delivered the opinion of the court:
Appellants make thirty-four assignments of error; and insist that none of the averments of the declaration, as set out in the statement preceding this opinion, were sustained by the evidence. After a careful examination of the evidence, we are satisfied that it tends to establish such averments, and, this being so, the judgments of the lower courts upon the questions of fact, embodied in such averments, are conclusive upon this court.
It is impossible for us'to consider separately each one of the assignments of error made by the appellants, but we will notice some of the main contentions of the ap*92pellants, arising out of the rulings of the trial court in reference to the admission and exclusion of evidence, and based upon the giving, refusal and modification of instructions.
The contract was fully performed; and, therefore, a recovery could be had under the common counts. Where the contract has been performed, and nothing remains to be done but to pay the amount due under it, a recoxrery may be had under the common counts, and the agreement may be read in evidence for the purpose of showing its terms and to measure the damages. (Shepard v. Mills, 173 Ill. 223; Foster v. McKeown, 192 id. 339). In the case of building contracts, a literal compliance therewith “is not necessary to a recovery, but it will be sufficient that there has been an honest and faithful performance of the contract in all its material and substantial particulars, and no omission in essential points, or willful departure from the contract; and mere technical and unimportant omissions will not defeat a recovery of the contract price, less any damages, however, requisite to indemnify the owner.” (Keeler v. Herr, 157 Ill. 57; Shepard v. Mills, supra; 2 Sutherland on Damages,—2d ed.— sec. 711). A recovery could also be had under the special count. The contention of the appellants is, that the appellee was bound to prove a strict compliance with all the provisions of the contract. It is true that, while the performance of the contract in all substantial particulars was established by the testimony, yet where there was departure from the strict letter of the contract in certain particulars, the proof tended to show a waiver on the part of the appellants. It seems tb be the contention of the appellants that proof as to waiver of a strict performance of the contract was not averred in the declaration, and, therefore, that the court erred in admitting proof of the same, and in calling the attention of the jury to the same in the instructions. There was no error in this respect for the reason that it was not *93necessary to allege in the special count that appellants waived some of the provisions of the contract, as waiver or estoppel may be proved without pleading it. (German Fire Ins. Co. v. Grunert, 112 Ill. 68; Continental Life Ins. Co. v. Rogers, 119 id. 474). The provisions of a written contract may be waived by a participation in acts done in disregard of it. (Foster v. McKeown, supra; Chicago and Eastern Illinois Railroad Co. v. Moran, 187 Ill. 316).
The defense, made by the appellants upon the trial below, was based mainly upon three grounds: First, that the contract for the construction of the party walls was not let separately to the lowest responsible bidder; second, that the contract did not provide for joint supervision by appellants; and, third, that the brick above the first floor level were not laid in mortar made by hydraulic cement.
Two contracts were made, one a separate contract between appellee and Snyder, the contractor, for the construction of the party walls, and the other a contract for the construction of the building of appellee. These contracts were executed at the same time. The specifications required that all bidders should put up a certified check for two per cent of the amount of their bids. Appellants, through the J.W. Evans Sons Company of which they were all stockholders and some of them officers, were bidders, and complied with the requirement in regard to the check, thereby recognizing that such requirement was reasonable. The evidence tends to show that Snyder was the lowest responsible bidder. It is true, that a contractor, named Clark, presented the lowest bid for the construction of the party walls, but his bid was not accompanied with a certified check, and when Clark discovered that Snyder’s bid for the remainder of the building was higher than his, and that he could not get both contracts, he refused to take, the contract for the walls alone, and withdrew his bid. In view of these facts it cannot be said that the contract for the party walls *94was not let to the lowest responsible bidder, inasmuch as, Clark being out of the way for the reasons stated, Snyder was the lowest responsible bidder. The proof shows that Snyder did put up a certified check in accordance with the requirements of the specifications. Nor can it be said that appellants had no notice and took no part in the bidding, inasmuch as J. W. Evans, who represented all of the appellants, examined the plans and specifications, and prepared and submitted a bid on behalf of the corporation above designated, which was rejected because it was too high.
By the terms of the party-wall contract it was agreed that the parties thereto should have joint supervision of the erection of the walls, and that the contract therefor should so provide; but the party-wall agreement did not specify particularly how the joint supervision should be exercised. Appellee employed an architect, and, in drawing the contract for the erection of the party walls between appellee and Snyder, the contractor, the architect inserted a provision giving appellants the right of joint supervision through him. No complaint is made as to the competency or honesty of the architect. The contract, thus made between apxoellee and Snyder for the construction of the party walls, was left with J. F. Evans as the representative of appellants, and no-objection was made by him to the provision that the supervision should be exercised through the architect, Miller. The evidence does not tend to show that the appellants claimed that they desired to exercise any joint supervision, except so far as such supervision was exercised by Miller, acting for both parties. The evidence does not tend to show that any complaint was made by appellants that they were deprived of the right of exercising a joint supervision with appellee over the work. ■ On the contrary, the evidence tends to show that several of the appellants actually participated in supervising" the construction of the wall. Before the walls were begun, J. F. Evans united *95with, and aided, the architect, Miller, in establishing the division line between the properties, and he performed this service for all of the Evans heirs, testifying that he represented them all at that time. The evidence also tends to show that, a few weeks after appellee began the erection of his building, the appellants began the erection of their building". The party wall was built for both buildings. The evidence tends to show that, during the erection of the buildings, instructions were given by .the foreman for the joists to be placed in the party wall according to the plans, prepared by the appellants for their own building. The evidence also tends to show that J. F. Evans, .who was frequently at the buildings while the walls were going up, and made no complaint of any kind, and made no request to be permitted to participate in the supervision, objected to certain bricks going into the walls; and changes were made by the foreman in charge of the work on appellee’s building, and also on the party walls, in order to enable the appellants to set their joists in the party wall. R. W. Evans testified that the plans for appellants’ building were prepared by him with the consent of all the appellants, and these plans called for the joists of the building of appellants to rest in the party walls. The construction of the two buildings, that of the appellee and that of the appellants, proceeded at the same time upon their adjoining lots, and the joists of appellants’ building were inserted in the party wall as the work progressed. Appellants, having thus accepted and used the party wall as it was built without making" any question as to its proper construction, should have objected at the time, if the walls were not being erected in a manner satisfactory to them.1 In view of the circumstances already narrated, there was evidence tending to show that appellants waived a strict performance of the terms of the contract with appellee, so far as the matter of joint supervision was concerned.
*96The party-wall contract provided that hydraulic mortar should be used in the construction of the party walls, while the contract for their erection specified lime mortar, instead of hydraulic cement mortar. Inasmuch as appellants examined the specifications and prepared and submitted a bid before.the contract was let, it is to be presumed that they knew that this mistake was made. Although presumed to know it, they never at any time made any complaint or objection while the construction of the walls was going on. The architect, however, testifies that, when his attention was called to this mistake in the specifications, hydraulic cement with a little lime was in fact used. There was other testimony, tending to show that the walls were laid with cement mortar; and so it was a question of fact for the jury whether the walls in question were laid in accordance with the terms of the contract sued on, or whether any deviation from those terms was waived by the appellants.
The facts of this case, as thus detailed, bring it within the rule announced by this court in Huck v. Flentye, 80 Ill. 258, where it was said: “It appears from the evidence that appellant and appellee were equally ready and anxious to re-build. The wall was as much a necessity for appellee as it was for appellant. It was built at the same time appellee’s building was, and appellee has had the same benefit of it that appellant has had. That appellee was ignorant of its being built is not pretended. The same architect made the plans for both buildings. Appellee was often present as the work progressed, and, at his request, work on the wall was delayed, to-give him time to put in his joists. Appellee says, that he never agreed to build any part of the wall or pay for its being built, and much stress is laid on this. Suppose that he did not. Appellant never agreed to build his part for him, and release him from liability. He knew the wall had to be built to enable him to complete his building, and that it had to be paid for. He did not forbid its *97construction or give notice that he would not be liable, and he had agreed that they would build together. The wall- was expressly built for the use of both buildings, and, as we understand the cases cited by appellant’s counsel, herein it differs from those. Appellant did-not here first erect his building, constructing one-half of this wall on appellee’s property without any agreement in regard thereto, and appellee afterwards erected bis building, but both buildings were erected, substantially, at the same time.” And in the Huele case it was said by the court, quoting from Greenleaf on Evidence, that “it is not necessary to the plaintiff to prove an express assent of the defendant, in order to enable the jury to find a previous request; they may infer it from his knowledge of the plaintiff’s act, and his silent acquiescence.”
We concur in the following views expressed by the Appellate Court in their opinion: “While the party-wall contract provided that hydraulic mortar should be used in the construction of the party walls, and the contract for their erection specified lime mortar, this 'fact must have been known to J. W. Evans when he prepared and submitted the bid of appellants’ corporation. Inasmuch as he made no objection at the time, we think that appellants are estopped from doing" so after having had actual notice through J. W. Evans of the fact, and permitting the lime mortar to be used. If he had, when the specifications were published, or at any time before the erection of the walls was begun, called the attention of appellee to the fact that the specifications were not in accordance with the party-wall agreement, appellee might readily have caused the error to be rectified, and he should, we think, have been afforded an opportunity to do so. ‘The law imposes upon a party subjected to injury from a breach of contract, the active duty of making reasonable exertions to render the injury as light as possible. If the. injured party through negligence or *98willfulness allows the damages to be unnecessarily enhanced, the increased loss justly falls upon him. ’ (Hartford Deposit Co. v. Calkins, 186 Ill. 104). To hold that appellants may evade payment of their proportion of the cost of the party walls solely because of an apparently inadvertent deviation from the contract, of which they, through their representative, were made aware in ample time to remedy the same, would be, in our opinion, contrary to the law and abhorrent to reason and justice.”
Under the terms of the party-wall agreement, it was evidently the intention of the parties that the one, first ready to proceed with the erection of his building, should at the same time erect the party walls, and, when they were erected, the other party should pay his ratable proportion of the expense, as specified in the contract, on demand. It is not stated that such proportion should be paid to the contractor, and, therefore, the contention of the appellants that, if they were bound to pay, they were only obliged to pay to the contractor, and not to appellee, is without force. No contract relation existed between appellants and the contractor, Snyder. They were not indebted to Snyder in any manner; nor can it be said that, by paying Snyder for the construction of the walls, appellee voluntarily made, or attempted to make, himself the creditor of appellants without their consent. Even if the contract contains no promise by appellants to pay appellee any portion of the cost of the construction, it may be said that it contains no promise that either party should pay the contractor. The fact, that the wall was built by appellee under the contract in pursuance of the desire of appellants to have it built, and of their willingness to pay a certain proportion of the cost, together with the fact that appellants and appellee erected their buildings at the same time, and appellants accepted and used the walls from day to day as the work progressed, raises an implied promise on the 'part of appellants to pay their proportion of the cost.
*99The rulings of the court below, and the action of the court in giving, refusing and modifying instructions, were all in accordance with the views herein expressed. We are, therefore, of opinion that no error was committed, which would justify us in reversing the judgment.
The judgment of the Appellate Court is affirmed.
Judgment affirmed.