West 3 Mechanical Contractors Ltd v Mizen Design Build Ltd

A fixed-price subcontractor’s obligation is determined by the contractual documents, including expressly incorporated amendments. A stated pipe size forming part of the agreed price was not merely a provisional allowance. A later drawing does not vary the subcontract where the contract requires a variation to be instructed, agreed in scope and value, recorded in writing, and signed before execution. Responsibility for whether the specified installation design satisfies the applicable pressure-drop criterion rests with the party having design responsibility. A subcontractor who installs the specified pipe in accordance with its contractual obligation is not in breach merely because the design proves technically inadequate.

West 3 was the mechanical subcontractor and Mizen the design and build contractor for a residential development. The subcontract required the installation of gas distribution pipework. Following a change to the location of the gas meters, the parties agreed that pipework would run from a basement meter room through risers to the flats, and West 3 priced the work on the basis of 1-inch TracPipe.

The installed system caused excessive pressure loss. Mizen relied on later revised drawings showing larger pipework and contended that West 3 was responsible for failing to install it. The court tried a preliminary issue concerning the contractual obligation, any variation, the technical adequacy of the original and revised systems, and whether the defect resulted from West 3’s breach.

1. Contractual obligation. The Agreement incorporated the original drawings subject to the amendments recorded at the pre-order meeting. Those amendments required TracPipe from the basement meter room through the risers into the flats. The subsequent exchange of emails formed part of the Agreement and established that the agreed price included 1-inch TracPipe. That was a firm contractual term, not a provisional or representative allowance.
2. Design responsibility. West 3 had no contractual design responsibility for selecting a pipe size capable of satisfying the 1 mbar pressure-drop requirement in para 5.2.2 of BS6891. The design obligation, including the obligation under para 5.1.1, rested with Mizen and its designers. West 3 was required to follow the drawings as amended.
3. No variation. Clause 8.1 provided the exclusive contractual mechanism for varying the Sub-Contract Works. A variation required an instruction, written agreement of both scope and value, and signature by an authorised representative of Mizen before execution. The later drawings, even if received, did not satisfy those requirements. The revised drawings were also not proved to have been issued to West 3.
4. Technical findings. The original system using 1-inch TracPipe would have failed to provide sufficient pressure to a substantial number of flats. The revised design was practically capable of implementation, but on the balance of probabilities would still have exceeded the 1 mbar pressure-drop limit in Flat 41 by a small margin.
5. Causation and declarations. West 3 installed the pipe required by the Agreement. The gas pressure defect therefore did not result, wholly or partly, from breach by West 3. The court proposed declarations confirming the contractual pipe size, absence of design responsibility, absence of variation, the technical findings, and the absence of breach.