Lintel (lintel level)

Walls cannot carry their own weight across an open doorway or window. A lintel bridges the gap and spreads the load to the wall on each side.

In Nigerian building. Most houses use cast-in-place reinforced concrete lintels. Builders often run one continuous band of concrete around the building at the head of the openings, tying the walls together; some call this a ring beam or a "full lintel". A "half lintel" covers only the openings and is cheaper. Which one a building needs is a design decision for the engineer.

The stage. After DPC, the blockwork rises. When it reaches the top of the doors and windows, the formwork and reinforcement go in and the lintels are cast. That is "lintel level". After it, the walls are taken up to roof height, or the upper floor is cast (decking).

Sellers and contractors use the stages to describe progress and to stage payments: "building at lintel level for sale".

Alternatives. The Nigerian Building and Road Research Institute has studied corbelled brick or block arches over small openings as a cheaper alternative to concrete lintels.

Where lintel level sits in the costs. By lintel level, most of the blockwork for a floor is done, so a large share of the walling cost has been spent. After it come the next major costs: the decking for an upper floor, or the roof structure and sheets for a bungalow. Builders' payment schedules often put a milestone at lintel level for this reason.

Buying a building at lintel level. Ask for the drawings and building approval the work follows, and have an engineer check the columns, lintels and blockwork before you pay.

Inspecting lintels. In a finished house, lintels are hidden behind plaster. Cracks that start at the corners of doors and windows are the usual sign of a problem and are worth an engineer's look.

• Across Nigeria: reinforced concrete lintels are standard in sandcrete block construction.
• Some rural and low-cost projects: precast lintels or corbelled openings are used to cut cost.