Cracking Along Bay Joints and Day Joints in Liquid Screed

A bay joint is planned in advance as part of the pour sequence, often using a proprietary joint former or formwork, so that the floor is divided into manageable, roughly square-ish sections. A day joint, by contrast, is usually less planned — it occurs where the pump breaks down, a delivery is delayed, or the working day simply ends before the area is complete.

Because a day joint edge is not always prepared to the same standard as a designed bay joint, it can be more prone to a visible line, a slight difference in surface level, or localised cracking along its length.

The first question is whether the crack runs exactly along the joint line or diverges from it — a crack that leaves the joint and travels into open screed suggests restraint or subfloor movement rather than a joint-related issue. The second is whether there is any vertical step or hollow-sounding area either side of the joint, which would point to a bonding problem at the interface between the two placements.

Where a floor is due to receive underfloor heating, joint positions also need to be considered alongside the pipe layout, since a poorly placed joint crossing directly over a manifold zone or pipe run adds unnecessary complexity to any future repair.

A competent installer plans bay sizes with reference to the room geometry, keeping joints away from doorway thresholds and heavy-traffic pinch points where possible, and aiming for roughly proportioned sections rather than long thin strips that are more prone to curling at the edges. Joint formers are usually removed or trimmed once the screed has set enough to hold its own edge.

Where a day joint becomes unavoidable, good practice is to cut back to a clean, vertical, well-compacted edge before resuming the pour, rather than simply abutting fresh screed against a feathered or already-drying edge, since a tapering edge bonds far less reliably than a square one.

Where a joint crack is confirmed as static and following the joint line, it is typically treated with a flexible sealant or resin appropriate to the flooring finish planned above it, rather than being ground out and rebuilt. Where movement is ongoing, monitoring over several weeks alongside temperature and heating cycle notes helps establish whether the joint is still settling or has a more persistent cause.

If several joints across a floor show similar cracking, this points toward a systemic issue with how the joints were formed or protected during the pour, which is worth raising with whoever managed the original installation rather than treating each crack in isolation.