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Dollar Rolls as MBS Financing, Not Direct Short Exposure

Article Quant Q&A · Author: Jojo
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Summary

The note explains why buying a TBA dollar roll should not be treated as simply shorting mortgage-backed securities. A roll combines two linked trades: buying or selling the front-month TBA and taking the opposite position in a later month. The example follows a front-month TBA short toward settlement, describing covering the short, delivering available pools, or using a June/July roll to move the exposure forward. This frames the roll primarily as a financing transaction.

The answer cautions that a view that a coupon is rich does not by itself imply a reason to buy the roll. Roll prices reflect both months’ TBA prices and can be affected by supply and demand, prepayment expectations, and other factors. The discussion is conceptual and gives no pricing formula or empirical evidence, so it clarifies exposure mechanics without quantifying when a roll trade may be attractive.

Key ideas

  • A dollar roll pairs transactions in two TBA settlement months.
  • Buying a roll is not equivalent to taking a direct short position in MBS.
  • A roll is generally viewed as a financing transaction.
  • Roll prices depend on both months’ TBA prices and market-specific factors.
  • A belief that a coupon is rich does not alone establish a case for buying the roll.

Tags

Fixed incomeDerivatives pricingMarket microstructure
Full text
# Economics of buying a dollar roll


# Economics of buying a dollar roll












This is a basic question. But I wanted to understand how a TBA short (buying the dollar roll) is economically short an MBS? For example, if an investor believes a specific coupon is rich, they could buy the dollar roll. How would they benefit from that coupons TBA reducing in price?

## Answer by Sharad (score 2, accepted)

https://quant.stackexchange.com/a/71376

Buying/selling dollar rolls does not have the same economic exposure as going long/shorting MBS. The following example hopefully clarifies this. Suppose you short (sell) a TBA on June 1st for the front month. Then, as you approach settlement day: (1) You can buy the TBA back and cover the short; (2) Plan to deliver pools (which you already have access to) to the buyer; and (3) Buy the June/July roll for that coupon which covers your short for the front month and rolls it forward to the back month. Note that the dollar roll consists of two simultaneous transactions: a purchase/sale in the front month coupled with a sale/purchase in the back month. The dollar roll is viewed as a financing transaction rather than a naked long/short exposure.

There is no straightforward connection between believing a coupon is rich and buying the roll. Rather, the roll price is determined by both front and back month TBA prices, which in turn are a complicated function of supply/demand technicals for the two months, prepayment expectations, and miscellaneous other factors.

Shown in full with attribution under the source's licence. Licence: CC BY-SA 4.0 (Stack Exchange)

This summary was written by Stratmill's research agent from the original; it is not a copy of the source.