Interim Payment Certificate in Construction: How QS Professionals Prepare and Certify the IPC

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The contractor's QS on a large infrastructure project had been preparing interim payment applications for eleven months without a significant dispute. The process had settled into a rhythm — measure the works on the last week of the month, compile the IPA, submit to the engineer by the agreed date, receive the IPC two weeks later, and confirm payment by day 56. Not every certified amount matched the application exactly, but the differences were small and the explanations were straightforward.

IPC number 12 was different. The application came to just over four million dirhams. The certified amount came back at three million, one hundred thousand. A reduction of nearly nine hundred thousand dirhams — approximately 22 percent of the submitted value — with no written explanation attached to the certificate.

Under the contract — a FIDIC Red Book 1999 — the engineer was required to certify the IPC within 28 days of receiving the payment statement, and was required to give written notice of any amount withheld and the reason for it. The notice of withholding had not arrived. The IPC carried a reduced number and a signature. Nothing else.

The QS had three decisions to make. Accept the certified amount and move on. Request the written explanation the contract required. Or challenge the reduction formally. Each path had a different commercial consequence and a different timeline. The choice depended on understanding exactly what the IPC process was supposed to do, what the contract required from the engineer, and what tools the QS had when the process did not work as intended.

The IPC is one of the most commercially significant documents on a construction project. It determines cash flow. It shapes the contractor's ability to pay subcontractors and suppliers on time. It reflects the commercial health of the project month by month. Understanding how to prepare one correctly — and what to do when the certified amount does not reflect the work done — is knowledge every QS working in construction needs before the first payment cycle begins.

The IPC Cycle — How Payment Flows on a Construction Project

On most construction contracts, payment does not happen in a single transaction at the end of the project. It flows continuously throughout construction through a monthly cycle of applications and certificates. The contractor applies for payment based on work completed. The engineer certifies what they consider due. The employer pays the certified amount within the contract's payment period. Then the cycle starts again.

This cycle exists because construction projects run for months or years, and contractors cannot finance the full cost of construction out of their own resources while waiting for a single payment at completion. The IPC cycle is the mechanism that keeps the contractor financially operational — and keeps the project moving. A project where the IPC process breaks down — where applications are submitted but certifications are delayed, or certifications are issued but not paid — is a project heading towards programme risk, subcontractor insolvency, and contractual dispute.

The FIDIC timeline in that table is the standard against which performance is measured on most international construction contracts. Under FIDIC Sub-Clause 14.7, if the employer fails to pay the certified amount by day 56, the contractor is entitled to financing charges on the overdue amount. If the delay continues, the contractor may suspend work under Sub-Clause 16.1 after giving notice. These are not theoretical remedies — they are practical commercial tools that a QS needs to know exist and be prepared to use.

The Interim Payment Application — What Every IPA Must Include

The Interim Payment Application is the document the contractor's QS prepares and submits to trigger the IPC cycle. Its quality directly determines the quality of the certified amount. An IPA with clear measurements, proper supporting documents, and accurate running totals gives the engineer the information they need to certify quickly and accurately. An IPA that is poorly structured, missing supporting documents, or arithmetically inconsistent gives the engineer grounds to return it or reduce it — and the QS limited basis to challenge the outcome.

The materials on site row in that table is worth particular attention. Many QS professionals on their first few projects either omit materials on site entirely or include them without adequate supporting documentation. The principle is straightforward — if the contractor has spent money purchasing materials that are sitting on site ready to be used, those materials represent a real financial commitment that the IPA can capture. The requirement is that the materials must be properly stored, clearly identifiable as belonging to this project, and supported by delivery notes and invoices. A site visit by the engineer or the client's QS to verify the materials is normal practice before certification.

The Running IPC Schedule — Why It Is the Most Important Document the QS Maintains

Across the life of a project, the IPC schedule tracks every certified amount, every retention deduction, every advance payment recovery, and every cumulative total from IPC 1 through to the final certificate. It is the single document that prevents double payment, confirms what has been paid, and provides the baseline for the final account.

A QS who does not maintain a meticulous running IPC schedule will eventually submit an application that conflicts with the project's payment history — either overclaiming because a previous payment was not properly deducted, or underclaiming because a previously approved variation was not carried forward. Either error creates a dispute that takes time to resolve and erodes the engineer's confidence in the contractor's QS function.

•       Update the IPC schedule after every certification: Not after payment — after certification. The certified amount is the contractual record. Payment may come days later

•       Track retention separately: Maintain a retention account showing the cumulative retention held, the first moiety released at practical completion, and the second moiety still outstanding during the defects liability period

•       Flag variations that are instructed but not yet valued: Include them in the IPA with a note that rates are under assessment — this protects the contractor's entitlement even when the rate negotiation is unresolved

•       Cross-reference each IPA to the programme: The progress of the measured works should align with the construction programme — an IPA that claims 70 percent completion when the programme shows 55 percent will draw scrutiny from the engineer

How the Engineer Certifies the IPC — and What QS Professionals Should Expect

The engineer's role in the IPC process is to independently assess the contractor's payment statement and certify the amount they consider due under the contract. In practice, the engineer's QS — or the client's QS — carries out the assessment, comparing the contractor's measurements against their own site records and BOQ data before recommending a certified figure to the engineer for sign-off.

On well-run projects, the contractor's QS and the client's QS agree the measured quantities together before the IPA is formally submitted — a joint measurement process that removes the uncertainty from the certification. This does not always happen on every project, but where it does, the IPC cycle runs faster and produces fewer disputes. A QS who requests joint measurement meetings with the engineer's team — and documents the agreed quantities from those meetings — is building the evidence base that makes their IPA harder to challenge.

What the Engineer Can and Cannot Do

The engineer can certify less than the IPA if they disagree with the measurement, consider certain items not yet complete, or determine that deductions apply under the contract — for liquidated damages, uncorrected defects, or other contractual adjustments. What the engineer cannot do under FIDIC is simply return a reduced number without explanation.

Sub-Clause 14.6 of the FIDIC Red Book requires the engineer to give written notice to the contractor if they intend to withhold any amount that the contractor has applied for, explaining the reason. This requirement exists to protect the contractor's ability to understand and respond to the reduction — and to give the contractor a fair basis for challenging it if the reason is not contractually valid.

When the IPC Cycle Breaks Down — Practical Steps for the QS

The infrastructure project QS from the opening of this article did not accept the reduced IPC without action. Within five days of receiving IPC 12, she sent a formal written request to the engineer under the contract's notice provisions, requesting the written explanation required by Sub-Clause 14.6 for the reduction of approximately nine hundred thousand dirhams from the submitted payment statement.

The explanation arrived ten days later. It covered four items. Two were measurement disputes — the engineer's QS had measured two concrete elements differently from the contractor's QS, and the difference in quantities accounted for roughly three hundred thousand dirhams of the reduction. One was a deduction for liquidated damages for a two-week programme delay that the contractor was contesting through a separate EOT claim. One was an item the engineer had excluded on the grounds that the variation instruction had not been formally issued — the work had been carried out following a verbal instruction that had not been confirmed in writing.

With the breakdown in hand, the QS knew exactly where to focus her response. The measurement disputes went into the next month's IPA with supporting measurement records from the joint site walk she requested immediately after receiving the explanation. The liquidated damages deduction was addressed through the EOT claim already in progress. The verbal instruction item required an urgent written request to the engineer to formalise the instruction — which, once issued, allowed the variation to be properly valued and included in a subsequent IPA.

None of this could have been managed without the written explanation. A QS who accepts a reduced IPC without understanding why it was reduced loses the ability to recover the difference through the normal IPC cycle — and may find at final account stage that items which were never formally disputed have been treated as accepted.

The actions that protect the contractor's commercial position throughout the IPC cycle:

•       Submit every IPA on time: Late submission delays the entire cycle — under FIDIC the engineer's certification period starts from receipt of the IPA, so a late submission means a late certificate and a late payment

•       Include all supporting documents with the IPA: Measurement sheets, progress photographs, delivery notes for materials on site, signed variation instructions, and the updated running IPC schedule — an IPA without supporting documents gives the engineer grounds to return it

•       Request written explanation for any reduction: Within five working days — do not let a reduced IPC pass without understanding the reason — the explanation is the starting point for recovery in the next cycle

•       Confirm all verbal instructions in writing before including them in the IPA: An instruction that cannot be traced to a signed document is an instruction the engineer can exclude from the certificate — follow up every verbal instruction with a written confirmation request

•       Maintain the running IPC schedule meticulously: Update it after every certification, track retention separately, and reconcile it against the payment register before every new IPA submission

The IPC process is the monthly heartbeat of the project's commercial management. For a complete guide to how variation orders — which feed directly into every IPA — are raised, valued, and tracked throughout construction, see our article on Variation Orders in Construction: A Practical Guide for Engineers and Contractors.