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Running Ductwork Perpendicular To Joists [2-Step Manual]

Excited to refurbish your home, only to realize that plumbing work is a pain? Well, chances are you’re facing difficulties in running pipework through your joists.

Are you planning on running ductwork perpendicular to joists?

If the answer is yes, there are rules to follow. You have to follow sequential steps such as planning before drilling. You have to choose the right tools and take proper safety precautions. The holes must be uniform and properly drilled as well.

This amount of information is unquestionably inadequate. So, we’ve put together a step-by-step guide for drilling holes into your joists. We’ve even offered some helpful hints and solutions too.

Let’s get to it!

Can You Run Ductwork Perpendicular to Joists?

Yes, it’s possible to run your ductwork perpendicularly. In fact, there are times that you have to do so. In that case, you have to drill a hole in them. 

However, if you have to run your pipes through, there are rules. These have to be followed to retain the structural integrity of the joists.

The handyman’s manual on joist drilling are mentioned below-

5 Basic Rules to Follow When Running Ductwork through Joists

Floor joists are timber, wood, or steel structural beams that support the floor. These beams are used to support the floor beds.

You can’t just run ductworks through your joists as you please. They are mostly dependent on the length/span of the board. The depth/width is also an important factor.

Let’s check these rules out.

Maximum Diameter of Holes

The general rule is you can have a hole that is 1/3rd of the depth. 

The depth is known as the width of the board. To break it down-

If you have a 2×6 (4.5-inch diameter) pipe, the maximum diameter would be 1 ½ inches.

Spacing Between Consecutive Holes

The maximum spacing between each consecutive hole is also limited. Each consecutive pipe should be 2 inches apart from the other. This barrier of wood between the pipes keeps it in place.

Spacing Between Hole and Edge

You cannot drill a hole anywhere within the width of the joist. There should be a minimum 2-inch gap from the top or bottom edge. Otherwise, the structural integrity will be compromised.

Notching Rules

This is the most important rule of the bunch. To understand this, you need to divide the joist into 3 parts.

You can have notches in the outer thirds of the joist. The maximum depth of each notch should not exceed ⅙th of the width. However, it’s best to avoid notches altogether.

However, you can never have notches in the middle 3rd of the board. This is how you properly notch a joist.

Stacking Pipes

This rule is regarding the placement of each pipe in the joist. You cannot have two pipes on top of each other. Stacking two pipes will cause more stress points to emerge.

You can however horizontally place each pipe next to each other.

We’ve covered the fundamentals, so let’s get to work!

Step-by-Step Guide for Running Ductwork Perpendicular to Joists

Now that we’ve gone over the rules, time to make a plan. We have a step-by-step blueprint of how to approach the task.

Step 1: Choose Your Safety Gear And Tools

You’ll need the proper gear to complete it. It is critical to have all of the necessary tools on hand ahead of time.

These include the drill machines and drill bits. Let us go a bit more in-depth about these two.

Drilling Gear

Joists or ducts that have stainless steel or heat-hardened metals require special drills. The go-to drill bits for these would be the M7 or M42 drill bits.

For wooden joists, the options are Forstner, Spade/Paddle, and Saw-hole drill bits.

Spade drill bits are the cheapest options used for electrical holes.

However, there’s a chance of a chip out on the joist. You can avoid that by using a plywood base and drilling slowly.

The best, albeit expensive alternative, is to use a Forstner Drill Bit. They bore clean cuts with minimal tear-out, which is perfect for ductwork.

The Saw-hole drill bit would be the middle ground between the two. It is slightly cheaper but almost as efficient as a Forstner. 

Dremel Drills are an excellent choice as well. However, there’s a guideline on cutting ductwork with Dremels.

Safety Precautions

Whenever you’re near a high-powered tool the chances of accident occurrence also rise. This risk is compounded if you’re working in a shop environment.

There are several ways in which you can get hurt at a construction job. Always wear the appropriate protection.

Whenever you’re working with drilling, wear goggles and gloves. 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 on to drilling the actual holes. Be it using a Jigsaw to cut ductwork or an angle grinder, safety is key. 

Step 2: Getting To Drilling the Joists

You need a power drill with the appropriate drilling heads. There are a lot of options for you to choose from. 

If you’re using the Hole Saw remember to be perfectly perpendicular to the joist. After ensuring that, place a sacrificial board under the joist.

Remember to use a clamp to hold your joist steady. 

You have to drill twice on the joist from opposite ends, for the Saw-Hole. This is because the Hole Saw drill bit cuts out a plug.

Repeat these steps to all the joists and make sure they are aligned perfectly. This would allow the pipes to enter smoothly later on.

Pro Tip: Use a framing square to mark the centers of the entry points. This will give you a more accurate reading throughout each beam.

To summarise, this is how you run ductwork perpendicularly through joists. We hope this article came in handy for resolving your joist issues.


Question: Can I reroute my existing ductwork?

Answer: Yes, you can but it is expensive to do so. It’s because all the ductworks are connected, hence the entire house’s ductwork has to be rerouted.

Question: When should I replace my Ductworks?

Answer: Ductworks should be replaced every 10-15 years before issues arise.

Question: What should I do with my old Ductworks?

Answer: The best option would be to recycle your old ducts. Especially the steel parts which will otherwise accumulate.

Bottom Line

Now you know how to plan on running ductwork perpendicular to joists. Hopefully, you have realized how to tackle the challenges associated with the task.

Let us know which suggestions came in handy when installing your ductwork. Till then, stay safe and good luck!