Are you concerned about the strength or integrity of your floor joists? Do you have bouncy, sagging or otherwise dilapidated floors? Then you may want to consider sistering your floor joists to add the necessary strength to bear the load. Sistering floor joists is necessary when bouncy, saggy and creaking or just plain weak floor structures exist in your home.
You can have a structural engineer come out to tell you what you need to do if you can spare about $500, or you can probably see for yourself what the problem is by the way your floor behaves. If your floor has obvious sag or it creaks, cracks and bounces when you walk on it, chances are you need to remedy the problem before it becomes worse or someone gets hurt.
What does Sistering Entail?
Sistering floor joists means to add new support joists right alongside of the original ones to strengthen your floor and structure. On sagging floors, it may be necessary to jack the floor beams up before sintering the joists.
Be careful if you have to do any jacking. You should only jack up a floor about 1/8 inch per day until It is where it needs to be, so jacking up a structure can often take many days to complete correctly. If you jack it up too fast, you will encounter cracks and splits in the walls and other upper structures. So, take your time and do it right or it will end up costing you a lot more in the end.
What you will Need:
First gather the tools and supplies for the job. For most jobs, you will need the following:
1. New floor joists: You will need as many joists as you feel need strengthened, so inspect the old floor and determine how many new joists you will need. Then determine what size they are. Most old floor joists will be 2 by 10 or 2 by 8. Measure their length as well. Some may require 16 foot joists while others may only be 10 feet or even 8 feet in some places.
4. Construction adhesive/wood glue
5. Hammer/nail gun
6. 3/8 inch or ½ inch lag bolts with large washers and nuts: This is one way to fasten new joists. You will probably use a combination of ways.
7. Floor jack: you may use a hydraulic jack or the manual type that acts as a beam to level the old joists before installing the new ones alongside them.
8. Quick clamps or c-clamps: you will need something to hold the new joist along side of the old ones before you permanently fasten them.
9. A 4 x 6 beam is often needed to place under the joists in order to jack them up to level position.
10. Pry bar and other tools to loosen any pipes and wires from the work area.
Step by Step
The basic process for sintering floor joists is as follows:
1. Clear the area where you will be adding new sister joists: You will need to remove any pipes and wires out of the way. The new joist need to go flush up against the old ones, so you have to at least clear enough space to get the joist in place. This sometimes is the hardest part of the job, depending on how many wires and pipes may be in the way. In many cases there will be obstacles that cannot be removed. In these instances, you may need to notch the joist to fit around the obstacle. This will weaken the new support joists and should be avoided wherever possible.
2. Place the new joists in place and clamp them with quick grips or c-clamps.
3. Now is when you want to do your leveling and jack up the joists if need be. Use a sturdy beam such as a 4 by 6 inch beam and run it across as many floor joists as you want to level at once. You can do up to four or five at a time in some cases, depending on your jack and leveling equipment. You should be doing this near the center of the floor. Jack it up until the joists will fit alongside of the old ones. Once the jack starts bearing a load and is raising the level of the old joists, you have to use some caution and common sense. For example, you do not want to jack up the entire structure too fast or you will have cracking and splitting going on in the upper walls and support structures. To avoid any cracking and splitting, you should only jack up the old joists about 1/8 inch per day, so this process could take several days depending on how much you need to raise the joists to make it level or to fit the new ones in place. If you have an exceptionally large floor area to work with, you may need to jack up the joists in more than one spot.
4. Once you have the new sister joists leveled and in place, you need to permanently attach them. There are a few different methods commonly used. A combination of construction adhesive and bolts with washers and nuts is probably the best. Apply construction adhesive in a zigzag pattern the entire length of the new joist and clamp it in place. Then you can use nails and/or bolts and washers to fix them in place permanently.
5. Now that you have the new sister joists in place and permanently attached, you should add some cross supports. Be careful not to get in the way of any pipes or wires that will need to be replaced. Often, when there is nothing in the way, you can use small sections of the same wood you used for the new joists to make cross supports. You should at least put cross supports in the middle or about every five to six feet. If there is too much plumbing or wiring in the way of putting up solid cross supports, you will need to put up scissoring cross supports. They are just two smaller pieces of lumber toe-nailed in the shape on an ex in between two floor joists.
6. Now is the time to remove any unwanted clamps, beams and jacks. Many people prefer to leave the center support beam and floor jacks in place if the work is done in a basement. They can offer great support for a first level floor.
7. Finally, replace all the plumbing and wiring you had to tear down to do the job and you should be done.
Now that you have a level floor, try walking across it and make sure it feels firm and doesn’t bounce and creak like before. If it still seems weak, you may need the additional support of some vertical beams to stop the bounce, but in most cases, the sister joists will do the job quite well when installed properly.