Managing Sub-Contractors on a Multi-Trade Project: Scope Control, Purchase Orders, and Cost Reconciliation for General Contractors

General contractors manage a more complex procurement problem than most people outside the industry appreciate. On a typical commercial fit-out or new-build project, a GC may be coordinating groundworks, structural steel, concrete, M&E, drylining, joinery, glazing, flooring, and decoration - each a separate sub-contractor, each with its own lead time, programme dependency, and cost risk. Sub-contractor packages can account for 40-60% of the total contract value on commercial projects. When the controls around those packages are loose - vague scopes, verbal agreements, unrecorded variations, and invoices approved before reconciliation - margin that looked healthy at tender can disappear quickly by final account.

This resource covers four operational areas where GCs most frequently lose control of sub-contractor costs: writing trade-specific scopes of works before booking, raising purchase orders against each sub-contract package, recording on-site instructions and day works correctly, and reconciling sub-contractor costs before invoicing the client.

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Trade-Specific Scopes of Works Before Any Booking

The most common source of cost disputes between general contractors and sub-contractors is a scope of works that was never written down clearly enough. A verbal brief at tender stage, followed by a generic "install as per drawings" instruction, leaves the boundary between what is included and what attracts an additional charge entirely undefined. That ambiguity always costs money.

Before booking any sub-contractor - even one you have worked with a dozen times before - a written scope of works for that trade package needs to be issued and acknowledged. The document does not need to be long. It does need to be specific.

For each trade package, the scope of works should cover: a description of the works by floor and zone referencing the current issue of drawings; the materials and equipment the sub-contractor is responsible for supplying; a list of explicit exclusions (what the GC will supply, what other trades will fix, what is outside the contract entirely); and any provisional items that are quantity-dependent or subject to a later instruction.

Provisional items deserve specific attention. If a floor area or element is provisional at tender - because the design is incomplete or owner decisions are pending - that should be declared in writing as a provisional sum rather than included in the fixed price. When the instruction arrives to proceed, the value is agreed at that point based on a schedule of rates or a quotation. This protects both parties and removes the ambiguity that generates claims.

The scope of works document should also confirm the programme obligations on the GC side: when the floor or zone will be ready for the sub-contractor to commence, what access constraints apply (working hours, site induction requirements, shared scaffold or hoist arrangements), and what the notice period is for any programme change. Getting these details agreed in writing before the sub-contractor mobilizes means any GC-driven programme disruption can be addressed commercially rather than discovered at the final account stage.

Procore's guidance on sub-contractor selection recommends obtaining a minimum of three bids per trade before awarding. Comparing bids on a like-for-like basis requires that each bidder has priced the same scope document. Without a written scope, bid leveling is guesswork - and the lowest number rarely turns out to be the cheapest when the scope gaps are filled in by change orders during the works.

Purchase Orders Linked to Trade Package and Job Record

Every sub-contractor package needs a purchase order raised against it before any work starts. The PO serves as the binding financial commitment - it defines the price agreed, references the scope document, and creates the record against which the sub-contractor's invoice will be checked at payment stage.

A PO for a sub-contractor package should contain: the job reference and project name; the sub-contractor's legal name and address; a description of the works that cross-references the scope of works document by version; the agreed contract sum, split by phase or milestone if the payment schedule is staged; the retention percentage being applied (typically 3-5%, matching the client's retention terms); any payment terms agreed; and the programme dates (start, completion, key milestones).

For fixed-price packages, the PO sum is the agreed lump sum or schedule of rates total. For day rate or time-and-materials arrangements - which are more common for enabling works, temporary services, or packages where the extent cannot be defined until the works open up - the PO should state the agreed day rate per operative role (laborer, tradesperson, supervisor), the agreed mark-up on materials if the sub-contractor is procuring, and a cap figure that requires a new instruction to exceed. Leaving day rate arrangements without a capped PO creates open-ended cost exposure that is very difficult to manage at invoice stage.

Once the PO is raised and sent, request written acknowledgment of it. If you are using a construction management system that links POs to jobs, the acknowledgment creates a clear record that the sub-contractor has received and accepted the commercial terms. If the sub-contractor comes back with amended terms, that needs to be resolved before mobilization - not after the work is done.

The booking confirmation - specifying start date, site induction requirements, access arrangements, and the GC contact for site queries - should be sent separately from the PO but reference it. This keeps the commercial document and the operational instruction clearly linked but distinguishable. The booking confirmation is the document you update when programme dates change. The PO is the document you reference when the invoice arrives.

On-Site Instruction Recording and Day Work Discipline

Scope creep is where many GC projects quietly lose money. The work that falls outside the original sub-contract - the extra strip-out discovered behind a false ceiling, the additional drainage run added mid-project, the rerouted cable containment after a late design change - is genuinely additional. But if it is not recorded correctly from the moment the instruction is given, recovering the cost becomes difficult.

The FIS CICV Best Practice Guide (February 2025) on variations and compensation events is direct on this point: verbal agreements are not enforceable. Written instructions are required. This applies equally to instructions from the GC to sub-contractors and instructions from the client or their representative to the GC. The principle is the same across the chain.

A verbal site instruction diary should be maintained by the site manager or project manager throughout the project. Every instruction given verbally - whether from the GC's own supervisor to a sub-contractor, or received from the client's contract administrator - is logged on the day it is given. The log should record: date and time, who gave the instruction, what was instructed, which trade or package it affects, and the estimated cost if known. This log becomes the basis for formal written instructions, which should follow within 24 hours.

Day work sheets are the mechanism for recording time-and-materials additional works on site. A valid day work sheet must contain: the date, the sub-contractor's name and operative names, the hours worked by each person and their role, the materials and plant used with quantities and unit costs, a description of the work done that references the verbal instruction or drawing change that authorized it, and a signature from the GC's site manager confirming the work was carried out. Unsigned day work sheets that arrive with the sub-contractor's interim application are genuinely difficult to dispute by then. The signature needs to happen on the day.

For any additional work above a defined threshold - typically $500-$800 is a reasonable floor for commercial projects - a written instruction and agreed price should precede the work rather than follow it. This is not always practical on live construction sites where speed matters, but it is the standard that protects both parties. Where the pre-agreed price is not practical, the written instruction confirming the work is additional and the basis of measurement (day work, schedule of rates, or a quotation to follow) should be issued the same day.

Sub-Contractor Cost Reconciliation Before Client Invoicing

The sequencing of when you check costs and when you bill the client matters significantly. Many GCs submit interim applications or raise invoices to the client, and then process sub-contractor interim payments afterward. This creates a gap where the GC does not know their actual committed costs at the point of billing, which makes job-level margin invisible until the final account is closed.

The stronger discipline is to reconcile committed sub-contractor costs before each application or invoice to the client. This does not mean waiting for all sub-contractor invoices to arrive before submitting your application - that would create a cash flow problem. It means tracking committed costs (raised POs) alongside certified costs (approved invoices) so you have an accurate picture of where the job sits at each billing cycle.

For each sub-contractor at interim billing stage, the check should cover: what value has been certified and paid to date on their package; what additional items have been instructed since the last certification that are not yet on an invoice; what day work sheets have been signed that will appear on the next application; and whether any of the sub-contractor's costs are in dispute. Completing this reconciliation before you finalize your own application means your certified value to the client is based on real committed costs, and your margin calculation is accurate rather than estimated.

When sub-contractor invoices arrive for payment, the three-way match process applies: the invoice total against the PO or sub-contract sum, the certified value against the application record, and the retention deduction against the agreed percentage. Any invoice that includes items not in the PO or not covered by a signed instruction should be held and queried in writing. Do not pay first and dispute later - recovery of overpayments on sub-contractor accounts is slow and expensive.

Back charges are the mechanism for recovering costs from a sub-contractor when the GC has had to remedy defects or incomplete work that the sub-contractor was contractually obligated to complete. Back charges must be documented: a written notice specifying the defect, a request to return and remedy with a defined deadline, and a written notification of the GC's intention to engage others if the deadline is not met. Without this paper trail, deducting a back charge from an interim payment is likely to generate a dispute or a retention-of-payment notice under the Construction Act.

How Zigaflow Supports General Contractor Sub-Contractor Management

Zigaflow's job management features give general contractors a single record for each project where sub-contractor POs, works orders, and delivery confirmations are linked together. When a sub-contractor package is confirmed, a purchase order is raised directly from the job record - carrying the job reference automatically. Interim payment schedules can be tracked against each PO so the GC can see certified vs paid vs outstanding across all packages at a glance.

For on-site instruction recording, the eForms App allows site managers to log day work sheets and variation records in the field on the day they happen, with supervisor sign-off captured digitally. These records are linked to the job and the relevant PO, so they are available when sub-contractor interim applications arrive and need to be checked.

When it is time to invoice the client, Zigaflow's job-level cost view shows committed PO spend and approved invoices side by side with the contract value. The three-way match on sub-contractor invoices - PO, completion record, invoice - is handled within the platform, and once matched, the invoice flows through to Xero, QuickBooks, or FreeAgent for payment processing. This closes the gap between field cost capture and accounts payable, so the job margin calculation is based on actual numbers, not estimates carried forward from the original tender.

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General contractors who tighten these four areas - written scope per trade, a PO per package, same-day instruction recording, and cost reconciliation before billing - are not doing anything unusual. They are applying the basic disciplines that protect margin on projects where sub-contractor cost is the primary variable. The specialty contractor gross margin benchmark sits at 15-25% (CFMA 2024), with the industry average just above 16%. The difference between a business at the bottom of that range and one at the top is rarely a single large loss. It is the accumulation of small, unrecorded, unrecovered costs across every project - the unsigned day work, the back charge never served, the sub-contractor invoice approved without a three-way match. Addressing these one at a time is how the margin improves.