Sistering Joists

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

Structural problems can refer to a variety of differing things, ranging from problems with the foundation itself to structural lumber that has been damaged by weather. Most of the time, it refers to structural wood elements that have been damaged by water, if it isn’t talking about problems with the foundation itself.

When people talk about “dry rot” in homes, what they’re really talking about is wood rot, which is caused by a combination of moisture, bacteria and air. It’s only dry at that moment. But it became rotted when it was 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 and split, just as if they weren’t connected to anything.

Ultimately, water is the biggest culprit in causing damage to most homes’ structures. Even wood eating insects are much more likely to attack moist wood, than they are 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 can happen which are hidden from sight or the home might not have been properly maintained by a previous owner. In these cases, damage is usually fairly severe by the time it is discovered. I recently inspected a home 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 structural members of a house, in order to strengthen them. While it can apply to doing that with new construction, it is usually applied to making repairs on a home, when structural elements of the home 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 in the floor. Left alone long enough, the floor could break and someone could fall through, possibly becoming injured in the process.

Making a repair to 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, once access to the bad floor joists has been attained, sistering them is fairly easy. 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 full 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 together.

Most people choose to use the same size dimensional lumber for the sistering joist, as was used for the original joist. If there is any doubt as to what the correct size 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 SizeJoist SpacingSelect GradeNo. 1 GradeNo. 2 Grade
2”x 6”1611’-4”10’-10”10’-9”
2”x 8”1615’-0”14’-5”14’-1”
2”x 10”1619’-1”18’-5”17’-2”
2”x 12”1623’-3”21’-4”19’-11”

Ideally, the sister joint should be both glued with construction adhesive and bolted through with at least 3/8” diameter bolts, forming a “W” pattern, with the bolts spaced about 8” apart. This should extend through the entire three feet of wood that is 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 the use of dimensional lumber to sister joists is common, it is not actually necessary. If you look at modern floor joists, they are rarely solid dimensional lumber anymore. That’s actually 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 not only are cheaper than solid lumber joists, but they provide less deflection, an important 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 actually produce a stronger joist than using 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 side that is more accessible. As with either the dimensional lumber sister or the dual ½” plywood sisters, both construction adhesive and 3/8” bolts should be used to connect the sister to the original joist.

sistered joists, sistering floor joists
Sistered joists, Pixel

What About Sistering Studs?

Sistering studs is even easier than sistering joists, in the case where there is a rotted or cracked stud in the home. In the case of a cracked stud, the hardest part will be aligning the pieces of the broken stud, so that the sister stud can be attached to it. 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 full length.

As with the joists, the sister stud should be both glued with construction adhesive and screwed. There is no need to use bolts. It may be necessary to clamp the pieces together first, in order 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 is going to take the full load from the old one, so it is important that it fit 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 also be used, in place 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 of plywood that’s just sitting around.

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