Dollar Rolls: MBS Financing, Drop Calculation & Break-Even Analysis
Dollar rolls are a specialized financing mechanism for agency mortgage-backed securities. Unlike standard repurchase agreements, dollar rolls allow the buyer to return “substantially similar” securities rather than the identical bonds, creating unique economics that every MBS investor should understand. This guide covers the drop calculation, implied financing rate derivation, break-even analysis, and when to choose dollar rolls over repo.
What Is a Dollar Roll in MBS?
A dollar roll is a repo-like financing trade specific to agency mortgage-backed securities (MBS). The seller delivers MBS for near-month settlement and simultaneously agrees to repurchase substantially similar MBS for far-month settlement — same program (or UMBS-deliverable cohort for Fannie/Freddie), coupon, and maturity, but potentially different underlying mortgage pools.
In a standard repo, you get back the identical securities and retain all coupon and principal payments. In a dollar roll, you get back substantially similar securities, and the buyer keeps all cash flows during the roll period. This makes dollar rolls more like a true sale and repurchase than collateralized borrowing.
Dollar rolls developed because agency MBS pass-throughs are fungible within a given coupon and issuer. Returning the exact same pool numbers is impractical when thousands of pools share identical characteristics. The TBA (to-be-announced) market provides the settlement infrastructure for dollar rolls — sellers specify the issuer, coupon, and face value, but not the specific pools until 48 hours before settlement.
Key differences from standard repo:
- Securities returned: Substantially similar (not identical)
- Coupon and principal: Retained by the buyer during the roll period
- Margin: No repo haircut, though TBA trades may have mark-to-market margin under FINRA Rule 4210
- Funding rate: Implied from the price drop, not explicitly stated
The Drop: MBS Dollar Roll Price Difference
The drop is the price difference between the near-month sale price and the far-month repurchase price. It represents compensation to the seller for giving up the securities and forgoing cash flows during the roll period.
MBS prices are quoted in 32nds. For example, 100-08 means 100 + 8/32 = 100.25% of par, or $1,002,500 per $1 million face value. In a normal (upward-sloping) yield curve environment, the drop is positive — the near-month price exceeds the far-month price.
The drop reflects multiple factors: the coupon carry foregone by the seller, expected prepayment rates, dealer balance sheet constraints, collateral scarcity in the TBA market, and fails pressure near settlement dates. When dealers urgently need specific coupons to cover short positions, the drop widens.
Implied Financing Rate on a Dollar Roll
The drop alone doesn’t tell you whether a dollar roll is attractive — you must calculate the implied financing rate and compare it to alternative funding sources like repo.
The cash flow timeline for a dollar roll seller:
- Near-month settlement: Receive sale proceeds (full market value, no haircut)
- During roll period: Buyer receives all coupon and principal payments
- Far-month settlement: Repurchase substantially similar MBS at the lower forward price
The challenge: prepayments are unknown until the roll terminates. The implied financing rate is an estimate based on projected prepayment speeds. Actual financing cost may differ once prepayments are realized.
Premium vs Discount MBS: How Par Price Affects Roll Economics
Whether an MBS trades above or below par significantly affects dollar roll economics because the buyer receives all principal payments at par during the roll period.
Premium MBS (Price > 100)
- Buyer receives principal at par, but securities trade above par
- Faster prepayments hurt the buyer
- The premium erodes faster than expected
- Seller benefits slightly from prepay offset
Discount MBS (Price < 100)
- Buyer receives principal at par, but securities trade below par
- Faster prepayments help the buyer
- Buyer gains the discount on returned principal
- Seller’s financing cost increases with prepays
Roll analysis must incorporate expected prepayment speeds (CPR or PSA). A slight difference in prepayment assumptions can materially change the implied financing rate, especially for premium MBS with high coupons.
Break-Even Drop: When to Roll vs Repo
The break-even drop is the drop value at which the implied financing rate on a dollar roll equals the prevailing GC (general collateral) repo rate for agency MBS. It provides a clear decision rule.
Drop > Break-Even: The roll is “special” — implied financing is cheaper than repo. Sell the roll.
Drop < Break-Even: Repo is cheaper. Finance via repo instead of rolling.
Roll specialness occurs when the implied financing rate is significantly below the GC repo rate. This typically happens when dealers need specific coupons to cover short positions, when TBA demand creates collateral scarcity, or when month-end balance sheet constraints reduce dealer willingness to hold inventory. Extremely special rolls can imply financing rates near zero or even negative.
Worked Example: GNMA 7.5% Dollar Roll Analysis
Setup: A portfolio manager holds $10 million face value of GNMA 7.5% 30-year MBS. She enters a 30-day dollar roll:
- Sell at 100-08 (100.25%) = $10,025,000
- Buy back at 100-00 (100.00%) = $10,000,000
- Drop = 8/32 = 0.25% = $25,000
| Component | Calculation | Value |
|---|---|---|
| Sale proceeds | $10M × 1.0025 | $10,025,000 |
| Drop value | 8/32 × $10M | $25,000 |
| Coupon foregone | 7.50% × $10M × (30/360) | $62,500 |
| Principal paydown (assumed) | Estimated at ~200 PSA | $50,000 |
| Principal impact | 0.25% premium × $50,000 | $125 (offset) |
| Net financing cost | $62,500 – $25,000 – $125 | $37,375 |
| Implied financing rate | ($37,375 / $10,025,000) × (360/30) | 4.47% |
Decision: If GC repo for agency MBS is available at 5.00%, the dollar roll offers a 53 basis point funding advantage. The roll is special — the manager should sell the roll rather than finance via repo.
Dollar Roll vs Repo Comparison
Understanding the key differences between dollar rolls and standard repo is essential for choosing the right financing approach.
| Dimension | Classic Repo | Dollar Roll |
|---|---|---|
| Securities returned | Identical | Substantially similar |
| Coupon/principal | Returned to seller (via margin adjustment) | Retained by buyer |
| Margin | Repo haircut (typically 2-5%) | No haircut (may have MTM margin) |
| Funding rate | Explicitly stated repo rate | Implied from drop |
| Settlement | DVP or tri-party | TBA settlement |
| Cash flow economics | Seller retains economic exposure | Transferred to buyer during roll |
| Accounting treatment | Usually secured borrowing | Depends on ASC 860 analysis |
| Eligible securities | Broad (Treasuries, agencies, corporates, etc.) | Agency MBS only (GNMA, FNMA, FHLMC) |
Common Mistakes
1. Assuming positive drop = cheap financing. The drop is only one component. You must calculate the implied financing rate by accounting for coupon foregone and principal impact. A large drop can still result in expensive financing if coupon forfeiture exceeds the benefit.
2. Ignoring prepayment impact. For premium MBS, faster-than-expected prepayments improve the seller’s economics (principal returned at par vs. premium price). For discount MBS, prepayments hurt the seller. Sensitivity analysis across prepayment assumptions is essential.
3. Comparing drop to repo rate directly. The drop is in price points; the repo rate is an annualized percentage. Converting the drop to an implied rate requires the full formula including coupon and principal adjustments. Comparing apples to oranges leads to poor financing decisions.
4. Mixing up “buying the roll” vs “selling the roll.” Terminology can be confusing:
- Selling the roll: You’re the MBS holder — sell near-month, buy back far-month (financing your position)
- Buying the roll: You’re taking the other side — buy near-month, sell far-month (providing financing)
5. Expecting the same pool back. Dollar rolls return “substantially similar” securities — same program, coupon, and maturity, but potentially different pool numbers with different prepayment characteristics. You may receive pools that prepay faster or slower than the ones you delivered.
6. Ignoring cheapest-to-deliver risk. The counterparty will deliver the least desirable pools within good delivery guidelines. Expect to receive pools with unfavorable prepayment profiles or higher remaining balances. The 0.01% delivery variance tolerance provides minimal protection.
Limitations
Agency MBS only: Dollar rolls are specific to Ginnie Mae (GNMA), Fannie Mae (FNMA), and Freddie Mac (FHLMC) pass-through securities that settle TBA. Corporate bonds, Treasuries, and non-agency MBS cannot be rolled.
Financing cost uncertainty: Unlike repo where the rate is fixed at inception, the actual financing cost of a dollar roll is unknown until termination. Prepayment rates determine the principal impact, and you don’t know actual prepayments until the roll closes.
Cheapest-to-deliver adverse selection: Counterparties deliver pools within the 0.01% variance tolerance that are least valuable to them. Over time, dollar roll participants systematically receive pools with less favorable characteristics.
Pool-specific portfolios: Dollar rolls are unsuitable for portfolios that require specific pool characteristics (seasoning, geography, loan balance). You lose control over exactly which pools you hold.
Accounting complexity: Whether a dollar roll qualifies as secured financing or a sale/repurchase depends on fact-specific analysis under ASC 860. Accounting treatment is not automatic — consult with advisors for proper classification.
Frequently Asked Questions
Disclaimer
This article is for educational and informational purposes only and does not constitute investment advice. Dollar roll economics depend on prepayment assumptions that may prove incorrect. Example calculations use hypothetical values. Accounting treatment is fact-specific under ASC 860 — consult advisors for proper classification. Always conduct your own analysis before executing dollar roll transactions.
