Butterfly Roofs in North London: Why the Lead Channel Matters

Tollington Surveyors

Sep 26

5 min read

Butterfly roofs, often referred to as London roofs, are a familiar feature of period properties across North London, particularly in Camden and Islington. They are commonly found on Victorian terraces dating from the later 19th century, including many houses built around the 1880s.

Unlike a conventional pitched roof, where rainwater generally runs towards gutters at the front and rear, a butterfly roof has two roof slopes falling inwards towards a central valley. This valley is traditionally lined with lead and forms a concealed drainage channel through the centre of the roof.

From street level, much of the roof can be hidden behind the front parapet. This is part of what gives many Victorian terraces their characteristic appearance, but it can also mean that problems developing within the central valley are not immediately obvious from outside.

The lead channel is the important part

The central lead channel collects rainwater from both roof slopes. On many traditional London roofs, the drainage arrangement is relatively simple, with the channel relying on a single outlet, often discharging via a hopper and rainwater downpipe.

This is one of the main vulnerabilities of the design. Leaves, moss and debris can restrict the outlet, causing water to back up within the channel during heavy rainfall.

Older leadwork can also suffer from fatigue cracking, splits, failed joints, defective flashings and historic patch repairs. Lead needs to be correctly detailed and laid with sufficient falls towards the outlet. Where a channel has sagged, distorted or been poorly repaired, areas of standing water can develop.

The problem with a concealed valley is that relatively small defects can allow water into the building below. Repeated wetting can eventually affect masonry, plasterwork and the supporting roof timbers.

What does a surveyor look for?

Inspection of a butterfly roof requires more than simply looking at the slates.

Internally, these roofs commonly create two separate roof voids, one on either side of the central valley. Access into these spaces is often restricted and, importantly, they may provide little or no direct view of the lead channel itself.

A dry-looking loft therefore does not necessarily mean that the central valley is watertight.

One of the most useful areas to inspect is the ceiling and wall junction directly beneath the central channel. We look carefully for historic water staining, discolouration, blistered decorations, softened plaster, cracking, salt deposits and areas of noticeably recent redecoration.

This is often where the building starts to tell you what has been happening above.

Where a lead channel has overflowed or leaked, water does not always appear directly beneath the original defect. Moisture can travel along roof timbers, masonry and ceiling construction before becoming visible internally. Previous staining can therefore provide useful evidence even where everything is dry on the day of inspection.

We also look for staining to rafters and joists, deterioration around the valley supports, distortion or sagging to the roof structure, dampness around parapet walls and evidence of previous repairs.

Externally, the condition of the lead channel, outlets, flashings and adjoining roof coverings is particularly important. Where direct access is not available, high-level inspection may be assisted by a drone, although this cannot replace close physical inspection where a defect is suspected.

The front parapet can provide clues

The butterfly roof is normally hidden behind the front parapet wall. Visible deterioration to this area can therefore provide useful clues about conditions behind it.

Open mortar joints, defective copings, weathered brickwork, staining or vegetation growth can all indicate prolonged exposure to moisture. Where the central channel or outlet is not draining correctly, saturation can also occur within adjoining parapet masonry.

Parapet walls are themselves particularly exposed parts of a building. Their copings, pointing and flashings need to prevent water entering masonry from above while the concealed roof drainage behind them needs to remain clear.

Not every defect means that the roof requires replacement. Many original butterfly roofs continue to perform satisfactorily when the leadwork, outlets, flashings, roof coverings and masonry are maintained correctly.

Butterfly roofs, loft conversions and mansard roofs

Butterfly roofs can also present opportunities for roof alterations and loft conversions, particularly where neighbouring properties have already established a pattern of roof extensions.

One option seen across London is to substantially alter the original roof and construct a mansard roof extension. A mansard typically provides much more usable internal space than attempting to create accommodation within the relatively shallow original butterfly roof.

Camden's planning guidance specifically recognises the possibility of mansard extensions to butterfly roofs where a roof extension is acceptable in principle. Its guidance describes retaining the existing parapet and forming the new roof behind it, subject to appropriate design and detailing. Business Portal

However, the presence of a butterfly roof should never be taken to mean that a mansard conversion will automatically obtain permission.

The established roof form of the terrace is important. Where neighbouring houses already have sympathetic mansard extensions, there may be an established precedent for roof alterations. Conversely, an intact and consistent historic roofline may be an important part of the character of the street, particularly within a conservation area. Islington's wider design guidance says the extent of existing roof alterations and the consistency of the terrace are relevant considerations when roof extensions are assessed. Islington Council Conservation areas require particular care. Islington has 42 conservation areas and additional controls apply to alterations in them; its guidance also confirms that external roof extensions in conservation areas require planning permission. Individual conservation-area guidance can impose considerably tighter restrictions.

This is why two apparently similar Victorian houses a few streets apart may have very different prospects for a mansard extension.

Before purchasing a property with the intention of carrying out a loft or mansard conversion, the planning history of the property and neighbouring terrace should therefore be checked. The relevant conservation-area guidance, any Article 4 Direction, Building Regulations requirements, structural implications, party wall matters and any restrictions affecting the property should also be considered.

The existing central valley drainage arrangement is especially important because a mansard conversion substantially changes the original roof structure and rainwater disposal. The new roof must be properly designed so that gutters, outlets, flashings and falls discharge water effectively without creating concealed areas where water can pond.

Where the property is leasehold or has been converted into flats, there are additional considerations. Ownership of the roof space, rights to alter the structure and the consent of the freeholder should be established before assuming that a roof conversion is possible.

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Contact us today to arrange a RICS HomeBuyer or Building Survey — our process is straightforward, our advice is independent, and you will always deal directly with a qualified local surveyor.