What is a dollar roll in the MBS market and how does it work as a financing tool?
I came across 'dollar rolls' in my CFA Level II structured products section. It seems similar to a repo but with MBS. How does it work, why is it beneficial, and what risks does it carry? I'd appreciate a numerical example.
A dollar roll is a financing transaction specific to the mortgage-backed securities (MBS) market. The investor sells an MBS for settlement in the current month and simultaneously agrees to buy back a 'substantially similar' (but not identical) MBS for settlement in a future month.
Key Difference from Repo:
In a repo, you get back the same security. In a dollar roll, you get back a similar security (same coupon, same agency, same original maturity, but different pool). This is because MBS are not fungible — each pool has different prepayment characteristics.
Mechanics:
- Investor sells $10M FNMA 5.5% MBS for March settlement at $102.50
- Investor buys back $10M FNMA 5.5% MBS (different pool) for April settlement at $102.00
Dollar Roll Benefit Calculation — Northgate Capital (fictional):
| Component | Amount |
|---|---|
| Drop (sell price - buy price) | $102.50 - $102.00 = $0.50 |
| Coupon foregone (1 month of 5.5%) | $10M x 5.5%/12 = $45,833 |
| Principal paydown missed | Assume $20,000 at par |
| Total cost of rolling | $65,833 |
| Interest earned on $10.25M cash (1 month at 5.0%) | $42,708 |
| Net dollar roll benefit | Drop x $10M/100 - Coupon - Paydown + Interest |
| = $50,000 - $45,833 - $20,000 + $42,708 = $26,875 |
Why Dollar Rolls Are Attractive:
- Implicit financing rate: The effective borrowing rate implied by the dollar roll is often below the repo rate, making it cheap financing
- Balance sheet management: The MBS is off the seller's books during the roll period
- Dealer demand: Dealers need specific MBS for TBA delivery; they pay up via favorable drop prices
- Liquidity: Dollar rolls are extremely liquid in the agency MBS market
Risks:
- Delivery risk: You get back a different pool that may have worse prepayment characteristics
- Market risk: If MBS prices move significantly, the agreed buy-back price may be unfavorable
- Accounting complexity: Treatment as financing vs. sale varies by standards
Exam Tip: The CFA exam may ask you to compute the implied financing rate of a dollar roll or determine whether rolling is more attractive than holding and financing via repo.
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