ALTHOUGH BIG BUILDERS MAY NOT HAVE the brawn to keep the roof from eventually caving in on the housing industry, they are flexing their muscles enough to keep speculative home buyers from entering the equation. According to a series of surveys by the NAHB conducted in March, April, and June of this year, 89 percent of the builders in hot housing markets reported that they were taking action to curb sales to investors.

A majority of builders (82 percent) said they sold only to owner-occupants. Of those that didn't, most put restrictions in place to protect themselves from buyers flipping homes. More than half prohibit the buyer from either putting the home up for sale before closing or selling the home within the first year after purchase, according to the survey. Others put builder buy-back clauses in the contracts, implement rental restrictions, charge more to finance the loan, or require owners to pay a fee if the home is sold within the first year.

This stance reflects builders' unease. Roughly 60 percent of single-family builders reported encountering some speculator activity to the NAHB, with more than half of those respondents agreeing that the activity drives up home prices.

The fear is that investors may dump the homes back into the market in a downturn, flooding the supply side of the business and driving down prices. High price appreciation in 55 metro areas and mortgage loan data showing an upswing in investor activity in those same markets give clout to the concerns.

But despite all the trepidation, a panel of large builders reported to the NAHB in April that only 3 percent of their single-family home sales during the previous six months were for investment.