CMBS Tranching and the First-Loss Waterfall

1. One mortgage pool, several risk positions

Tranching changes who absorbs losses first; it does not change the pool's aggregate loss.

Securitization flow

Commercial mortgages

CMBS mortgage pool

Rated tranches

Investors buy different slices of the same collateral according to their risk and return preferences.

Priority rule

Payment priority runs downward; loss priority runs upward.

SeniorPaid first · Last loss · Lower required yield

MezzanineIntermediate protection, risk, and yield

JuniorPaid last · First loss · Higher required yield

2. Build and price the capital stack

Edit the pool assumptions, tranche shares, or buyer yields. Dollar amounts are in millions.

Pool principal ($MM)

$

Pool coupon

%

Loan term

years

Untranched price ($MM)

$

Recovery on defaulted loans

%

Face-weighted target yield — —

Illustrative tranched price — Course spreadsheet benchmark: $529.1 MM

Price gain from tranching — —

Purchaser net cash flow — Undiscounted pool cash inflows less price

Tranche assumptionsShare and target buyer yield

Tranche	Pool share	Face value	Buyer yield

Entered shares—

Capital stack and current lossesSenior at top · Junior at bottom

The tranche sizes are normalized for the visual if entered shares do not sum to 100%.

Pool price = PV of annual pool interest + PV of balloon principal

Following the course example, the page rounds the face-weighted target yield to one decimal percentage point before pricing the interest-only pool. The spreadsheet's $529.1 million value is a rounded trial-and-error benchmark.

3. Send a loss through the first-loss waterfall

Choose a pool default rate. Recoveries reduce the effective principal loss before it reaches the bonds.

Pool default rate 5.0%

Exact default rate

%

Effective pool loss — —

First-loss tranche — Lowest priority in the stack

Most senior tranche hit — None means all tranches remain intact

AAA subordination — Losses absorbed below AAA before attachment

Loss absorption by trancheRed overlay = principal loss

Waterfall calculationAttachment and detachment points

Tranche	Face	Attach	Detach	Loss	Loss rate	Remaining

Attachment is the amount of subordinate principal that must be exhausted before this tranche begins to lose principal.

4. See each tranche's nonlinear loss exposure

A tranche is protected below its attachment point, loses principal between attachment and detachment, and is exhausted above detachment.

—

5. Connect tranching to issuance economics

Tailoring bonds to investors can increase proceeds, but the benefit must exceed added structuring and issuance costs.

Illustrative pool sale price$MM per pool

Untranched

—

Tranched

—

Incremental sale proceeds—

Additional issuance cost per pool$3.0 MM

Net incremental proceeds per pool—

Course lender-return illustrationFour pools per year · 10-year horizon

Untranched lender IRR — $8 MM issuance cost per pool

Tranched lender IRR — $11 MM issuance cost per pool

Annual untranched net cash flow—

Annual tranched net cash flow—

Course benchmarks8.56% vs. 14.84%

6. Three conclusions to carry forward

Use the tool to separate collateral risk, security design, and economic value.

Losses are redistributed

Subordination protects senior bonds by concentrating early losses in junior bonds. The pool's total loss is unchanged.

Ratings reflect position

Attachment, detachment, collateral quality, and structural protections shape credit risk. The illustrative labels here are not agency ratings.

Value creation is conditional

Tranching can raise sale proceeds when investor demand lowers required yields, but higher transaction costs and model risk can offset that gain.