This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
It hardly needs to be said that a contract may at any time be discharged by all the parties thereto. It is desirable to have the discharge, like the contract, clearly made in writing, so as to cover all questions of further rights and liabilities.
If a party to a contract clearly waives his right under a certain clause, and the other party, relying on such a waiver, does not fulfil that clause, the resulting variation from the contract terms is excused. Thus if a building contract makes an architect's certificate a condition precedent to payment, and if the owner gives the contractor to understand that the certificate will not be required, this waiver renders the production of;he certificate unnecessary as a condition of payment. Waiver may be found in the conduct, as well as in the words, of a party. The question whether words or conduct amount to a waiver, is one for the jury to decide.
Contracts may always be modified by agreement of the parties. The subsequent agreement must be a good contract according to the ordinary rules, in order to effect a change in an existing contract. A waiver may also practically modify a contract. It is important, however, for the sake of avoiding complications and disputes to have a contract always kept clear and readily provable. Hence it is only a reasonable precaution to have any modifications of a written contract put in writing. One advantage of this is that the written expression of the understanding is likely to be made more definite, thus avoiding disputes on the ground of inexactness, as well as on the ground of misunderstanding.