A rotted floor rafter, or joist, can be a structural problem that causes your floor to be unsafe. Several things can cause this kind of rot, such as water damage, insects, mould and deterioration from ageing. Replacing a floor joist is a moderately physically demanding job that requires some carpentry experience.
Inspect the old rafter or joist for the cause of the damage. If it is water damage, be sure the leak or seepage problem is corrected. Insect damage requires extermination of the termites or other pests and making sure all old nests or materials are removed. Corrected whatever caused the damage or the problem will resurface with the new joist.
Consult your lumber yard or home building supply to get the correct joist size. It is advisable to have a structural engineer or builder help you with this step if you are not comfortable that you have the experience to determine the correct size. A floor rafter or joist is a load-bearing structural component, and it must meet the right size and material specifications to make sure you don't have floor failure. Once you have purchased your new joist, get it to the installation location by either removing a basement window or pushing it through the access hole in your crawl space.
Put the new joist anchors in place on the ends of the new joist, but don't fasten them. Lay the new floor joist in place next to the old rotted joist, but on its side not standing as it will be when installed. Rest it on the sill plate where it will be attached so it is ready to lift when the old joist is released.The sill plate is the wood sill that sits atop the foundation wall and anchors the floor to the foundation. The old joist and all the other joists are anchored to this sill.
Place hydraulic jacks at either end of the existing rotted floor joist, or on the adjacent floor joists to either side of the rotted one, but not on the rotted timber. Hydraulic jacks are posts that act as temporary lifting columns for jacking up a floor or structure. They have a screw lift that can be turned to lift a floor in small increments for a job such as this one. Lift the jacks slowly, approximately 1/4 inch at a time. Lift one, then move to the opposite end of that joist and lift it. Continue to rotate areas where you are lifting by moving to the joist on the other side of the rotted joist and lift it, and then the far end of that beam so you slowly lift both ends evenly and release the pressure on the rotted joist.
Tip up the new floor joist so it is in the position it will be in when installed. You may need to knock it in lightly with a hammer. A little tension is fine but if it is wedged too tightly, lift the floor jacks one more turn so you can get the new joist into place.
Cut the old floor joist to remove it. If the old joist is supported by a floor column, cut the joist on either side of the column and remove that block, then the remaining sections. Cut it into pieces to make removal easier, if necessary, but do as few cuts as possible to avoid damaging the new joist or any other parts of the floor. Knock the joist out with a hammer when it is loose enough to remove, and remove the old anchors.
Push the new joist into place where the old, rotted one was removed. Use the hammer to knock it into place, if necessary. Check for level and add wood shims to any gaps between the floor and the joist. Fasten the anchors to the sill plates at either end with the 16 d box nails, also known as 16 penny nails.
Release the floor jacks using the same system as before, lowering by 1/4 inch at a time and rotating from one end of each beam to the next to release them all evenly. Check the new floor joist again for level when the old one has been removed.