Extensive water damage and rot may require the complete replacement of floor joists, but a homeowner with some carpentry skills may enlist a plumber to locate and fix the source of water causing the joist to rot before replacing rotten joists, according to SFGate. The website also recommends using certain safety equipment, such as safety glasses, a dust mask, work gloves and a work light.
After accessing the area under the subfloor through the crawl space at the side of the house, measure the distance from the points where the ends of the rotten joist attach on a sill plate or on a girder at each end. Do this before measuring the side of the joist from the lower edge to the upper edge to determine the size of framing lumber for the replacement. After obtaining the right size replacement joist, the carpenter must cut the joist to the correct size and then maneuver it to the installation point. Use a framing hammer to fit the joist on the edge alongside the rotten joist with one end on a sill plate or a girder. The next step involves connecting each end of the new joist to a sill plate and a girder with framing angles and hanger nails. The last step involves cutting the rotten joist vertically near each end and removing the rotten section with a reciprocating saw that has a wood-cutting blade.