# CMBS: What Commercial Mortgage-Backed Securities Are and How They Work Published: 2026-03-19 Author: Warren Team URL: https://www.heywarren.com/blog/cmbs --- **CMBS** (Commercial Mortgage-Backed Securities) are bonds backed by a pool of commercial real estate loans, mortgages on office buildings, hotels, shopping centres, apartment complexes, industrial properties, and other income-producing real estate. CMBS allow banks and lenders to originate commercial real estate loans and then package them into securities sold to institutional investors, recycling capital for further lending. Investors in CMBS receive principal and interest payments from the underlying mortgage pool; in return for taking credit risk, they earn yields above comparable-maturity US Treasuries. The CMBS market is a critical source of commercial real estate finance, approximately $600–$800 billion in CMBS was outstanding in the US as of 2024. Understanding CMBS structure, tranching, and risks is important for commercial real estate investors, fixed income analysts, and anyone following the commercial real estate debt market. ## How CMBS Are Structured **Conduit CMBS** (the most common type): A bank or investment bank originates 50–300 commercial mortgages, pools them together, and transfers the pool into a Special Purpose Vehicle (SPV). The SPV issues multiple classes ("tranches") of bonds with different risk/return profiles: ![Commercial mortgages are pooled into an SPV, which issues tranched bonds sold to investors.](data:image/svg+xml,%3Csvg%20xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg%22%20viewBox%3D%220%200%201090%20125%22%20width%3D%221090%22%20height%3D%22125%22%20role%3D%22img%22%3E%3Ctitle%3EFlow%20diagram%3C%2Ftitle%3E%3Crect%20width%3D%22100%25%22%20height%3D%22100%25%22%20fill%3D%22%23f8fafc%22%2F%3E%3Crect%20x%3D%2230%22%20y%3D%2225%22%20width%3D%22170%22%20height%3D%2275%22%20rx%3D%2210%22%20fill%3D%22white%22%20stroke%3D%22%232563eb%22%20stroke-width%3D%222%22%2F%3E%3Ctext%20x%3D%22115%22%20y%3D%2258.5%22%20text-anchor%3D%22middle%22%20font-family%3D%22system-ui%2C-apple-system%2Csans-serif%22%20font-size%3D%2214%22%20font-weight%3D%22600%22%20fill%3D%22%230f172a%22%3ELender%3C%2Ftext%3E%3Ctext%20x%3D%22115%22%20y%3D%2278.5%22%20text-anchor%3D%22middle%22%20font-family%3D%22system-ui%2C-apple-system%2Csans-serif%22%20font-size%3D%2211%22%20fill%3D%22%2364748b%22%3EOriginates%20loans%3C%2Ftext%3E%3Cline%20x1%3D%22205%22%20y1%3D%2262.5%22%20x2%3D%22237%22%20y2%3D%2262.5%22%20stroke%3D%22%2364748b%22%20stroke-width%3D%222%22%2F%3E%3Cpolygon%20points%3D%22244%2C62.5%20235%2C57.5%20235%2C67.5%22%20fill%3D%22%2364748b%22%2F%3E%3Crect%20x%3D%22245%22%20y%3D%2225%22%20width%3D%22170%22%20height%3D%2275%22%20rx%3D%2210%22%20fill%3D%22white%22%20stroke%3D%22%232563eb%22%20stroke-width%3D%222%22%2F%3E%3Ctext%20x%3D%22330%22%20y%3D%2258.5%22%20text-anchor%3D%22middle%22%20font-family%3D%22system-ui%2C-apple-system%2Csans-serif%22%20font-size%3D%2214%22%20font-weight%3D%22600%22%20fill%3D%22%230f172a%22%3ELoan%20Pool%3C%2Ftext%3E%3Ctext%20x%3D%22330%22%20y%3D%2278.5%22%20text-anchor%3D%22middle%22%20font-family%3D%22system-ui%2C-apple-system%2Csans-serif%22%20font-size%3D%2211%22%20fill%3D%22%2364748b%22%3E50%E2%80%93300%20mortgages%3C%2Ftext%3E%3Cline%20x1%3D%22420%22%20y1%3D%2262.5%22%20x2%3D%22452%22%20y2%3D%2262.5%22%20stroke%3D%22%2364748b%22%20stroke-width%3D%222%22%2F%3E%3Cpolygon%20points%3D%22459%2C62.5%20450%2C57.5%20450%2C67.5%22%20fill%3D%22%2364748b%22%2F%3E%3Crect%20x%3D%22460%22%20y%3D%2225%22%20width%3D%22170%22%20height%3D%2275%22%20rx%3D%2210%22%20fill%3D%22white%22%20stroke%3D%22%232563eb%22%20stroke-width%3D%222%22%2F%3E%3Ctext%20x%3D%22545%22%20y%3D%2258.5%22%20text-anchor%3D%22middle%22%20font-family%3D%22system-ui%2C-apple-system%2Csans-serif%22%20font-size%3D%2214%22%20font-weight%3D%22600%22%20fill%3D%22%230f172a%22%3ESPV%3C%2Ftext%3E%3Ctext%20x%3D%22545%22%20y%3D%2278.5%22%20text-anchor%3D%22middle%22%20font-family%3D%22system-ui%2C-apple-system%2Csans-serif%22%20font-size%3D%2211%22%20fill%3D%22%2364748b%22%3ESpecial%20Purpose%20Vehicle%3C%2Ftext%3E%3Cline%20x1%3D%22635%22%20y1%3D%2262.5%22%20x2%3D%22667%22%20y2%3D%2262.5%22%20stroke%3D%22%2364748b%22%20stroke-width%3D%222%22%2F%3E%3Cpolygon%20points%3D%22674%2C62.5%20665%2C57.5%20665%2C67.5%22%20fill%3D%22%2364748b%22%2F%3E%3Crect%20x%3D%22675%22%20y%3D%2225%22%20width%3D%22170%22%20height%3D%2275%22%20rx%3D%2210%22%20fill%3D%22white%22%20stroke%3D%22%232563eb%22%20stroke-width%3D%222%22%2F%3E%3Ctext%20x%3D%22760%22%20y%3D%2258.5%22%20text-anchor%3D%22middle%22%20font-family%3D%22system-ui%2C-apple-system%2Csans-serif%22%20font-size%3D%2214%22%20font-weight%3D%22600%22%20fill%3D%22%230f172a%22%3ETranches%3C%2Ftext%3E%3Ctext%20x%3D%22760%22%20y%3D%2278.5%22%20text-anchor%3D%22middle%22%20font-family%3D%22system-ui%2C-apple-system%2Csans-serif%22%20font-size%3D%2211%22%20fill%3D%22%2364748b%22%3EAAA%20%E2%86%92%20Equity%3C%2Ftext%3E%3Cline%20x1%3D%22850%22%20y1%3D%2262.5%22%20x2%3D%22882%22%20y2%3D%2262.5%22%20stroke%3D%22%2364748b%22%20stroke-width%3D%222%22%2F%3E%3Cpolygon%20points%3D%22889%2C62.5%20880%2C57.5%20880%2C67.5%22%20fill%3D%22%2364748b%22%2F%3E%3Crect%20x%3D%22890%22%20y%3D%2225%22%20width%3D%22170%22%20height%3D%2275%22%20rx%3D%2210%22%20fill%3D%22white%22%20stroke%3D%22%232563eb%22%20stroke-width%3D%222%22%2F%3E%3Ctext%20x%3D%22975%22%20y%3D%2258.5%22%20text-anchor%3D%22middle%22%20font-family%3D%22system-ui%2C-apple-system%2Csans-serif%22%20font-size%3D%2214%22%20font-weight%3D%22600%22%20fill%3D%22%230f172a%22%3EInvestors%3C%2Ftext%3E%3Ctext%20x%3D%22975%22%20y%3D%2278.5%22%20text-anchor%3D%22middle%22%20font-family%3D%22system-ui%2C-apple-system%2Csans-serif%22%20font-size%3D%2211%22%20fill%3D%22%2364748b%22%3EInstitutional%20buyers%3C%2Ftext%3E%3C%2Fsvg%3E) *Commercial mortgages are pooled into an SPV, which issues tranched bonds sold to investors.* **CMBS Tranching Example** (simplified $1 billion pool): | Tranche | Rating | % of Pool | Yield Spread | Description | |---|---|---|---|---| | AAA (senior) | AAA | 65–70% | T+120bps | First to receive payments; last to absorb losses | | AA | AA | 8–10% | T+180bps | Senior mezzanine | | A | A | 5–7% | T+250bps | Mezzanine | | BBB | BBB | 4–6% | T+400bps | Junior mezzanine | | BB/B | Below investment grade | 3–4% | T+600bps+ | Higher yield; first to absorb losses above equity | | Equity (B-piece) | Not rated | 3–5% | Highest | First-loss piece; absorbs first losses | **The B-piece buyer**: The unrated first-loss piece (B-piece) requires a sophisticated "B-piece buyer", a CMBS investor who performs detailed due diligence on every loan in the pool and negotiates to remove problem loans before closing. B-piece buyers earn above-market returns for taking the first-loss risk. ## CMBS vs. RMBS (Residential MBS) | Feature | CMBS | RMBS | |---|---|---| | Underlying collateral | Commercial real estate (office, hotel, retail, multifamily) | Residential mortgages | | Number of loans | 50–300 (large loans, individually analyzed) | Thousands (small loans, statistically analyzed) | | Loan size | $5M–$500M+ per loan | $100K–$1M typical | | Prepayment protection | Lockout, yield maintenance, defeasance | Callable; significant prepayment risk | | Recourse | Typically non-recourse | Recourse to borrower in most states | | Analysis approach | Asset-by-asset underwriting | Pool-level statistical modelling | ![CMBS pools contain far fewer but far larger loans than RMBS pools, requiring asset-by-asset underwriting.](data:image/svg+xml,%3Csvg%20xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg%22%20viewBox%3D%220%200%20800%20210%22%20width%3D%22800%22%20height%3D%22210%22%20role%3D%22img%22%3E%3Ctitle%3EComparison%3C%2Ftitle%3E%3Crect%20width%3D%22100%25%22%20height%3D%22100%25%22%20fill%3D%22%23f8fafc%22%2F%3E%3Ctext%20x%3D%22230%22%20y%3D%2257.5%22%20text-anchor%3D%22end%22%20font-family%3D%22system-ui%2C-apple-system%2Csans-serif%22%20font-size%3D%2214%22%20font-weight%3D%22600%22%20fill%3D%22%230f172a%22%3ECMBS%20loans%3C%2Ftext%3E%3Crect%20x%3D%22240%22%20y%3D%2225%22%20width%3D%2215.750000000000002%22%20height%3D%2255%22%20rx%3D%226%22%20fill%3D%22%232563eb%22%2F%3E%3Ctext%20x%3D%22267.75%22%20y%3D%2257.5%22%20font-family%3D%22system-ui%2C-apple-system%2Csans-serif%22%20font-size%3D%2214%22%20font-weight%3D%22700%22%20fill%3D%22%232563eb%22%3E175%3C%2Ftext%3E%3Ctext%20x%3D%22230%22%20y%3D%22152.5%22%20text-anchor%3D%22end%22%20font-family%3D%22system-ui%2C-apple-system%2Csans-serif%22%20font-size%3D%2214%22%20font-weight%3D%22600%22%20fill%3D%22%230f172a%22%3ERMBS%20loans%3C%2Ftext%3E%3Crect%20x%3D%22240%22%20y%3D%22120%22%20width%3D%22450%22%20height%3D%2255%22%20rx%3D%226%22%20fill%3D%22%237c3aed%22%2F%3E%3Ctext%20x%3D%22702%22%20y%3D%22152.5%22%20font-family%3D%22system-ui%2C-apple-system%2Csans-serif%22%20font-size%3D%2214%22%20font-weight%3D%22700%22%20fill%3D%22%237c3aed%22%3E5.0K%3C%2Ftext%3E%3C%2Fsvg%3E) *CMBS pools contain far fewer but far larger loans than RMBS pools, requiring asset-by-asset underwriting.* **Prepayment protection**: Unlike residential mortgages, CMBS loans typically include lockout periods (no prepayment for 2–5 years) and yield maintenance or defeasance provisions that make prepayment very expensive, protecting investors from reinvestment risk. ## Key CMBS Metrics for Analysis **LTV (Loan-to-Value)**: Lower LTV = safer. CMBS loans typically originated at 50%–75% LTV. Higher LTV tranches are riskier. **DSCR (Debt Service Coverage Ratio)**: NOI / Debt Service. A DSCR of 1.25× means the property generates 25% more cash than needed to service the loan. DSCR below 1.0× means the property can't cover its debt payments. **Occupancy and tenant quality**: For office and retail CMBS, occupancy rates and the credit quality of anchor tenants directly affect cash flow reliability. **Debt yield**: NOI / Loan Balance. A debt yield of 8%+ is generally considered safe; below 6% indicates high leverage. ## CMBS Market Stress: 2008 and Post-COVID **2008–2009 financial crisis**: CMBS experienced severe losses as commercial real estate values fell 40%+. Lower-rated tranches were wiped out; even some BBB tranches suffered significant losses. The CMBS market froze completely, and it took 2–3 years to recover. **2020–2022 COVID stress**: Hotel and retail CMBS were severely stressed as occupancy collapsed. Special servicers (who manage defaulted loans) became very active. Many CMBS loans were modified; some hotels and mall properties went through REO (Real Estate Owned) disposition. **2024–2025 office stress**: The post-pandemic decline in office occupancy has stressed office-heavy CMBS pools. Urban office CMBS has seen significant special servicing transfers and losses, particularly in gateway cities with high vacancy. ## Authoritative Sources For deeper background and primary-source data on this topic, the following authoritative sources are useful starting points: - [Consumer Financial Protection Bureau](https://www.consumerfinance.gov/) - [HUD, Department of Housing and Urban Development](https://www.hud.gov/) ## Conclusion CMBS pool commercial real estate loans and tranche them into rated bonds with different risk/return profiles. The AAA senior tranche provides highly rated, yield-above-Treasuries exposure to commercial real estate cash flows; lower tranches provide higher yield with higher risk. CMBS analysis requires understanding loan-level underwriting (LTV, DSCR, property type) as well as structural protections. For related real estate finance concepts, see our guides on [asset-backed securities (ABS)](/blog/asset-backed-securities-abs) and [what is real estate NOI](/blog/what-is-real-estate-noi). Warren at [heywarren.com](https://heywarren.com) helps real estate investors, fixed income analysts, and commercial real estate professionals understand CMBS structure, credit analysis, and the dynamics of the commercial mortgage-backed securities market. --- ## Related Reading **More from Warren**: - [Asset-Backed Securities (ABS): What They Are and How They Work](/blog/asset-backed-securities-abs) - [What Is Real Estate NOI: How to Calculate Net Operating Income](/blog/what-is-real-estate-noi) - [PIK Interest: What Payment-in-Kind Means in Leveraged Finance](/blog/pik-interest) **Authoritative sources**: - [SIFMA, CMBS Market Data](https://www.sifma.org/resources/research/us-mortgage-related-issuance-and-outstanding/) - [Trepp, CMBS Analytics](https://www.trepp.com/) - [SEC, Asset-Backed Securities Regulation](https://www.sec.gov/rules/final/2014/33-9638.pdf)