Structured & Insurance-Linked
How to Underwrite an RMBS Tranche
Underwrite an RMBS tranche by modeling the mortgage collateral loan by loan, then running projected prepayments, defaults and severities through the deal's structure. Focus on borrower FICO, CLTV and documentation type, third-party due diligence findings, and the tranche's credit enhancement under shifting-interest or sequential rules. For agency RMBS, prepayment risk dominates instead of credit.
What makes rmbs tranche underwriting different
Residential mortgage-backed securities are bonds backed by pools of home loans. Agency RMBS carry a government-sponsored guarantee, so the analysis is about prepayment and interest rate risk. Non-agency RMBS, including prime jumbo, non-QM, re-performing and credit risk transfer deals, expose investors to borrower defaults as well. What makes RMBS distinct is the long life and borrower prepayment option: when rates drop, borrowers refinance and bonds bought above par lose value, and when rates rise, bonds extend. Credit losses depend on home prices, borrower equity and how servicers handle delinquent loans, including advancing and modifications.
Senior non-agency investors look at credit enhancement, the shifting-interest mechanism that locks out subordinate principal for years, and third-party due diligence on the loans. Subordinate investors focus on loss timing and severity because they absorb the first losses. Agency investors model prepayment speeds by coupon, loan size, geography and servicer. Issuers and warehouse lenders focus on loan eligibility and the gain on sale achievable at securitization. Re-performing loan deals add delinquency history and modification terms to the analysis, and servicer advancing rules affect cash flow timing to every tranche.
Key metrics
Credit enhancement
Subordination and overcollateralization below the tranche determine loss absorption capacity.
Weighted average FICO and CLTV
Borrower credit and equity are the main drivers of default and severity in non-agency pools.
Prepayment speed (CPR)
Prepayments change the bond's life and yield and, in shifting-interest deals, the share of principal going to subordinates.
Projected default and loss severity
These determine whether losses reach a given tranche under base and stress cases.
Due diligence exception rate
Third-party review results show how well loans were underwritten and documented.
Delinquency pipeline
Loans 60 or more days delinquent foreshadow future losses and can trip performance triggers.
How to underwrite it, step by step
- Review the offering documents, loan-level data and third-party due diligence reports.
- Stratify collateral by FICO, CLTV, DTI, documentation type, occupancy, loan purpose and geography.
- Build prepayment, default and severity assumptions and stress them by home price scenario.
- Model the waterfall including shifting interest, step-down tests and servicer advancing.
- Calculate tranche yields, weighted average life and break-even losses under each scenario.
- Evaluate the servicer, representation and warranty framework and enforcement mechanism.
- Compare tranche spread to comparable deals and set position size.
Due diligence checklist
- Prospectus or offering memorandum
- Loan-level data file
- Third-party due diligence reports
- Pooling and servicing agreement
- Representation and warranty provisions
- Rating agency presale reports
- Monthly remittance reports
- Servicer and master servicer information
- Agreed-upon procedures report
- Home price index data for key markets
Key risks and mitigants
Home price declines increasing severity
Stress home prices by region and model higher severities in downside cases.
Fast prepayments on premium bonds
Model yield across prepayment scenarios and avoid paying large premiums for refinance-sensitive collateral.
Extension when rates rise
Assess weighted average life sensitivity to slower prepayments.
Weak representation and warranty enforcement
Review the breach review process and independent reviewer triggers.
Servicer advancing and modification practices
Diligence servicer policies and model the effect of advancing stop rules and modifications on cash flows.
Typical financing and deal structure
Non-agency RMBS commonly use a senior-subordinate structure with shifting interest, where subordinate bonds receive little or no prepayment principal for an initial lockout period so credit enhancement builds for senior bonds. Step-down tests allow principal to flow to subordinates once delinquency and loss thresholds are met. Non-QM and re-performing deals often use sequential pay structures with overcollateralization and excess spread. Credit risk transfer deals reference agency pools and pay based on realized losses. Agency RMBS are pass-throughs or are restructured into collateralized mortgage obligations with planned amortization and support classes. Servicers advance delinquent payments up to recoverability limits.
Red flags
- High due diligence exception rates or waived findings
- Collateral weaker than comparable recent deals at similar spreads
- Weak or vague representation and warranty enforcement
- Servicer with poor loss mitigation history
- Rising early delinquencies in the issuer's prior deals
Frequently asked questions
What is the difference between agency and non-agency RMBS?
Agency RMBS are guaranteed by Fannie Mae, Freddie Mac or Ginnie Mae, so investors take prepayment and interest rate risk but little credit risk. Non-agency RMBS have no such guarantee, so investors also bear borrower default losses. Non-agency analysis requires loan-level credit modeling and careful review of deal structure and representations.
What is shifting interest in RMBS?
Shifting interest is a structure that directs most or all prepayment principal to senior bonds for a set period, so subordinate bonds remain outstanding and senior credit enhancement grows as a percentage of the pool. After the lockout, principal can shift to subordinates if performance tests are met. It protects senior investors from adverse selection late in the deal's life.
Why does prepayment risk matter in RMBS?
Borrowers can usually prepay at any time, often by refinancing when rates fall. For bonds bought at a premium, faster prepayments reduce yield. When rates rise, prepayments slow and bonds extend, which lowers value in a rising rate environment. Prepayment modeling is therefore central for both agency and non-agency investors.
What is non-QM RMBS?
Non-QM RMBS are backed by mortgages that fall outside qualified mortgage rules, such as loans to self-employed borrowers using bank statement income, investor property loans or loans with higher debt-to-income ratios. They can perform well with strong borrower equity, but investors analyze documentation type and underwriting practices closely.
Related terms
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Try Underridge freeEducational content, not investment, credit, legal or tax advice. Benchmarks are common practitioner rules of thumb and vary by lender, market and cycle.