Glossary
Brick bonds and matching them on a London extension
The pattern in which bricks are laid (Flemish, English, stretcher). Important for conservation-area matching.
The pattern in which bricks are laid (Flemish, English, stretcher). Important for conservation-area matching.
Brick bond is the pattern in which bricks are laid. It matters in planning because matching the existing bond is one of the details conservation officers check, and it matters structurally because solid-wall bonds such as Flemish and English tie the two leaves together, while stretcher bond indicates a cavity wall.
Stretcher bond — all headers hidden — is the signature of a cavity wall and is near-universal after about 1930. Flemish bond, alternating headers and stretchers in each course, and English bond, alternating courses of headers and stretchers, both indicate a solid wall and are typical of Georgian and Victorian stock.
Reading the bond from the street tells you the wall construction before anyone opens anything up, which in turn decides the Part L insulation strategy and whether a new opening needs a cavity closer or a solid-wall lintel bearing.
In a conservation area, and on any locally listed building, new brickwork is expected to match the existing in bond, brick type, mortar colour and joint profile. Applications are commonly conditioned to require a sample panel on site for approval before the main brickwork proceeds.
Getting this wrong is one of the most visible ways an extension reads as an addition. Matching a soft red London stock in Flemish bond with a machine-made wirecut in stretcher bond will be noticed.
The elevation drawings should annotate the bond, the brick specification and the mortar mix and joint. A lime mortar — commonly NHL 3.5 — is specified for historic solid walls, because a cement mortar is harder than the brick and drives decay into the brick face.
A worked Barnet example of brick bond: with 20 conservation areas — the Hampstead Garden Suburb Conservation Area and the Mill Hill Conservation Area among the best known — London Borough of Barnet (Barnet Council) sees brick bond constantly, and there is no borough-wide Article 4 direction, so permitted development generally survives outside conservation areas. Where brick bond pushes a Barnet scheme out of permitted development, the route becomes a £548 application decided in 8 weeks rather than a £249 prior approval in 6 weeks.
| Cavity wall indicator | Stretcher bond |
|---|---|
| Solid wall indicators | Flemish bond, English bond, header bond |
| Typical solid wall thickness | 215 mm (one brick) or 328 mm (one and a half) |
| Historic mortar | Lime, typically NHL 3.5 |
| Standard brick format | 215 × 102.5 × 65 mm |
| Typical condition | Sample panel approval before main works |
Look at the bond. Visible headers in a regular pattern usually mean solid; all stretchers usually means cavity.
In a conservation area, closely. Elsewhere, 'similar' is normally the wording used in the condition.
It is best avoided. Cement is harder than a soft stock brick and causes spalling.
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