How To Box Out Floor Joists?

Do you need to install plumbing in the floor but can't because a floor joist is in the way? Do you want to know how to box out your floor joist so you can install the plumbing? You’ve come to the right place, because we have researched this topic and have tips for you.

There are different ways to box out your floor joist, depending on where you need to place it. The different techniques make use of a double joist system.

We'll talk more about double joists in the succeeding sections. Read on to learn about the different ways to box out your floor joist to make way for plumbing installation.

Plumbing And Floor Joists

Most of the time, installing pipes that will go through the floor can be adjusted so that they will not hit any of the floor joists. However, there are instances where you cannot adjust the pipes. A good example is when you install a bathtub in the only available area in the bathroom.

Plumber joining plastic pipes with cement for drains under a concrete

Fortunately, there are several techniques that you can use to box out the floor joists so that you can install the pipes and not compromise the support of the floor joists. Compromising joists can lead to floors or walls falling.

We’re going to talk about several techniques, so let's get right to it.

How To Box Out Floor Joists

Some of the techniques for boxing out floor joists are more appropriate for certain tasks than others. When picking a technique, match it with the situation that you have.

Whenever you need to box a joist for any type of installation through the floor, it will always involve three joists. In situations where you need to install a large duct that will go through the floor, it will involve four or more joists.

You can adapt the techniques below to whatever number of joists you need to box out.

If the installation requires the removal of blocking, you will need to remove the blocking first, then install new blocking after installation.

Blocking for floor joists consists of short pieces of lumber that are normally as thick and wide as the floor joist. You install blocking perpendicularly across the distance between two joists. Blocking prevents the lumber in joists from bending or buckling when you apply vertical compression on them.

Joist Boxing For Holes Close To Rim Joist

PVC waste water system rough plumbing drainage pipe and fitting

This technique is best for installations that are close to the rim joist. If the plumbing that you need to install is less than a foot away from the rim joist under the floor, then this is the ideal technique.

Making Measurements

  1. Mark the area on the floor where you need to make a hole.
  2. Measure the size of the hole that you need and mark that on the floor.
  3. Double-check the location and the measurement, then use an oscillating saw to cut a hole in the floor.
  4. Measure the distance from the header joist to the end of one of the joists.
  5. Measure a space equal to the thickness of two joists from the edge of the hole that is farthest from the rim joist. Mark this for when you cut the joist later.
  6. Get joists that have the same width as the joists that your house already has.

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Installing Additional Joists

  1. Cut two joists that match the length that you need.
  2. Install the joists on the side of the existing joists that are facing the joist that you will cut. The new joists should be facing each other once you cut the middle joist.
  3. Use a double joist hanger to fasten the new joists on the rim/header joist. Use a structural flathead wood screw to fasten the hangers.

Correct hanger and joist sizes, How To Box Out Floor Joists?

    • Use a 2 x 8 hanger if you’re using a 2 x 8 joist. Use a 2 x 6 hanger if you have a 2 x 6 joist.
  1. Install double joist hangers on the opposite end of the new joists.
  2. Measure the distance between the new joists.
  3. Cut two pieces of joists that equal the distance between the two new joists.
  4. Cut the middle joist following your marking to give way to the pipe that will go through the hole.
  5. Install the two pieces of combined joists perpendicular to the joist that you cut off.
  6. Install double joist hangers on both sides of the two new perpendicular joists.
  7. Use a single hanger to connect the joist that you cut to the new perpendicular double joists.

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Joist Boxing For Holes Far From The Rim Joist

This method is ideal for holes that are far from the rim joist.

Installing Double Joists

  1. Follow Steps 1 to 4 of the section “Making Measurements” above.
  2. Follow Steps 1 to 5 of the section “Installing Additional Joists” above.
  3. Cut four joists equal to the distance between the two new joists.
  4. Measure the total thickness of two joists.
  5. Measure a distance equal to the thickness of the two joists from both sides of the hole that is along the length of the joists. Mark this distance on the middle joist.
  6. Cut the middle joist on your markings.
  7. Install two of the joists perpendicularly on one end of the joist that you cut. Use a double joist hanger to fasten the two joists.
  8. Repeat the same steps for the other two joists on the other cut end.
    • You can increase the distance of the double joist supports away from the hole if you need space around the edge of the hole.
  1. Use a single hanger to connect the two ends of the joist that you cut to the two new perpendicular double joists.

Joist Boxing For Small Holes

In some situations, you may not need a big hole. You can use one of the first two methods depending on the distance of the hole from the wall. This is an alternative to the first method if the distance of the small hole from the wall is more than a foot but less than two feet.

Keep in mind that if the hole is more than 3 to 4 inches, you need to apply one of the first two methods.

Double Joist For Small Holes

  1. Follow Steps 1 to 4 of the section “Installing Double Joists” above.
  2. Cut the middle joist at a distance equal to twice the thickness of a joist from the edge of the hole farthest from the rim/header joist.
  3. Remove the part of the joist that you cut coming from the hole to the rim/header joist.
  4. Measure the distance from the face of the perpendicular double joist to the rim/header joist.
  5. Cut two joists that are equal to this length.
  6. Install one of the joists from the rim joist to the perpendicular double joist. Position it to the left side of the hole. Use a single joist hanger to keep the joist in place on the rim joist side and another hanger on the side of the perpendicular double joist.
  7. Repeat the same step to install the other joist on the opposite side of the hole.

Boxing Floor Joists For Horizontal Plumbing

Plumbing in the laundry room

If the pipe comes from the floor below the joist or from a nearby wall and needs to go through the joist, you can still use any of the methods above. You will still need to cut the joist to let the pipe through.

The methods above will allow you to transfer the load of the joist to the perpendicular double joist, which will, in turn, distribute that load to the two new joists.

Here is a method that could make your job easier. Note that this method is specific to this situation.

Joist Is Parallel To An Adjacent Wall

If the joist that you need to cut is parallel to a wall, you can install two 4 x 4 vertical supports on that wall. Screw the joist to that support. The vertical supports will take the load of the cut joist.

If the joist is long, you can install two or three supports along the length of the wall to support the cut joist. If the cut is close to the middle of the joist, you need support along the entire length of the joist every 12 to 16 inches.

Always remember to use structural flathead wood screws for this. Use three to four screws per vertical support.

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Plumber repairs the pipeline in the floor

There are different methods of boxing floor joists. Certain methods work best for specific situations.

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