Butterfly Roof: How It Works, Pros, Cons and Drainage

A butterfly roof is a V-shaped roof where two slopes tilt inward towards a low central line, like a butterfly’s wings raised upwards. Instead of rainwater running off the outside edges, it collects in a central valley and drains away at the ends. The shape is popular in modern and contemporary architecture because it creates striking lines and room for tall windows.

This guide explains how a butterfly roof works, its pros and cons, the roof coverings used, the drainage issues to watch and whether it suits UK weather. For an overview of other roof shapes, see our guide to roof types explained.

What Is a Butterfly Roof?

A butterfly roof is made of two opposing sloping surfaces, each rising towards the outside edge of the building and falling towards the middle. It is the opposite of a gable roof, where the two slopes rise to a central ridge. You can think of it as two mono pitch roofs facing each other. Our mono pitch roof guide explains how each half works.

The low point in the centre is the valley, and it normally carries a gutter or channel that moves water to downpipes at the ends of the roof or through scuppers.

How a Butterfly Roof Works

1. Two opposing slopes: each side tilts inwards at a gentle pitch.
2. Central valley: water from both slopes runs to the lowest point.
3. Internal gutter or channel: a lined channel carries water along the valley.
4. Outlets: downpipes at the ends, or overflow scuppers, take the water away.
5. Waterproof covering: a membrane or metal covers the slopes and the valley.

Because the central valley has to carry water from the whole roof, its size and detailing are critical. This is where most problems start.

Advantages of a Butterfly Roof

• Distinctive modern look. The V-shape is bold and contemporary.
• High windows and light. The raised outer edges allow tall clerestory windows, which bring daylight deep into the building.
• Rainwater harvesting. The central channel collects water in one place, which makes it easy to store for garden use.
• Good for solar panels. Each slope can face the sun if the building is oriented well.
• Raised ceilings. Interior spaces can feel taller at the edges.
• Works at low pitch. The slopes can be shallow, which keeps the overall height down.

Disadvantages of a Butterfly Roof

• Drainage risk. The central valley takes the water from the whole roof, and a blockage or undersized outlet can cause overflow into the building.
• Leaks at the valley. It is the most exposed detail and needs careful waterproofing.
• Snow and leaves. They collect in the valley, so regular clearing is needed.
• Higher cost. Internal gutters, special detailing and structural support cost more than a simple pitched roof.
• Specialist design. The roof needs proper engineering for loads and drainage.
• Less common in the UK. Fewer roofers have experience with it, so choose carefully.

Is a Butterfly Roof Suitable for UK Weather?

It can be, but the UK’s rainfall, wind and leaf fall mean that the design must be right. A butterfly roof needs generous drainage capacity, overflow protection and easy access for maintenance. In heavy rain, a small channel can fill quickly, and without an overflow, water can run back into the building. With good design and regular care, the roof can work well, and many modern UK homes use it successfully.

Roof Coverings for a Butterfly Roof

Because the pitch is usually low, the covering must be suitable for shallow slopes and for the valley.

• Single ply membrane: seamless, flexible and well suited to the valley. See our guide to flat roof materials for the main options.
• EPDM rubber: common on residential butterfly roofs, with large sheets and few joints.
• Standing seam metal: a sleek, modern covering that works at low pitch. See our guide to the standing seam roof.
• GRP or lead in the valley: often used as the lining for the central channel.

Butterfly Roof Insulation

A butterfly roof is normally built as a warm roof, with insulation above the deck, to keep the structure warm and reduce the risk of condensation. Tapered insulation can be used to form the slopes and the fall to the valley. Our guide to warm roof construction explains the layers.

Drainage: The Most Important Part

On a butterfly roof, the central gutter is part of the roof, not just an accessory. It needs:

• Enough width and depth to carry the water from both slopes in heavy rain.
• A continuous fall towards the outlets, so water does not stand in the channel.
• Properly sized outlets, with leaf guards and easy access for cleaning.
• An overflow route, such as a scupper, so excess water leaves outside the building.
• A well-detailed lining, with welded or seamless joints in the valley.

Our guide to roof valleys and guttering explains how valleys carry water and what can go wrong.

Butterfly Roof vs Other Roof Types

Planning and Building Regulations

A butterfly roof is an unusual shape, so planning officers may look closely at how it fits with the street. In conservation areas, listed buildings and many suburban streets, the shape might not be allowed. Building regulations apply to the structure, insulation, drainage and fire safety. Check with your local planning authority and building control before you finalise the design.

Common Butterfly Roof Problems

• Blocked outlets: leaves and debris stop the water from draining.
• Ponding in the valley: poor falls or sagging structure leave water standing.
• Failed valley lining: joints and seams split or open.
• Overflow into the building: an undersized channel or missing scupper.
• Heavy snow loading: snow collects in the centre and adds load.

Check the valley and outlets at least twice a year, and after storms.

Questions to Ask Your Architect or Roofer

• How was the size of the central channel calculated?
• Where will the overflow go if the outlets block?
• What covering and lining will be used in the valley?
• How will I get safe access to clean the roof?
• What is the guarantee, and does it cover the drainage detail?
• Has this roofer built a butterfly roof before?

Frequently Asked Questions

What is a butterfly roof?

A butterfly roof is a V-shaped roof where two slopes tilt inward towards a central valley, which drains the water to the ends of the roof.

Why is it called a butterfly roof?

The V shape looks like the raised wings of a butterfly.

Are butterfly roofs prone to leaks?

The central valley is the weak point, so poor design or blocked drainage can cause leaks. With good detailing, overflow protection and maintenance, they can perform well.

Are butterfly roofs allowed in the UK?

Yes, but planning permission may be harder to get in conservation areas and traditional streets. Check with your local planning authority.

What is the best covering for a butterfly roof?

Seamless coverings such as single ply membrane, EPDM or standing seam metal are common choices, with a well-detailed lining in the valley.

Can a butterfly roof collect rainwater?

Yes. The central channel makes it easy to collect water in one place for storage and garden use.

Final Thoughts

A butterfly roof offers a striking look, tall windows and easy rainwater collection, but it demands careful design and regular maintenance. Pay close attention to the size of the channel, the outlets and the overflow, choose a seamless covering and use a roofer who has built one before.