Structural wooden beams in houses and other buildings can weaken with age or start to sag because they are supporting a much heavier load than they were meant to carry. Weak beams may be uncovered during a home remodeling or when converting a non-residential structure into a home. It may be cheaper and more practical to reinforce an existing wooden beam rather than replace it.
The simplest and least expensive way to stabilize a weak existing beam is to install a new support column at the point where the sagging is greatest. The weak beam is jacked up to allow installation of a new steel, wood or masonry column. This reduces the beam span, reducing the load on the weak beam. Adding new support columns can be difficult in any place other than a basement or crawlspace.
Another way to reinforce a beam is with a flitch plate. In this method, a steel plate is sandwiched between two identical wooden beams and the beam-and-steel sandwich is bolted together. The steel plate should be as long as the beam, with bolt holes punched or drilled through the steel. Half-inch carriage bolts are spaced 16 inches apart, with two bolts at the end of the beam. The end bolts should be 2 inches in from the beam's end. The bolts are staggered along the beam with one bolt near the top and the next near the bottom. This method transfers the load from the wooden members to the steel plate through the bolts and friction between the steel and the wood. Alternatively, steel flitch plates can be installed on each side of the weak beam.
You also can adopt the flitch plate principle without the steel by placing a new "sister" beam of the same size as the old one next to the weak beam. The weak beam is jacked up, if needed, to level it, and construction adhesive is spread along the face of the old beam. The sister bean is pressed against the adhesive, and the two beams are bolted together every 16 inches. A sister beam can be installed on both sides of the weak beam for even more strength.
Another way of reinforcing a weak wooden beam is through the use of steel bars and epoxy. This technique requires access to the top of the beam. An inch-wide slot is cut down the length of the beam, with the depth of the cut approximately three-fourths the depth of the beam. Structural epoxy resin is poured into the cut, a high-tensile steel reinforcing bar is laid down, more epoxy is poured, and another bar is laid down until the slot is filled. Once the epoxy cures, this technique roughly doubles the strength of the beam.