Unilaterally Amending Payment Terms By Conduct Alone…Nice Try!

As a contractor, you might find yourself in a situation where the method for calculating payment originally agreed to in the contract becomes unworkable with the actual project scope or existing work-site conditions. Perhaps it would be beneficial to you if the contract contained different payment terms. While a contract can be changed through verbal or written agreement between the parties, can a party, by its actions alone, agree to revise payment provisions unilaterally imposed by another?

This issue was dealt with in Whissell Contracting Ltd v Calgary (City), 2017 ABQB 644. Whissell entered into a subcontract with SNC-Lavalin Graham Joint Venture ("SNC") to perform certain work for the City of Calgary. The subcontract provided that payment would be calculated on a unit rate basis. Instead, Whissell began invoicing SNC on a force account basis claiming that soon after beginning work on the project, the scope of work and site conditions changed greatly from those