With mortgage interest rates rising so quickly—nearly a full percentage point in the past month—getting an interest rate that sticks has taken on greater significance.

Interest rates have risen for seven consecutive weeks, from the 5.21 percent market bottom on June 12 to 6.34 percent on Aug. 7, according to Freddie Mac.

A rate lock can help hold the line. A traditional rate lock is a lender's guarantee that your mortgage carries a specific interest rate, points and other costs.

The lock is good for a specific period—if you fail to complete your home purchase or refinance before the clock runs out, and interest rates rise, brace yourself to pay a higher rate. If interest rates fall during the lock period, you can't take advantage of them unless you rewrite the lock and pay additional costs.

Only a "float down" option grants you a lower rate if rates fall within a given window of time. Unless specified otherwise, float downs stick you with the higher rate if rates rise during the lock period.

Everything depends on the language in the rate-lock agreement.

"Each lender has their own policy," said Stephanie Noryko, a broker with Granite Financial in Cupertino.

"If the loan does not close on time, some lenders will automatically extend your lock, some will continue to extend the lock until the loan closes. Other lenders may give you one free extension. Still others will charge you a percentage of the loan amount for an extension," said Noryko.

With so many possibilities, a written lock contract is mandatory—at the time you receive the lock, not later. Oral agreements can be hot air.

The contract should lock in as many costs as possible, the interest rate as well as points. The agreement should include your name; time when rate lock is set; effective date of agreement; lock cost; what rate and other loan terms are locked; the lock's expiration date and time; and any post-lock options.

Lock as soon as you see the desired rate or "on application"—when you first apply for the mortgage—so that your rate is locked as you spend time getting the application approved. That's particularly important if you barely qualify at today's rates and an increase would make buying unaffordable: lock as soon as possible.

If, however, you can withstand a rise in rates, you could benefit financially from delaying the lock.

"If rates stay flat, the consumer with a rate lock loses because of the cost of the lock. The only time a rate lock is good is if interest rates increase by more than about 0.125 percent during the lock period," said Richard Calhoun, owner broker of Creekside Realty in San Jose.

"Rates go up and down every day. Even with the currently higher rates, if someone has a 40-day escrow, there is likely a period where rates dip. If you catch a dip, then locking makes sense. The only time lock rates makes sense is if you know that rates will increase or if you cannot afford the higher payments," Calhoun added.

Lock periods should be long enough to allow for settlement, contingencies and other potential delays. Locks average 30 days, but range from 15 to 60 days.

• Before choosing a lock-in period, determine the average time for loan processing. Ask your lender to estimate the time necessary to process your loan.

• Once you lock in a rate, make sure your loan is approved and closed before the commitment expires. Quickly submit the application and other required documents.

• Locks cost money. Shop around for both the terms of the lock contract and its cost. Some lenders charge an up-front, nonrefundable fee even if the loan doesn't close. Others might levy the fee at settlement. The fee could be a flat fee, a percentage of the mortgage amount, a fraction of a percentage point or a higher interest rate. The costs vary depending upon the length of the lock-in period, the options you choose and mortgage program.

• If you have a floater, it's your job to keep an eye on the market.

Real estate writer Broderick Perkins, executive editor of San Jose-based DeadlineNews.Com, writes regularly for
Saratoga News.