It is important that your floor joists always stay in good condition. If they crack or rot away, the support keeping your floor from sagging is gone. Additionally, the flooring can become weak and unsafe. While replacing floor joists is a complex process, it is well within the capabilities of a dedicated homeowner. What follows should give you all of the information you need to accomplish this task.
Step 1 - Remove the Floorboards
Whether you are working on joists under the floor in your home, or joists outdoors such as underneath a deck, you will not be able to access them normally. To replace floor joists, you must first remove the floorboards that cover them.
Get out your crowbar and your hammer. Put the tip of the crowbar between the boards and push it in, using the hammer to tap on the end. This should lever the boards apart, and allow you to wiggle the crowbar further in to pry the boards free. Repeat until you remove everything around the anticipated area.
You should expect to damage some of your floorboards, as this method is hard to use without doing so. Because of this, you should target your efforts carefully. If you are replacing joists in your floor, chances are they are visibly damaged. Remove the floorboard closest to the damage, and then use the hole you make exactly to see which ones need replacing. Do not remove any floorboards that are not absolutely required, and always start with the most obvious choice.
Step 2 - Mark the Old Locations
Now that the joists you want to replace are visible, it is time to remove them. However, before you do this, it is important that you take note of their location. Joists will usually be 16 inches apart, but there are also some cases in which they will be 24 inches apart instead.
Step 3 - Remove the Old Joists
Select a joist, and select an end. Make a cut all the way through the joist using your saw, staying as close to the end you have selected as possible without hitting any nails or the joist hanger if it is present. Make another cut at the other end using the same strategy.
Repeat the process for all of the pieces you wish to replace. Then, you can remove the old joists by tapping on them with your hammer until they pop out of place. Whether joist hangers are present or not, remove the old nails with your hammer, and then remove the ends of the old joists.
Step 4 - Replace the Old Joists
Begin by placing your new joist hanger at the location you previously marked. Line up the top of the ledger board. Hammer the speed prongs in to temporarily secure the joist hanger in place while you work, and then use nails to attach it to the ledger board. The nails should be 16d galvanized for outdoors, and common for indoors.
Measure the area for your joist to be sure your new lumber piece will sit the dimensions. If it's too long, simply measure the wood, mark it, and trim it accordingly.
Now, you can insert your new joist into the hanger, and nail it in securely. Regular lumber, 2x6 or 2x8 are pretty standard, can be used indoors, but make sure you're using a material that can stand up to the weather outside, such as pressure-treated wood.
If you are lucky, you will be able to use your old floorboards to cover the repair, in which case you have only to arrange them correctly and nail them. If not, you will have to purchase and install replacements.