I needed to lift a section of decking that had subsided a couple of inches, and I didn’t have a jack handy to bear the considerable weight. Instead, I made an A-brace, as shown in the drawing, out of a couple of short lengths of 2×4 and a 1x brace.
To make the A-brace, face-nail a pair of 2x4s at one end with three 16d nails through each side. The 2x4s should be 2 in. to 3 in. longer than the height you want to reach. Spread the unnailed ends of the 2x4s so that they form an inverted V beneath the appropriate beam and toenail one of the 2×4 legs to a secure 2×6 or 2×8 base. Forcing the legs together raises the load. Toenailing the other leg to the base, along with a cross-brace, secures it.
Gerry Magid, Jamaica Plain, MA