Board-marked concrete records the timber it was cast against: every knot, every joint, every plank. Done well, it is one of the warmest materials we know. Done badly, it looks like a car park. This is a short guide to how we get it right — and how we cut its carbon in half.

Choosing the boards

We use rough-sawn spruce, 22 mm thick and 120 mm wide, used no more than three times so the grain stays sharp. Boards are wetted the day before the pour so they do not draw water out of the concrete and leave pale, dusty patches on the finished face.

Laying out the pattern

Every board joint is drawn. We set out the boards on elevation drawings before anything is ordered, lining joints up with window heads, floor levels and corners. On site, the pattern is the first thing a visitor reads, so it deserves the same care as a façade.

The mix

Our standard mix replaces 50 percent of the Portland cement with GGBS, a by-product of steel making. It cures a little slower and a little lighter in colour, but it cuts the embodied carbon of the concrete by around 45 percent and gives a beautiful soft grey.

Test panels first

Before any pour, the contractor casts a full-height sample panel on site. We test release agents, vibration times and curing blankets until the client, the engineer and the architect all agree. The approved panel stays on site as the benchmark for every wall that follows.

Pouring and vibrating

Pours are planned in lifts of no more than 1.2 metres, vibrated carefully so the concrete fills every grain without bleeding through the joints. The worst defects — honeycombing and cold joints — almost always come from rushing this step.

Curing and protecting

The formwork stays on for at least three days, then the wall is wrapped in wet hessian for a week. Finished walls are protected with boards until the building is closed in. Concrete is surprisingly fragile in its first month.

Repairs and patina

Small blemishes are part of the material, and we leave most of them. Larger repairs are made by a specialist with a mortar matched to the test panel. Over the years, the surface softens and darkens slightly, and we think it gets better with age.

Where we use it

We use board-marked concrete where a building meets the ground — plinths, retaining walls, stairs and cores — and keep timber for the rooms people touch. The contrast between the two is one of the quiet signatures of our work.