Drill Perfect Holes in Ceiling or Floor Joists (3 Easy Steps!)

Last Updated on October 9, 2022 by Barry Gray

Drill Holes in Ceiling Joists

You often require drilling holes in ceiling joists to pass any electrical or plumbing runs through the ceiling or floor.

I noticed that quite a few people have a common concern that drilling holes in joists will make them weaker. I’m here to let you know that you can safely drill holes in joists by following a few simple guidelines.

Step 1: Identify Joist Type

Two common types of joists are used in the construction of residential homes. These are called (1) Dimensional Lumber, which has a cross-section of a basic rectangle. And then there’s also the (2) I-Joist, also known as a Structural Joist. Pictured below is the dimensional lumber on the left, with the I-Joist on the right.

It is important that you identify exactly what type of joists you have (it will be one of the two), as the rules are different for both of them.

Step 2: Measure Holes

This is an important step. In order to keep the integrity of the joists, you will need to follow some rules when drilling through them. The sizing and positioning of holes are a crucial step so it’s best you follow this step carefully. Each joist type has a different set of rules.

Dimensional Lumber

These joists are quite easy to drill holes in, as long as you stick to the guidelines below.

  • Size: The diameter of the holes cannot exceed a third of the joist depth. This means that if you are drilling a hole in a joist that’s 10 inches high (254mm), the diameter of the hole cannot exceed 3.3 inches (83mm).
  • Position: The holes must be at least 2 inches (50mm) away from the top or bottom edge of the joist. If you intend to drill multiple holes in the joist, then two holes must be at least 2 inches (50mm) apart from each other.

I-Joist/Structural Joist

These joists are a bit more tricky to drill holes in. Below are the basic guidelines.

  • Size: Keep holes in these joists to a maximum of 1.5 inches in diameter (38mm). If you’re drilling a hole in the center of the joist then you can drill up to a maximum of 4 inches in diameter (100mm).
  • Position: You can drill holes anywhere between the flanges of the I-Joist. The flanges are the thicker ends at the top and bottom of the joist. Do not drill holes within 6 inches (152mm) from either end of the joists. Keep holes 8 inches (203mm) apart from each other if there are multiple holes in the joist.

If you intend to drill any holes larger than 4 inches (100mm) in diameter, please first check with the joist supplier/manufacturer.

Step 3: Drill

I’m pretty certain that none of you need to be advised on how to drill holes. A normal drill is required and if you are drilling small holes like 1/4 inch or so, then a normal wood drill bit would be perfectly fine.

If you are drilling holes on the larger side (if you’ve never seen a drill bit the size of the hole you want), then you can simply use a hole saw attachment for your drill. If you don’t have one you can get it for under $10 and it looks like this (link to Amazon).

You’re Done!

Once you have followed the steps in this article, then you can relax because the job would be done and you would not need to worry about messing up the strength and integrity of your joists.

Good luck on your project.  Hopefully, these tips I shared will help.

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Barry Gray

Hi, I’m Barry. I’ve loved woodworking and bringing things back to life for more years than I care to remember. I hope my passion for tools comes across loud and clear in everything you read here on The Tool Square.