SEATTLE—The gap between rental expense and mortgage expense continues to widen, with Zillow saying that rent payments take up a bigger slice of monthly incomes than ever in most major metropolitan areas. Renters in the US making a median income could expect to put 30.2% of their paycheck toward rent in the second quarter of this year, up from 29.5% a year ago and the highest average on record, according to Zillow.

By contrast, the US average across the 35 largest metros for a monthly mortgage payment is 15.1% of monthly income, or 50% less than the average rent payment's share. The rate of mortgage affordability stayed basically the same over the past year, Zillow says.

In some metro areas—including Chicago, Dallas-Fort Worth, Philadelphia, Houston, Miami-Fort Lauderdale and Detroit—the gap between mortgage payments and rental payments is greater than 100%. Only in San Jose, CA is it cheaper to rent (41.5% of monthly income) than it is to buy (41.9%).

Both on a national level and in all but one of the 35 metros Zillow covers, rental payments' share of monthly income is higher than the average between 1985 and 2000. During those years, renters could expect to spend about 24.4% of their income on paying the rent. Mortgages, on the other hand, take a smaller share than they did historically: the long-term average was 21.3%, according to Zillow.

“Our research found that unaffordable rents are making it hard for people to save for a down payment and retirement, and that people whose rent is unaffordable are more likely to skip out on their own healthcare,” says Svenja Gudell, chief economist at Zillow. “There are good reasons to rent temporarily—when you move to a new city, for example—but from an affordability perspective, rents are crazy right now. If you can possibly come up with a down payment, then it's a good time to buy a home and start putting your money toward a mortgage.”

Assuming that interest rates increase over the next year, and with them mortgage rates, it will still be cheaper to buy than rent in most major metros, according to Zillow data. The exceptions will be four of the five California metros Zillow covers, led by Los Angeles, where a 6% interest rate will mean a mortgage payment comprising 50.2% of monthly income.

The Zillow findings dovetail with research issued last month by MPF Research, which found that apartment rent growth had reached a 15-year high. “While apartment rents are rising at a significant pace all across the country, the nation's average price increase is being skewed quite a bit by the surging rents in the Western region of the country,” MPF VP Greg Willett said in July.