First Fix

First fix is the initial stage of electrical, plumbing, or mechanical installation work, carried out while the building structure is still open before walls, ceilings, or floors are enclosed. It covers all the concealed elements built into the fabric of the building: cable runs, conduit, back boxes, and wiring looms in electrical work; pipe runs, fittings, and manifolds in plumbing and heating. Nothing visible is installed at first fix. Everything installed at first fix will be hidden once the finishing trades complete their work.

The First Fix Window and Programme Timing

First fix is time-critical on any construction project because it must be complete before plastering, drylining, or floor-finishing trades can proceed in each zone. On a commercial new-build, the first fix window for electrical work typically opens once the structure is watertight and internal partitions are erected, and closes when the plasterer arrives. Miss that window - because the electrician was delayed, materials did not arrive, or the programme was not co-ordinated - and boarding or plastering cannot start. Every day of delay at first fix ripples through the programme.

This is why trade contractors need clear programme information ahead of time: not just when to start, but when each zone will be accessible, which other trades are working in the same areas simultaneously, and when first fix in each zone needs to be complete so the finishing trades can proceed. Co-ordination between electrical, plumbing, and mechanical services contractors at first fix stage prevents costly clashes between pipe runs and cable trays or competing trades occupying the same ceiling void.

Documentation and Sign-Off at First Fix

Most contracts require an inspection or sign-off before walls and ceilings are enclosed. On electrical installations, this typically means a continuity and insulation resistance test on concealed circuits before boarding or plastering. On larger projects, the main contractor or clerk of works may require photographic evidence of the installation before closing in.

This documentation is also self-protection. If a fault is later traced to a concealed element and the contractor cannot produce records of what was installed and tested, they face a difficult defect dispute. Photographing the installation zone by zone, referenced against the drawings, takes minutes on site and can resolve a claim months or years later.