Sculpted vs annuity repayment

The mechanical difference

An annuity (or level-payment, mortgage-style) structure pays the same total debt service every period. Interest falls as the balance amortises, so principal rises to keep the sum constant. It is the default repayment shape in corporate lending, and there is nothing wrong with it when income is flat.

A sculpted structure sets each period's debt service to that period's CFADS divided by the target DSCR. The payment rises and falls with the project's cash. DSCR is then constant instead of the payment being constant.

The two structures answer different questions. An annuity asks "what does a level repayment of this debt look like?" Sculpting asks "what is the most debt these cash flows can carry at this covenant?" In project finance, the second question is the one that matters, because the debt quantum is the output of the model, not an input to it.

What happens to DSCR on an annuity

Take the worked example from the calculator: a 12-year renewables profile with CFADS ramping from 32 to a peak of 48.9 and then declining, a 1.35x covenant and a 6.75% coupon. Sculpted, it supports 264.18.

Now repay that same 264.18 on a level annuity of 32.82 per year. Debt service no longer moves with cash, so DSCR is simply CFADS divided by a constant — it traces the shape of the cash flows:

DSCR ranges from 0.98x to 1.49x, breaching the covenant in 3 of 12 years (years 1, 2, 12). Those breaches are not theoretical. A DSCR breach traps distributions, and a deep enough one is an event of default. No lender would sign this structure at this debt quantum.

Why the weakest period caps everything

So size the annuity down until it never breaches. Because the payment is constant, the constraint is a single inequality that has to hold in every period:

The largest permissible level payment is set entirely by the worst period. Here that is year 1, with CFADS of 32.00, which caps the payment at 23.70. Multiply by the annuity factor and the debt tops out at 190.81.

Every other year then services 23.70 while comfortably able to afford far more. In the peak year the annuity structure runs a DSCR of 2.06x against a 1.35x covenant — that headroom is capacity the sponsor has paid for and cannot use.

Sculpting removes the single-constraint problem entirely. Each period contributes its own capacity at exactly the target ratio, so the weak year constrains only the weak year.

Side by side

Where annuity is the better answer

Sculpting is not free, and it is not always right. The honest trade-offs:

• Sculpting raises absolute interest cost. A back-loaded principal profile means the balance stays outstanding longer. In the table above the sculpted structure pays more total interest — it is carrying more debt for longer. That is the price of the extra capacity, not a hidden cost.
• It concentrates refinancing and forecast risk. Sculpted debt leans on later-period CFADS that has not happened yet. If the production or price forecast for years 8–12 is soft, a sculpted structure is more exposed than a front-loaded one.
• Documentation is heavier. A sculpted profile means a bespoke amortisation schedule annexed to the facility agreement, and usually a re-sculpting mechanic on defined events. Annuity repayment is one line.
• Flat CFADS makes it pointless. If the cash flows are genuinely level — a fully availability-based PPP, for example — the sculpted and annuity profiles converge and the extra complexity buys nothing.

The rule of thumb: the more variable the CFADS profile, the more sculpting is worth. Run your own numbers through thecalculator and read the uplift figure — if it is under a couple of percent, the structuring effort probably is not worth it.