What is a dollar roll in the MBS market and how does it work as a financing tool?

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:

1. Investor sells $10M FNMA 5.5$102.50**
2. Investor buys back $10M FNMA 5.5$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$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:

1. Implicit financing rate: The effective borrowing rate implied by the dollar roll is often below the repo rate, making it cheap financing
2. Balance sheet management: The MBS is off the seller's books during the roll period
3. Dealer demand: Dealers need specific MBS for TBA delivery; they pay up via favorable drop prices
4. 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.

Practice MBS financing strategies in our CFA Level II bank.