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What Is the Building Code For Sistering Joists and Why You Should Know It

Joists are an integral part of many homes, but over time they can bow, twist, and sag. When this happens, other components like walls and the ceiling usually do the same.

Joists are designed to carry a specific load and must be built in a way that will support it, or it will fail. If a joist falls victim to age, poor design, or disrepair, the solution is often to “sister” the joist.

Source: hubpagespro.com

Sistering a joist describes adding another identical joist like a sandwich, which doubles the thickness of the joist. Most building codes, including the most used, the IRC, do not directly address sistering a joist, but they do address girders, which are similar.

Today we will discuss how a joist should be sistered using typical code requirements and general engineering principles as a guide.

What Does Sistering a Joist Mean?

For lack of a better analogy, you can think of sister joist as an identical, conjoined sister. For example, let’s say your joist is in the floor, making it a floor joist. If your floor joist measures 2” by 8” (approximately) , you have 2” x 8” floor joist. To install a sister joist, you will make another joist exactly like the one in the floor and attach it with nails, screws, or bolts.

Why Would I Sister a Joist?

When you sister a joist, you are in effect building a double girder. Most people will think of a girder as a long piece of steel used to build skyscrapers. Girders are very strong, and are used to provide support over an unsupported span, like a bridge. In the case of a floor joist, it is acting as a single girder and provides support to the floor above it.

Over time, joists can sag under weight and allow a gap to form between the ceiling and adjacent walls. Sistering a joist essentially adds the full strength of a new joist to whatever support the original joist is still providing. Professionals generally consider nailing two boards together a double girder, while adding an identical board to an existing joist, a sister.

Will Sistering a Floor Joist Fix My Sagging Floor?

Adding a sister to a sagging floor joist is the best way to fix a sagging floor however, there is a correct method to use. Sistering a floor joist (or joists) usually requires some prep work before installing the sister onto the floor joist. You don’t want to just add a sister to a sagging floor joist because that will make the sag permanent.

First, you will need to raise the original floor joist to its original position. Most pros and do-it-yourselfers use telescoping support jacks for this function. Telescoping support jacks have a threaded rod on the end that extends the length of the jack with every turn. Telescoping jacks are handy because they function as both the jack and the support post simultaneously.

What Are the Rules When Installing a Sister Floor Joist?

To add a sister to a floor joist, you need to follow some rules to make sure your sister does its job. Professional carpenters follow these rules to make sure they are following building code, and because it’s the best way to do it. You can think of a sister joist as a splint on a broken finger. First, you have to straighten the finger (floor joist) before you apply the splint (sister).

Make Sure the Sister Joist Has Bearing Support

The first step in adding a sister joist is to make sure you have bearing support wherever the sister joist will be installed. Bearing support refers to the posts, girders, concrete, or other materials used to support the floor system from below. In most cases, a girder is installed underneath the floor system to support the floor joists.

Wherever your sister ends, you will need support from below in the form of a concrete post, telescoping support jack, or other support. In other words, your sister joist must have support anytime two ends butt together. Generally, just make sure your sister joist is the same length as your original and you should avoid needing additional posts.

Always Drill Holes and Cut Notches In the Correct Place

A common amatuer mistake is to make notches or holes in a floor joist or sister, but in the wrong location. Placing these notches or holes in the wrong place effectively removes the strength of the joist. Always follow these rules when cutting holes or notches in a floor joist:

  1. Never Remove More Than 1/6th the Width Of the Joist.

    You never want to cut a notch or drill a hole that removes more than one-sixth of the joist material. For example, if you have a 2” x 8” floor joist and you cut a 4” notch in the edge of it, from an engineering standpoint, you now have a 2” x 4” board supporting your floor where there should be a 2” x 8” joist.

    To illustrate, if you have a 2” x 8” joist, any notch you cut must not exceed 1/6th of this width. So, you can safely make about a 1” notch in a 2” x 8” floor joist or sister, as long as it is not in the middle third of the span.
  1. Never Cut a Notch In the Middle Third Of the Span.

    If you need to run a cable or pipe through a sister joist, never put the notch in the middle third. For example, if your sister joist will span 60”, don’t put a notch further than 20” from either end. Always use the outer two thirds of the joist and never exceed more than ⅙ the width of the board.

    Notches and holes occur in floor joists and sisters on occasion, although avoiding them is best. The important thing to note is that if your plans call for a 2” x 8” and you cut a 2” notch in the edge, you have violated the code because you have effectively installed a 2” x 6” where a 2” x 8” is specified.
  1. Never Drill a Hole Within 2” Of the Edge Of the Sister Joist

    You shouldn’t drill a hole close to the edge of a board because you effectively make the board smaller. Just like taking too big of a notch, this weakens the sister joist. If you need to drill a hole, always make sure it is at least 2” from the edge. Always put holes as close to the center of a sister joist as possible.
  1. Never Drill a Hole Larger Than ⅓ the Width Of the Sister Joist.

    If you must drill a hole to accommodate a large pipe or vent, never cut a whole larger than ⅓ the width of the sister joist. For example, if your sister joist is 2” x 8”, you can safely drill a 2 ½” hole in the center. However, a 3” hole, even directly in the center, would not work as it would exceed ⅓ of the width of the sister joist.

Install the Correct Fasteners For the Sister

When installing a sister joist, make sure your fasteners are appropriate for the project. When adding a sister (or sisters) to a joist, it is important to connect all the boards to each other, not just the one beside it. In some situations, you may need to add two or even three sisters to a joist if the situation is severe.

Never use #12 or #16 nails if you are adding more than one sister to a joist. Although these are commonly used throughout a home, they are not long enough to penetrate three sister joists. The best solution when using more than one sister joist is to bolt all the sisters and joist together with appropriately sized galvanized carriage bolts.

You also don’t want to use a lag bolt for sister joists if you can avoid it. The reason is because the sister is only as useful as it is tight. Lag bolts can easily strip trying to draw the sister and joist together. Carriage bolts require a washer and nut, so they can be tightened much more effectively without damaging the lumber.

To make your connections even stronger, don’t align the carriage bolts evenly. For example, mark the locations for the carriage bolts on one edge with odd numbers, like one foot(12”), three feet(36”), five feet(60”), seven feet(84”), and so on. On the other side, start with even numbers, like 2 feet(24”), four feet(48”, and so on. 

From an engineering standpoint, this pattern spreads the load more effectively. If you are installing only one sister joist, you can use #12 or #16 sinker nails to connect the sister using the same pattern. You can also use screws, as long as they are 2 ½”-3” long. Most pros use 3” galvanized wood screws with a square drive head, but Phillips or Torx screws are also fine. Just make sure the sister joist is as tight as you can get it.

Orient the Sister Crown Up

The pros will usually select a sister joist for its shape and crown. Ideally, they are looking for a board with a slight crown, which means the edge of the board is slightly high in the middle. An example of an exaggerated crown would be the rockers on a rocking chair. 

The pros look for a board with a slight crown because it will be slightly stronger in the downward direction. By putting the crown up, the sister is a little stronger from the top down, which is where it needs to be the strongest. You only want a slight crown, about a ¼”, because a severe crown may cause a hump in your finished floor.

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