Rent checks are getting bigger.

Rents in April were 4% higher than a year before, according to a report from Zillow. That's the fastest increase in two years, outpacing home prices which rose by 3%, the report showed.

Rental affordability is getting worse, and it doesn't look like it will be improving anytime soon. "This is here to stay," said Svenja Gudell, senior director of economic research at Zillow. "We will continue to see rental increases, and affordability will worsen before it gets better."

Rents have been steadily rising since 2000 and unlike home prices, which took a hit when the bubble burst, there's been no disruption to slow rent prices, explained Gudell.

"Income hasn't kept up with rental increases. You are having to spend much more your monthly income on rent and it's a concern. It's a national concern."

Rents increased faster than home values in 20 of the 35 largest markets.

But even as rents climb, there is still strong demand for rentals, creating a housing crunch in some cities.

"Places hard hit like Seattle, San Francisco and Denver are having a hard time keeping up and building enough units to satisfy demand," said Gudell.

Renters in San Francisco face a nearly 15% rise in payments while rents in Denver went up 11.6% in April from last year, according to Zillow's Rent Index. Only two cities on the list experienced a drop in rent from last year: Chicago and Minneapolis.

Low mortgage rates make home buying an attractive option, but large rent checks can make saving for a down payment tough. On a national level, homeowners can pay an average of 15.3% of their income on mortgage payments each month, the report found, while renters will dish out roughly 30% on rent. Gudell said tenants in high-income areas can expect to pay closer to 50% of their income on rents.

"There are bunch of things keeping renters on the sidelines and that means usually the folks that would be normally making the switch to become homeowners are still taking up the rental units."

Here's how much rent prices rose in the largest markets in the country in April from last year, according to Zillow's Rent Index:

San Francisco, 14.9%

San Jose, 12.9%

Denver, 11.6%

Kansas City, Mo., 9.5%

Portland, Ore., 8.6%

Charlotte, N.C., 6.6%

Austin, Texas, 6.2%

Cincinnati, Ohio, 6.2%

Seattle, 6.2%

Houston, 6.1%

Detroit, 6.0%

Sacramento, 5.9%

Los Angeles, 5.6%

Dallas-Fort Worth, 5.5%

Phoenix, 5.4%

Boston, 5.2%

San Diego, 5.1%

Atlanta, 4.9%

San Antonio, Texas, 4.6%

St. Louis, 4.5%

Pittsburgh, 4.4%

Riverside, Calif. 4.2%

Tampa, Fla., 4.1%

New York, 3.4%

Miami-Fort Lauderdale, 3.2%

Philadelphia, 2.8%

Baltimore, 2.7%

Columbus, Ohio, 2.6%

Cleveland, Ohio, 2.4%

Las Vegas, 2.1%

Washington, D.C., 2.1%

Orlando, 2.0%

Indianapolis, 1.5%

Minneapolis-St. Paul, -0.3%

Chicago, -1.0%