Local Topic
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SOUTHWARK — Extending under the butterfly roof: Peckham’s M-shape problem (Doc A)
Peckham and Camberwell’s Victorian terraces hide their roofs behind straight parapets, and behind the parapet lies the borough’s signature quirk: the butterfly. Two pitches falling inward to a central valley — an M in section — draining through the middle of the house.
Charming, historic, and the single most important fact about extending these particular terraces, because every rear addition negotiates with a roof that sheds its water into its own spine.
The valley owns the strategy
On a conventional terrace, a rear extension meets a roof draining front and back — simple abutments. The butterfly drains to its centre, through a valley gutter running the house’s depth to an outlet at the rear — usually exactly where you’re about to build.
Extend across the back and the valley’s discharge, its overflow route and its future maintenance access all get redesigned, not obstructed.
An extended or re-routed outlet, overflow provision so one blocked hopper doesn’t fill the valley like a bath, and the NFRC-grade lining details these long internal gutters depend on.
The number of Southwark loft and rear projects that have discovered the valley’s importance via a ceiling stain is not small; the survey-first habit exists for this stock.
Structure follows the same logic. The butterfly’s inward pitches bear on the party walls and the central spine differently from a conventional roof, so opening the rear wall for the extension — Approved Document A work with an engineer’s numbers as ever. Gets designed with the roof’s actual load paths understood, not assumed from a standard detail.
Working with the M
Handled knowingly, the butterfly becomes a design gift. The parapet line gives rear extensions a clean datum to work beneath — flat-roofed additions sit naturally under the M’s outer shoulders.
And the valley’s rhythm across a terrace run is genuine townscape, which Southwark’s conservation appraisals for these streets recognise.
The Victorian Society’s affection for the type is well founded. Some schemes even borrow the language, echoing the M in the extension’s own roofline to knowing effect.
Design an access hatch to the valley while you’re at it. Future you, up there in November, sends thanks.
The rule for this stock compresses to one line: find the water before you draw the walls. The butterfly has been managing its odd geometry for 150 years.
Extend with its logic — outlet, overflow, access, then architecture — and it will manage yours for the next hundred too.