Prices for existing single-family homes tumbled at a record annual rate in August, according to the S&P Case/Shiller home-price index.

Both the 10-City and 20-City Indexes clocked in record declines. The 20-City Composite Index fell 16.6% in August, though S&P noted that “the acceleration in decline was only moderate.” The 10-City Composite was off 17.7%.

“Given deep discounting due to foreclosures and the wide imbalance between home sales and inventory, we expect home prices to fall further,” said Michelle Meyer of Barclays Capital Global Economics in a research note.

Every region reported negative results for a fifth straight month. Both the 10-City and 20-City indexes have seen year over year declines for 20 straight months.

"Based on the monthly trend, national home prices, likely fell at a faster pace in Q3 than Q2, even after accounting for seasonal distortions. We expect the downward pressure on home prices to continue through the end of next year."

Home builder stocks declined in morning trading; Hovnanian Enterprises (HOV) was off 16%, Centex (CTX) was down 9.1%, Ryland (RYL) fell 8.6% and D.R. Horton (DHI) declined 7%. As long as prices of existing homes are plummeting and inventories remain high, home builders are likely to struggle.