parapet wall. Behind that wall the roof slopes down to the middle, not up.
This design works well when it’s looked after. When it isn’t, the central valley gutter becomes the weak spot of the whole house.
How a butterfly roof works
A butterfly roof has two slopes that fall inwards, usually towards the party walls on each side and down to a central valley. Rain runs into a valley gutter that runs from front to back. At the rear it drains through an outlet, often into a hopper and downpipe.
At the front, a parapet wall hides the roof from the street. There’s often a gutter behind the parapet too. Both gutters are traditionally lined with lead, laid in sections with steps (drips) so water can’t run back under the joints.
Why valley gutters leak
A valley gutter collects water from both slopes, so it carries more than any other part of the roof. Common faults include:
- Blockages. Leaves, moss, silt and slipped slates collect in the gutter and at the outlet.
- Splits and cracks in the lead. Lead expands and contracts with heat. Sections laid too long, or fixed too tightly, can crack over time.
- Failed patches. Bitumen or tape repairs over old lead may hold for a while, then trap water.
- Poor falls. If the boards under the lining have sagged, water sits instead of draining.
- Blocked rear outlets and hoppers. Water backs up along the whole length of the gutter.
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Signs of a problem inside the house
Leaks from a valley gutter usually show on the top floor, along the centre line of the house. Look for staining or bubbling paint on ceilings, damp around the loft hatch, or marks on the rear wall below the outlet. Water can travel along timbers before it drips, so the stain may not sit right under the fault. Our guide to why roof leaks are hard to find explains why.
Outside, look from a back window at the rear outlet and downpipe. Water spilling down the wall in heavy rain suggests a blockage.
What to check before buying
Butterfly roofs are easy to miss on a viewing because you can’t see them. Before you exchange, it’s worth knowing:
- whether the valley gutter is lead or a later replacement material
- whether there are signs of past leaks on top-floor ceilings
- whether the rear outlet, hopper and downpipe are clear
- the condition of the slopes, party-wall upstands and chimneys either side.
A pre-purchase roof survey covers these points and gives you a written report to take to the seller.
Keeping the valley clear
Most valley gutter problems start with debris. Having the gutter cleared regularly, especially after autumn leaf fall, is the simplest protection. Don’t climb onto the roof yourself. Parapets, valleys and old slate are no place to work without proper access. A gutter survey or a general roof survey will show what state the valley is in now.
Quick answers
Is a butterfly roof a bad thing?
No. It’s a traditional design that works well when the valley gutter, outlet and slopes are kept in good order. Problems usually come from neglect.
How do I know if my house has a butterfly roof?
From the street the front looks like a flat parapet with no visible slope. From the back you’ll often see a V shape, with the outlet in the middle.
Can a drone see into the valley gutter?
Often, yes, where a flight is possible. A drone can photograph the valley from above without anyone standing on the roof. Rules and weather decide whether it can fly.
Related services
More advice
- Victorian Terrace Roofs in London: Common Defects
- Lead Flashing Problems on Older Roofs
- Gutter and Downpipe Problems That Cause Damp
- Why Roof Leaks Are So Hard to Find
- 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.