Cutting Holes In Joists For Duct – 3 Step Guide To A Perfect Fit

We may get commissions for purchases made through links in this post.

Ecstatic about fixing the pipework in your house, only to find it bothersome? Well, it’s probably you’re having a tough time with your joists.

Is cutting holes in joists for duct difficult?

It is wise not to drill holes in as it makes the beam unstable. However, if you must make a hole there are rules. You have to follow sequential steps such as planning before drilling. The holes must be uniform and a lot of precautions should be taken.

This much info is surely not enough. We’ve put together a step-by-step process of drilling holes into your joists for plumbing. 

Let’s not wait and get to it!

4 Codex Of Joists

Joists are key structural beams of timber, wood, or steel made to support the flooring. 

The floorbeds are mounted on top of these beams in contrast to overhanging beams. In comparison to the pros and cons of cantilever beams, joists are appropriate.

On that note, you may ask yourself “Is it wise to drill holes in your supporting beams?” The answer is no. You can drill a hole, but it cannot be just anywhere within the beam.

There are rules when it comes to notching and drilling holes in joists. Failure to do so would result in flooring flaws. Curious about the rules?

Let’s check them out.

Maximum Diameter of Holes

The regulations regarding the size of the holes are quite clear. The determinant is the depth or width of the joist.

The general rule is you can have a hole the size of a quarter of the depth. The depth is the width of the board. To break it down-

If you have an 8 inch (203.2 mm) wide joist the maximum diameter would be 2 inches (50.8 mm).

Spacing Between Holes

The Maximum spacing between each consecutive hole is also limited. Each of the holes should be 3 diameters apart.

If you have a 12mm hole, the next hole would be 36mm from it. It could be either left or right from that hole.

Center Lining

The next commandment of joist-drilling is the placement of the holes. You cannot cut a hole randomly in the wood.

The holes have to be placed in the centerline of the beam. A centerline should be drawn across the span of the board. The middle of each hole should align with the centerline.

Otherwise, it would shift the center of mass of the entire board. This would have disastrous outcomes.

Boundaries of Hole Placement

The final rule regarding joist drilling is the limitation of holes within the board. You have to keep a bit of space between the very ends of the beam.

The holes should be located between 0.25 and 0.4 of the span of the joist. It shouldn’t be before the 0.25 mark or after the 0.4 mark.

This is to leave ample space for corrections later on. These spaces can be utilized later on. Having these spaces allows for retrofitting a vent to your existing ductwork.

Now that we’ve covered the basic rules, let’s start drilling!

Drilling Holes Into The Joists For Duct

After taking into consideration the rules of joist-drilling, you should draw up a plan. Down below, we have a step-by-step blueprint of how to approach the task.

Without further ado, let’s get to it!

Step 1: Plan The Runs

Proper planning of the duct lining would help you get a bearing on things.

You should take into account the drawing of the building. This is crucial as you have specific outlets within the buildings. Working around those layouts will save you a lot of trouble.

Plan according to where the major water outlets are. Next, take into account where the bathrooms and sinks lie above the floor. There should be different duct lines of the sink and bathroom.

Lastly, make sure you have different duct lines for the hot/cold water. After you’ve taken care of the crucial planning, we move onto the next step.

Step 2: Choose Your Safety Gear & Tools

Since this job requires precision, you need the right tools to carry it out.

It’s important to have all the tools ready beforehand. Not having the right tools would result in sub-par outcomes.

Firstly, it’s best to wear the appropriate protective gear. This includes gloves, helmets, and goggles.

If you’re unsure about which gloves to invest in, take a look at our picks below-

Glove 1
Glove 2

Now that you are well-protected, we move to the next step, the tools. 

You need a power drill with the appropriate drilling heads. Make sure it’s an angled drill, as it’s narrower and easy to handle. 

Dremel drills are an excellent alternative. You can even cut your ductwork with Dremel.

Anyway, here are two tips in preparing to drill into your joists:

Tip 1: Make sure you have a cord extension. A longer cable will give you better mobility.

Tip 2: Put on weights on the side of your ladder to make it more stable.

Let’s move on to drilling the actual holes.

Step 3: Drill In The Holes

Remember, you must drill the holes with accuracy. Marking the holes beforehand will work wonders on your accuracy of drilling.

It’s important to note that you should line the adjacent holes perfectly. The holes in each joint should be properly lined. This would make it easier for the pipes/ducts to run through.

This also ensures space for ductwork insulation in garages, basements, etc.

Here are two tips when it comes to drilling holes:

Tip 1: Make sure you reinforce the joist using plywood. Gluing a piece of plywood across the holes would help secure it more.

Tip 2: Make sure to drill the holes with ample space. If your duct has a radius of 15mm, drill a 19mm to accommodate for movement.

To summarise, these are the processes when it comes to drilling holes in your joists. We hope this article came in handy for resolving your joist issues.


Question: Will screwing into the joists weaken the joist?

Answer: Yes, improperly screwing holes or cutting out notches cause stress points to emerge. These imperfections cause the bottom edges to tense up and squeeze the top edges.

Question: How do a Joist and Truss Differ?

Answer: The joist actually supports the heavy load which your floor bears. A truss, on the other hand, supports your roof. Simply put, joist is for the floor and truss is for your roof.

Question: Why should I use a Joist over Trusses?

Answer: Trusses play a role in house fires, as they falter faster compared to joists. Firefighters prefer joists because joists take longer to burn.


Now you know how to plan on cutting holes in the joist for the duct or pipeline. Hopefully, you have realized how to resolve these issues.

Good luck cutting the holes!

Richard Allen