FI 19 – Mortgage-Backed Security (MBS) Instrument and Market Features

Mortgage-backed securities (MBS) are bonds built by securitizing mortgage loans: the pool of loans is moved into a special purpose entity, and the entity issues securities whose payments come from the loan cash flows. The complication is that those cash flows are not fixed. A borrower may repay early, pay late, or fall behind, and the pace depends on household income, the loan terms (the original rate versus current rates, and any prepayment penalty), and whether the property is sold. Because scheduled and actual payments rarely match, an investor pricing an MBS has to make assumptions about how prepayments will unfold.

Prepayment risk is the risk that principal, or some part of it, is returned on a schedule that differs from the contract. It has two sides, and both are driven by the general level of interest rates.

Contraction risk and extension risk

When rates fall, borrowers of fixed-rate mortgages refinance into cheaper loans, so realized prepayments run ahead of the forecast and the security pays off sooner than expected. This is contraction risk: principal comes back faster than scheduled, which hurts the investor twice. The returned money has to be reinvested at the new, lower rates, and the early repayment caps how much the bond price can rise. Refinancing is especially heavy in the United States, where mortgages rarely carry prepayment penalties; penalties are far more common elsewhere.

When rates rise, the opposite happens. Borrowers hold on to their low-rate loans and may postpone buying a new home, so prepayments come in below forecast and the security lasts longer than expected. This is extension risk: the stretched-out cash flows now have to be discounted at higher rates, so their present value falls just when the investor would rather have the cash to reinvest.

Time tranching

A securitization structure can shift prepayment risk from one investor to another rather than remove it. The tool is time tranching: the issuer creates bond classes with different expected maturities. In a sequential arrangement, all principal repayments flow to the first tranche until it is fully retired, then to the next tranche, and so on down the line. Investors then pick the tranche whose expected life suits them, though they still bear the chance that prepayments arrive faster or slower than assumed. A single deal often layers time tranching on top of credit tranching (a subordinated, waterfall structure), so a note can be defined by both its expected maturity and its seniority.

A mortgage loan is secured by a specific real estate property and commits the borrower to a set series of payments. The lender holds a first lien and a security interest in the property, so if the borrower stops paying as agreed, the lender can foreclose, take possession, and sell the property to recover what is owed. Mortgage loans are the foundation of the world’s biggest ABS market, that for MBS.

Loan-to-value and debt-to-income

The loan advanced is usually smaller than the purchase price; the gap is the down payment. The loan-to-value ratio (LTV) equals the mortgage amount divided by the property value. A lower LTV means more borrower equity, which makes default less likely and leaves the lender better covered if a sale is forced. LTV is not static: it moves as principal is paid down and as the property value drifts up or down. It matters in both residential and commercial lending.

Capacity to keep up with payments is judged separately. In residential lending the measure is the debt-to-income ratio (DTI), monthly debt payments divided by monthly pre-tax gross income. A low DTI signals room to service the debt comfortably; a high DTI warns that the borrower may be overextended relative to income. Lenders prefer to see a low DTI before advancing a loan.

Prime, subprime, agency, and non-agency

In the United States, lenders sort mortgages by borrower credit quality. Prime loans go to strong borrowers: solid employment and credit history, low DTI, ample equity, and a first lien on the property. Subprime loans go to weaker borrowers, often with high DTI or high LTV, and can be secured only by a second lien ranking behind other debt.

RMBS are the bonds created by securitizing residential mortgages. Across four markets, the United States, Canada, Japan, and South Korea, the dividing line is whether a government or quasi-government body stands behind the securities. US residential securitizations fall into three buckets: guaranteed by a federal agency, guaranteed by a government-sponsored enterprise (GSE), or issued by a private entity with no such guarantee. The first two are agency RMBS; the third is non-agency RMBS. Federal-agency securities are backed by the full faith and credit of the US government, which effectively guarantees timely interest and principal. GSE securities carry the GSE’s guarantee rather than the government’s, and the GSE charges a fee for it. Non-agency issuers, banks and other private businesses, obtain credit support instead through subordination, guarantees, pool insurance, or letters of credit; that market largely vanished after the global financial crisis amid tighter regulation.

Contingency features: prepayment and recourse

The borrower’s most important right is the prepayment option, the ability to pay down more principal than the schedule requires, in part or in full, before maturity. For the lender or investor this is exactly what makes cash flows uncertain. Lenders can blunt it with a prepayment penalty that compensates for the rate difference when a borrower prepays after rates fall; such penalties are common in Europe but rare in the United States.

The other key feature is recourse. If a default forces a sale and the proceeds fall short of the balance owed, a recourse loan lets the lender pursue the borrower for the shortfall, while a non-recourse loan confines the lender to the property alone. US residential mortgages are non-recourse in many states, whereas most European residential mortgages are recourse. The distinction bites hardest for underwater loans, where the balance exceeds the property value (LTV above 100%): under non-recourse rules a borrower may choose a strategic default and hand back the keys even while able to keep paying, because the lender cannot reach the borrower’s other assets.

A mortgage pass-through security is created when a lender pools mortgages and sells securities backed by them. The pool’s monthly cash flows, principal, interest, and prepayments, flow through to the security holders once fees are deducted. A pool can hold several thousand loans or only a handful. Once a mortgage backs a pass-through, it is said to be securitized.

The fees cover servicing: collecting payments, forwarding proceeds, sending notices, tracking balances, starting foreclosure when needed, and supplying tax information, plus any charge for guaranteeing the issue. Because these charges come out of the mortgage interest, the security’s coupon, called the pass-through rate or net coupon, sits below the average rate earned on the underlying loans. In the structure shown in the source, a weighted average coupon of 4.55% on the loans, less servicing fees of 0.55%, leaves a 4.00% pass-through rate.

Weighted average coupon and weighted average maturity

The loans in a pool are heterogeneous: they differ in balance, rate, and remaining term. Two pool-level summaries capture this. The weighted average coupon (WAC) weights each loan’s rate by its share of the pool’s current balance. The weighted average maturity (WAM) weights each loan’s remaining months to maturity by the same balance share.

Because scheduled repayments and unscheduled prepayments both occur over an MBS’s life, the legal maturity says little about when the bond will actually be repaid. Market participants therefore lean on the weighted average life (the average life), an estimate of how long the security will be outstanding if current rates hold and expected prepayments are realized. Two bonds with the same 30-year legal maturity, one an option-free corporate bond and one an MBS, do not carry the same interest rate risk, because only the MBS is reshaped by prepayments.

Collateralized mortgage obligations (CMOs) securitize pass-through securities or several loan pools and then carve the combined cash flows into bond classes, or tranches, with different exposures to prepayment risk. Tranching cannot make prepayment risk disappear, but it can move that risk around, sheltering some tranches at the expense of others and so trimming the uncertainty over the size and timing of each investor’s payments. The features are tuned to institutional needs, and as a rule the more senior a tranche, the less prepayment risk and default risk it carries.

Sequential-pay CMO

In a sequential-pay structure the tranches are retired in order, a direct application of time tranching. Every principal payment goes to Tranche A until its balance reaches zero, then to Tranche B until it is retired, and so on. Consider a CMO backed by USD100 million of collateral with a 4% pass-through coupon, a 4.55% WAC, and a 360-month WAM, split into three tranches.

Ordering the principal payments protects the short tranche, A, against extension risk, because the longer tranches B and C stand behind it; in return, B and C gain protection against contraction risk from A. The exact monthly principal is still unknown, since it depends on the collateral’s actual prepayment speed, but the average lives differ enough to serve different investors. An endowment funding near-term projects favors the short average life of Tranche A; a pension fund expecting a wave of retirements in later years favors the longer Tranche B; and a long-term investor comfortable with more risk takes Tranche C.

Other CMO tranches

The same cash flows can be sliced in further ways, some through time tranching and some by separating interest from principal.

• Z-tranche (accrual bond): pays no interest until a preset date; during the accrual period its principal is credited with the stated coupon. Usually the last tranche in a sequence, it frees up cash for the other tranches and spares its holder reinvestment risk when yields fall. Average lives often exceed 20 years, which makes it risky and hard to value.
• Principal-only (PO): receives only the pool’s principal repayments. Since the holder collects face value through scheduled principal and prepayments, PO value is highly sensitive to prepayment and interest rates, rising when rates fall or prepayments accelerate.
• Interest-only (IO): receives only interest and has no par value; it is the companion of the PO. Faster prepayments cut its cash flows, so it is often used to hedge interest rate risk.
• Floating-rate: pays a rate tied to an index or market reference rate, usually with a cap and a floor. It can be built as an inverse floater, whose rate moves opposite to rates, and is used to hedge interest rate risk.
• Residual: collects whatever remains after every other tranche is paid. It suits investors able to bear and hedge the risk, such as hedge funds and long-term institutions; banks tend to avoid it because of capital requirements.
• Planned amortization class (PAC): an evolution of the sequential CMO. A PAC promises scheduled, fixed principal payments as long as the pool’s prepayment rate stays within a set maximum and minimum band. Inside that band, all prepayment risk is absorbed by an accompanying support tranche, giving PAC investors far more predictable cash flows.

A commercial mortgage-backed security (CMBS) is backed by mortgages on income-producing real estate: shopping centers, offices, warehouses, hotels, apartment blocks, and health care facilities. The loans are repaid out of rent and the other income each property produces. The loans in the pool were written either to finance an acquisition or to refinance an existing mortgage. The securitization and structuring mechanics closely resemble RMBS, but two features set it apart: call protection and a balloon maturity provision.

Call protection

Unlike RMBS, CMBS shield investors from early prepayment, which makes them trade more like corporate bonds. Protection comes at two levels. Structural call protection works through sequential-pay tranches: no junior tranche receives any principal until the senior tranche above it has been completely retired, and principal losses fall on the junior tranches first. Loan-level protection rests on covenants, through three mechanisms:

• Prepayment lockout: a contractual ban on any prepayment during a stated period.
• Prepayment penalty points: preset penalties a refinancing borrower must pay the CMBS entity, where one point equals 1.00% of the outstanding loan balance.
• Defeasance: prepayment is allowed, but the borrower must buy a portfolio of government securities that exactly replicates the loan’s remaining scheduled principal and interest, including the balloon balance. When the last payment is met the portfolio is worth zero, and the cost of assembling it is the cost of defeasing.

Balloon maturity and its risk

Most residential mortgages are self-amortizing, but commercial loans usually are not. They pay interest and some principal along the way, then repay the large unamortized balance in a single balloon payment at maturity (often at year 5 or year 10). A borrower can fail to make that payment for several reasons: an inability to refinance because debt service coverage or LTV has deteriorated, a lender unwilling to extend given the economy, or a property that cannot be sold for enough to clear the balance. Balloon risk is the risk of this default at maturity. The lender may then grant a workout period and modify the terms, which lengthens the loan, so balloon risk is a form of extension risk.

Concentration risk and the DSC ratio

An RMBS pool may hold thousands of small, similar residential loans, so any single default barely moves the security. A CMBS pool may hold only a few large commercial loans, so one default can hit investors hard. This concentration risk forces CMBS investors to study the individual loans, the specific properties, and the owners, not just the deal structure. The two key credit indicators are the LTV, already covered, and the debt service coverage (DSC) ratio, the property’s annual net operating income divided by its debt service.

Net operating income (NOI) is rental income minus cash operating expenses minus a non-cash replacement reserve for the property’s depreciation; it excludes loan principal and interest, capital expenditure, depreciation, and amortization. Debt service is the annual interest plus principal repayment. A DSC above 1.0 means the property’s cash flows cover debt service while keeping the property in its original condition, and the higher the ratio, the safer the loan.