Slight increases in mortgage interest rates and a slower rise in family income pushed the typical family's ability to afford a house down a little in August, the National Association of Realtors reports.

"Affordability conditions are drifting down right now," said John Tuccillo, the association's chief economist. "But sales are up and I think that indicates the desire of people to get into the market."The August "affordability index" dropped a half percentage point to 109.0 from the revised July level of 109.5, the association said. That rate is 0.8 of a percentage point below the 109.8 index of August 1987.

The figure meant a family earning the national median income of $31,684 had 109 percent of the income needed to qualify for a conventional loan covering 80 percent of the national median existing-home price of $91,300, the association said.

From July to August, the national median home price increased $600 to $91,300, the association said.

The August index of 109 is above the record low level of 63.9 recorded in September 1981 and well below the peak index of 153.1 in February 1972, the association said.

"We anticipate a further slight decline in the August index as interest rates continue to rise, but the index is likely to stabilize for the remainder of the year as home prices begin to moderate," Tuccillo said.