Structural Mechanics of Securitised Credit

Structural Mechanics: Cash Flows, Credit Enhancement and Tranching

Securitised credit products provide investors with the opportunity to gain exposure to diversified pools of loans through instruments such as mortgage-backed securities (MBS), commercial mortgage-backed securities (CMBS), asset-backed securities (ABS), and collateralised loan obligations (CLOs). While the underlying collateral differs across these products, the core structural mechanics of securitisation are broadly consistent.

The securitisation process

The securitisation process begins with loan originators pooling together loans with similar characteristics, such as mortgages, auto loans, or consumer loans. Once the loan pool reaches a sufficient size, the issuer engages an investment bank to structure and distribute the securitised products. A special purpose vehicle (SPV) or trust is then established to hold the underlying loans. This legal separation removes the assets from the issuer’s balance sheet and ensures that cash flows are dedicated to investors in the securitisation.

To finance the loan pool, the trust issues interest-bearing securities that are sold to capital market investors in discrete slices, known as tranches. During the structuring phase, tranche sizes, pricing, and features may be adjusted in response to investor demand. Over time, borrowers’ interest and principal payments are passed through the structure and distributed to investors according to a predefined payment hierarchy.

Securitised product tranching and credit enhancement

A typical securitisation structure is divided into three tiers:  junior, mezzanine, and senior tranches. These tranches differ in their priority of claim on cash flows, exposure to losses, and expected returns.

The more senior tranches (typically rated AAA, AA, A, and BBB) sit at the top of the capital structure and receive cash flows first. In exchange for this seniority and lower expected loss exposure, senior tranches offer the lowest returns.

Mezzanine and junior tranches (typically rated BB or below) rank lower in the payment hierarchy and are designed to absorb losses first if defaults occur in the underlying loan pool. Investors in these tranches are typically compensated for taking on greater risk through higher expected returns.

This tiered structure provides credit enhancement, as losses are absorbed by subordinated tranches before reaching senior tranches. As a result, senior tranches benefit from structural protection even when some underlying loans default.

Credit enhancement can also arise from excess cash flow generated within the securitisation. For example, in subprime auto ABS, borrowers typically pay higher interest rates than prime borrowers. This generates additional cash flow within the securitisation, which may be used to absorb losses and reduce the risk to senior bondholders.

Similarly, CLOs are structured to offer significant levels of credit support to senior tranches. Coverage tests are used to assess whether sufficient cash flow is being generated from the underlying loans to service the senior tranches. As a result, senior CLO tranches are typically well supported by subordinated positions in the capital structure. Historically, even during periods when leveraged loan default rates were elevated (such as during the global financial crisis), no AAA-rated senior CLO tranche has defaulted ^Footnote1.

Payment and yield compensation

Senior securitised tranches tend to offer attractive risk‑adjusted returns due to the multiple layers of credit protection beneath them. Despite their higher credit quality, AAA-rated securitised credit has often offered yields comparable to A rated corporate bonds, reflecting structural complexity and lower market utilisation relative to traditional corporate credit.

For investors willing to assume additional risk, lower-rated subordinated tranches can offer higher yields than similarly rated high yield corporate bonds. These higher returns generally reflect investors’ position lower in the capital structure, where losses are absorbed first, and should be assessed alongside the embedded credit and structural risks described in the tranching framework above.

Maturity and repayment profiles

Securitisation tranches may carry either fixed or floating interest rates, typically matching the interest rate characteristics of the underlying loan pool. Unlike government or corporate bonds, which have a single stated maturity date, securitised products often have more complex repayment profiles.

Most securitisations are backed by amortising loans where principal is repaid gradually over time alongside interest. This is particularly evident in residential mortgage markets, where monthly payments include both components. As a result, investors receive partial principal repayments throughout the life of the investment rather than a single payment at maturity.

Given this structure, investors generally focus on a tranche’s expected weighted-average life (WAL) rather than its stated maturity date. WAL represents the weighted average timing of all individual principal repayments and provides a more accurate measure of cash flow exposure than stated maturity alone.

Treatment of defaults

In securitisations with large numbers of underlying borrowers, the default of an individual loan does not automatically result in a default of the securitised product. Losses are first absorbed by the most junior tranches and only migrate up the capital structure if defaults accumulate.

A securitised product is generally considered to be in default only when losses reach the most senior tranche. This loss‑allocation mechanism underpins the credit stability of senior securitised bonds.

At the underlying loan level, borrowers that default typically enter a workout or loan modification process. This may involve adjusting loan terms to improve affordability or extending the loan’s maturity, sometimes in exchange for a higher interest rate to compensate the lender for increased risk.

For a single-asset/single borrower (SASB) CMBS, where only one borrower is involved, securitised product holders typically work with a third party, known as a special servicer, to determine a resolution to the default that is intended to be favourable to the bondholders.

Other securitisations, such as agency MBS, do not expose investors to credit losses from borrower default. In these structures, any default on the underlying mortgage is absorbed by the issuing agency, with the underlying loan removed from the pool at par. In effect, a default on a loan in an agency MBS pool behaves economically like a prepayment.

It should be noted that permanent losses for senior securitised bondholders have been rare historically, given the significant levels of subordination and credit support embedded in modern securitisation structures.