Ask an owner how much the project will cost and you'll usually get a precise answer. Ask how much cash it will need in March, and the answer is often a shrug. Yet timing matters as much as the total. Lenders release funds against a draw schedule, boards approve annual capital budgets, and treasury teams need to know when the big cheques are coming. A project that's on budget but needs $3 million more than planned in one quarter is still a problem.
The tool that answers the timing question is the cash flow S-curve. This guide explains where it comes from, how to read planned against actual spend, how front-loading distorts it, and how owners can use it for funding draws and board reporting.
What an S-curve is, and why it has that shape
An S-curve plots cumulative cost against time. Spend starts slowly during mobilization, early site work and foundations. It accelerates through structure, envelope and the peak of mechanical and electrical rough-in, when the most trades are on site at once. Then it flattens during finishes, commissioning and closeout, when the work is detailed but lower in value.
Plot the monthly spend on its own and you get a rough bell shape. Add each month to the running total and the bell turns into an S. The steep middle is where most of the money goes. In the illustrative 14-month project below, months 5 to 9 alone account for 57% of the construction value.
The shape itself is less important than what it represents: a time-phased budget that ties dollars to the work in the schedule. That's what makes it a planning tool rather than a picture.
Where the curve comes from: the cost-loaded schedule
The most reliable S-curve comes from a cost-loaded schedule, where each activity in the contractor's CPM schedule carries a share of the contract value. When the schedule says drywall on level 3 happens in May, the dollars for that drywall land in May too.
The U.S. Government Accountability Office's Schedule Assessment Guide treats resource and cost loading as a best practice. It notes that cumulative cost profiles reveal peaks and troughs in funding needs that could cause cash flow problems. Its worked example shows a labour cost spike being smoothed by moving nearly $14,000 of work with available float from one month into the next. That's the practical use: once costs are tied to activities, the team can see a funding crunch coming and resequence work with float to soften it.
Public owners who rely on cost-loaded schedules set firm rules for them. The U.S. Unified Facilities Guide Specification for project schedules (UFGS 01 32 01.00 10), used on federal construction, requires that activity values add up to the contract price, that cost loading be reasonable and without front-end loading, and that the approved schedule be the basis for determining what the contractor has earned each period. It also sets minimum values for closeout items such as as-built drawings and the final punch list, so the last few percent of the contract isn't billed before the work is done.
Most private Ontario projects don't go that far. Progress billing is often based on a schedule of values that sits apart from the schedule. That works, but it means the owner's S-curve is built from two sources that may not agree. If the contractor's schedule shows structure finishing in month 7 and the schedule of values bills 90% of structure by month 5, one of them is wrong.
View the data
| Month | Planned spend | Cumulative |
|---|---|---|
| 1 | $0.2M | $0.2M |
| 2 | $0.8M | $1.0M |
| 3 | $1.4M | $2.4M |
| 4 | $1.8M | $4.2M |
| 5 | $2.2M | $6.4M |
| 6 | $2.4M | $8.8M |
| 7 | $2.4M | $11.2M |
| 8 | $2.3M | $13.5M |
| 9 | $2.1M | $15.6M |
| 10 | $1.7M | $17.3M |
| 11 | $1.3M | $18.6M |
| 12 | $0.9M | $19.5M |
| 13 | $0.4M | $19.9M |
| 14 | $0.1M | $20.0M |
The owner's cash flow is bigger than the contractor's curve
The contractor's S-curve covers the construction contract. The owner's funding need is broader. A useful owner S-curve layers in:
- Soft costs: design fees, permits, project management, testing and inspection, legal and financing costs. Design fees are heavily front-weighted, so they shift the curve earlier.
- Owner-supplied items: furniture, IT, security systems, signage and equipment. These often land in a lump near the end and can create a second peak.
- Holdback: under Ontario's Construction Act the owner retains holdback from each progress payment and releases it later. The cash leaves later than the cost is incurred. Our guide to construction holdback in Ontario explains the release rules.
- Payment timing: work done in May is invoiced at month end and paid within the statutory deadlines. Our post on Ontario prompt payment rules covers that clock.
- Contingency drawdown: change orders don't arrive evenly. Plan for contingency to be used through the busiest part of the job, not spread flat.
Keep two curves if your finance team needs both: a cost curve (value of work done) for project control, and a cash curve (when money leaves the bank) for treasury and lenders. Mixing them is a common source of confusion at board meetings.
Planned versus actual: reading the gap
Once the job starts, the monthly report should plot actual cumulative spend against the baseline S-curve. The gap between them is a signal, but it needs interpreting.
Actual spend below plan is usually not savings. It most often means the work is behind schedule. Money that wasn't spent this month will be spent later, often at a higher cost because the project runs longer. A project that's 10% under its cash flow curve at month six is more likely late than cheap.
Actual spend above plan can mean the work is ahead of schedule, which is good, or that costs are running over, or that billing is ahead of the work. You can't tell which from spend alone.
That's why spend curves need a third line: the value of work actually completed. That's the idea behind earned value. Our guide to earned value management for owners explains planned value, earned value and actual cost, and how to turn them into cost and schedule indices. For cash flow purposes, the key point is simple: compare money paid with work in place, not just with the plan.
Front-loading: when billing runs ahead of the work
Front-loading is when a contractor's schedule of values or cost loading puts more value on early activities than they're really worth. Mobilization, general conditions, bonds, early trades and deposits are the usual places. The result is a billing curve that runs ahead of the work in place.
Contractors have understandable reasons. Construction is paid in arrears, holdback ties up cash and early costs are real. Some front-loading is normal and fair, especially for mobilization and genuine material deposits. The problem is when it goes too far.
For the owner, heavy front-loading means:
- Less leverage late in the job. If most of the money has been paid by the time finishes and deficiencies come around, there's little left to motivate completion.
- More exposure if the contractor fails. If a contractor defaults with the job half done but two-thirds paid, the remaining contract balance may not cover the cost to complete with a replacement.
- A misleading S-curve. Spend tracks the plan nicely while the work quietly falls behind. The board sees a green light that isn't real.
View the data
| Line at month 6 | Cumulative | Finish |
|---|---|---|
| Baseline plan | $8.8M | Month 14 |
| Billed (paid) | $9.0M | |
| Work in place | $7.6M | Month 16 (forecast) |
The fix starts before award. Review the schedule of values line by line against the estimate and the schedule. General conditions should be billed over time, not as a lump at the start. Mobilization should be a small, defined amount. Closeout items, commissioning, as-builts and operations manuals should carry real value so they get done. The UFGS approach of setting minimum values for closeout items is worth borrowing.
Using the S-curve for funding draws and the board
Construction lenders commonly fund against a draw schedule supported by a cost consultant's certification of work in place. A credible baseline S-curve, built from the cost-loaded schedule and the full project budget, tells the lender and the owner's finance team when each draw is likely and how big it will be. When the forecast changes, the draw schedule should change with it, before the shortfall happens.
For boards and capital committees, the S-curve is one of the clearest single charts in a project report. Show three things: the approved baseline, actual cumulative cost to date, and the current forecast for the rest of the job. If the forecast curve has shifted right, the project is late. If it ends higher, the project is over budget. If both, you have the conversation you need to have. Our monthly report template shows where it fits in an executive report.
On programs we've supported, the most useful habit was re-forecasting the curve every month from the updated schedule, rather than leaving the baseline curve in the report and plotting actuals against it until the gap became embarrassing.
Common mistakes
- Drawing the curve with a template instead of the actual schedule. Standard curves are fine for an early feasibility estimate. They're not good enough once a schedule exists.
- Leaving out soft costs and owner items. The construction curve looks fine while the owner's total cash need is understated.
- Never re-baselining after major changes. If scope or the schedule has changed significantly, the original curve no longer describes the plan.
- Treating underspend as good news without checking schedule progress.
What owners should do
- Require a cost-loaded schedule or a schedule of values tied to the schedule in the contract, with activity values adding up to the contract price.
- Review cost loading for front-loading before you accept the baseline, and set meaningful values for closeout and commissioning items.
- Build an owner-level cash curve that includes soft costs, owner-supplied items, holdback and contingency, separate from the contractor's cost curve.
- Plot planned, actual and forecast every month, and compare payments with work in place, not just with the plan.
- Update the draw schedule and board forecast whenever the curve moves, so funding is in place before it's needed.
Go deeper: cash flow and the S-curve during construction and costs, rates and accrual in Microsoft Project.
The bottom line
An S-curve turns a budget into a timeline of cash. Built properly from a cost-loaded schedule, it tells owners when money is needed, shows early whether work is keeping pace with spending, and exposes front-loading before it becomes a problem. Built from a template and never updated, it's just a nice shape in a report.
Want an independent check of your project's cash flow forecast or schedule of values? Book a 30-minute call with Benjamin R. Consulting.
Sources: U.S. GAO, Schedule Assessment Guide: Loading Activities with Resources; Unified Facilities Guide Specifications, UFGS 01 32 01.00 10 Project Schedule (WBDG); Navon, R., Cash flow forecasting and updating for building projects, Project Management Journal (PMI); Levelset, Front-loading a schedule of values. The S-curve examples are illustrative. This article is general information, not financial or legal advice.