(Bloomberg) -- Lumber futures posted their biggest-ever weekly loss, extending a tumble from all-time highs reached last month as sawmills ramp up output and buyers hold off on purchases.Prices in Chicago fell 18% this week, the biggest decline for most-active futures in records going back to 1986. Lumber has has now dropped almost 40% from the record high reached on May 10.Sawmills appear to be catching up with the rampant homebuilding demand in North America that fueled a months-long rally, br