How to Sister Joists for Improved Structural Support and Stability

Working on older homes presents the carpenter or woodworker with unique problems. As homes age, a number of things can go wrong with them, some of which can be quite serious. Of these, structural issues are generally the worst.

Structural problems can refer to various things, ranging from issues with the foundation itself to structural lumber damaged by weather. Most of the time, it relates to structural wood elements damaged by water, if it isn’t discussing problems with the foundation itself.

When people talk about “dry rot” in homes, they’re really talking about wood rot, which is caused by a combination of moisture, bacteria, and air. Wood is only dry at that moment, but it becomes rotten when it is wet.

But rotting isn’t the only thing that can happen to structural members in a home; there are also potential problems with wood warping, cupping, twisting, and splitting. By and large, the closer connections between different pieces of wood are, the lower the chance of those things happening. But rot can also damage fastener connections, making them virtually worthless. In that case, wood pieces can warp, twist,t and split as if they weren’t connected to anything.

Ultimately, water is the biggest culprit in causing damage to most home structures. Even wood-eating insects are much more likely to attack moist wood than to infest dry wood. A good roof, proper sealing, and maintaining the paint job on the home are all important parts of protecting it from moisture.

Even with proper maintenance, there’s always a potential for damage to the home’s structure. Things hidden from sight can happen, or a previous owner might not have adequately maintained the home. In these cases, damage is usually reasonably severe when it is discovered. I recently inspected a house where over a dozen floor joists were destroyed by water damage, requiring pulling up the floor in a large room and sistering all the joists.

How to “Sister” Floor Joists

Sistering is the name given to adding material to the structural members of a house to strengthen them. While it can be used for new construction, it is usually used for home repairs when structural elements become damaged.

Damaged floor joists can be a serious problem, as they ultimately hold up the entire home. Perhaps more noticeably, weak floor joists can create sagging areas and soft spots. Left alone long enough, the floor could break, and someone could fall through, possibly injured.

Repairing this requires access to the floor joists themselves. If the home has an unfinished basement, that’s not a problem. But if it is post-and-beam construction, there may not be enough space under the floor to work. In that case, the area under the home may need to be excavated, an excruciating project that must be done by hand, or the flooring may need to be pulled up to gain access.

In either case, sistering them is relatively easy once access to the lousy floor joists has been attained. The more common way to do this is to attach an additional piece of the same-size dimensional lumber to the side of the existing joist. In this case, it is very important that the tops of the joists are level with each other; or, more correctly, the new joist must be at the level that the top of the old joist should be.

Ideally, new joists attached to old ones should run the entire length of the old ones, but that’s not always possible. In cases where it isn’t, the new joist should go at least three feet past the ends of the damaged area, at both ends, providing plenty of good material to attach the two.

Most people use lumber of the same size and dimensional as the original joist for the sistering joist. If there is any doubt as to what the correct size of dimensional lumber would be for a particular span, refer to the chart below, which contains the data from the building code.

		Lumber Grade
Nominal Joist Size	Joist Spacing	Select Grade	No. 1 Grade	No. 2 Grade
2”x 6”	16	11’-4”	10’-10”	10’-9”
24	9’-11”	9’-7”	9’-1”
2”x 8”	16	15’-0”	14’-5”	14’-1”
24	13’-1”	12’-4”	11’-6”
2”x 10”	16	19’-1”	18’-5”	17’-2”
24	16’-8”	15’-0”	14’-1”
2”x 12”	16	23’-3”	21’-4”	19’-11”
24	20’-3”	17’-5”	16’-3”

Ideally, the sister joint should be glued with construction adhesive and bolted through with at least 3/8” diameter bolts, forming a “W” pattern, with the bolts about 8” apart. This should extend through three feet of wood on either side of the damaged section. Screws can be used in place of bolts, but if they are, then use three times as many screws as bolts.

Sistering Joists with Plywood

While using dimensional lumber to sister joists is everyday, it is unnecessary. If you look at modern floor joists, they are rarely solid dimensional lumber anymore. That’s a very inefficient design, using more lumber than necessary. Today’s floor joists are usually either wood I-beams or open web floor trusses. These options are cheaper than solid lumber joists and provide less deflection, a vital factor for ceramic tile floors, granite countertops, and other modern building materials.

Scabbing ½” CDX construction plywood to both sides of the damaged floor joist can produce a stronger joist than solid dimensional lumber. This should be attached with construction adhesive and either screw or bolts, just as if it were dimensional lumber. If bolts are going to be used, then a few screws to hold the plywood in place, while the bolt holes are drilled and the bolts are installed, can help considerably.

Another alternative, especially if both sides of the existing joist are not readily accessible, is to use a single layer of ¾” CDX plywood on the more accessible side. As with either the dimensional lumber sister or the dual ½” plywood sisters, construction adhesive and 3/8” bolts should be used to connect the sister to the original joist.

What About Sistering Studs?

Sistering studs is even easier than sistering joists when a rotted or cracked stud is in the home. In the case of a cracked stud, the most challenging part will be aligning the pieces of the broken stud so that the sister stud can be attached. All that’s required is an additional 2”x 4” stud. Rather than only going three feet past the ends of the damage, the stud should go the entire length.

As with the joists, the sister stud should be both glued with construction adhesive and screwed. There is no need to use bolts. Clamping the pieces together first may be necessary to align the cracked stud. It can also be helpful to screw the crack closed, although it would be inadvisable to think that the screwed together stud is strong.

In this case, the new stud will take the full load from the old one, so it is important that it fits snug between the top plate and floor plate. If it doesn’t have to be driven into place, it’s probably a bit short.

Three-quarter-inch CDX plywood can be used instead of a 2”x 4”. But in this case, it will probably be cheaper to use a 2”x 4” stud than to rip a piece of plywood unless there’s a scrap piece just sitting around.