Floor joists are important elements of every floor frame, and they’re perhaps the most important structural component of your floor, carrying the weight of everything that comes on top. They’re crucial for load distribution so that there is no part of the floor is overburdened.
Whether you need to troubleshoot the floor joists or make any structural changes or additions, it can be rather challenging to locate them, especially if you’re unaware of their direction.
In this guide, we will talk about different methods you can use to find floor joists under different types of flooring, including hardwood and carpet.
How to locate floor joists
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While there are some recommendations and standards when it comes to the positioning of floor joists, it is usually different in every household. Of course, this will also depend upon the building codes and other local requirements.
Depending on the type of flooring, there are different methods to locate the floor joists. Some of them may require a bit more experience than others, and they can also vary in reliability and accuracy. Nevertheless, we will discuss the best methods you can use.
1. How to find floor joists under the subfloor
Usually, people want to find floor joists in order to get rid of that annoying squeaky sound or replace or repair their subfloor. Regardless of your plan, if you want to locate the floor joists under the subfloor, you can use different methods.
The first and the most common method would be using a stud finder, which is a specific tool made for this kind of task. However, sometimes the stud finder may not be the most accurate solution, which is where other, more “traditional” methods come into play.
The first alternative would be to use a hammer or a magnet to locate the floor joists underneath the subfloor. If this doesn’t work, or you simply don’t know how to utilize this method, you can always resort to math – measuring the space between floor joists.
2. How to find floor joists under hardwood
The first step to finding floor joists under hardwood is using a stud finder to locate the joists, even roughly, which is the key in determining floor joist direction.
The next step would be to remove the molding located on the edges of the room and then use a drill to create a small hole at the edge. Make sure that this hole is located where the molding will cover it once it is reinstalled, as you don’t want to ruin your flooring.
You want to continue drilling until you’ve reached solid wood, which is actually the floor joist. Now you want to move on to the next joist, which will be located 16 or 24 inches apart, depending on how the joists were installed.
Once you reach the second joist, measure the distance between the two joists so you know where you can find the rest. Calculate and measure the distance between the joists, and then mark them so you know exactly where the rest of the floor joists are located.
In case you have access to your crawl space or basement, you could achieve this by drilling a hole through the subfloor. If you use this technique, measure the distance between the joists from beneath and mark them accordingly.
By simply shining a light on the joists from your subfloor, you will be able to detect the nail heads securing the floor joists.
3. How to find floor joists under a carpet
While the electronic, magnetic, and traditional options work on hardwood, they may not be as effective on carpeting, especially if you have a thicker subfloor. Luckily, there are high-quality electronic stud finders that offer a deep scanning option, which is what you need.
Unfortunately, the sound test using a hammer will not be as reliable if you have carpeting, so it is pretty much off the table. Also, a magnetic stud finder isn’t a good option, either, as you cannot lay them flat even if the magnets were strong enough to locate the nails.
If you’ve decided to purchase a high-quality stud finder, it may even have the option of thick material scans, which is exactly what you need to scan the floor through your carpeting. Now you want to place the stud finder as flat as possible against the surface.
Press the locate button on your electronic stud finder (or a different corresponding button) and it should start beeping. This doesn’t mean that it has located the joist, as this is just the initial sound it makes, and you should not move it until it stops beeping.
Once the beeping sound has stopped, start moving the stud finder across the surface of the room. If you’re using a high-quality stud finder, it will indicate the direction of the joists with arrow lights, pointing you to the floor joists.
The arrows should start blinking fast or become brighter once you’ve reached the joists. In some cases, the stud finder will even make a different sound once the joists have been located. Once you reach the very center of the joist, there should be a red light on the finder.
Of course, different stud finders will come with different mechanisms, so make sure to read the manual and find what type of alert your stud finder has.
How to use a stud finder to locate floor joists
Stud finders are tools that can either be electronic or magnetic, and their sole purpose is to locate the floor joists, whether it be by detecting their edges (electronic), or the nails securing the framework (magnetic).
Electronic stud finder
Even though they can sometimes fail, electronic stud finders are still the most reliable option, especially if you don’t have much experience with how the subfloor structure works and other tools such as hammers.
In order to ensure that the electronic stud finder is showing accurate information, it is necessary to position it correctly. Once you’ve laid it onto the floor surface, all you need to do is press the button and the finder will begin inspecting and reading the subfloor.
In case you press the button before positioning the device onto the floor surface, the reading won’t be accurate. This type of electronic tool inspects the density of the surface, locating the joists underneath.
In case you’ve pressed the button before placing the device onto the floor, you can always restart it and start over. It is important to follow these steps in order to get an accurate reading and locate the joists before removing the flooring.
In order for the device to work properly, it is necessary to position it parallel to the floor surface, sliding it left and right slowly until you’ve detected the floor joists. Depending on the device, it may detect either the edges or the center of the joist.
In case the device detects only the outer edges, you’ll have to move it back and forth until you’ve found both sides. The easiest way to mark these edges is to use a tape and place it on both sides you detect. Of course, choose a tape that won’t damage your floors.
Magnetic stud finder
In case you’re using a magnetic stud finder, the process will be a bit different. While the electronic stud finder will detect density changes and locate the floor joists, a magnetic stud finder, on the other hand, detects metal nails securing the floor joists.
When using a stationary magnetic stud finder, you will feel a pull when you reach a spot underneath a metal nail is located. Just like with the electronic stud finder, you’re supposed to drag the tool across the floor surface until you feel that magnetic pull.
A moving magnetic stud finder, on the other hand, will make a loud noise once the metal nail is located, and you won’t have to search for a magnetic pull. However, magnetic stud finders aren’t as reliable as the electronic ones, as they only locate the nails.
What this means is that you’ll get a rough estimate of where the floor joists are located. Still, finding floor joists is much more than that, as you need to locate the edges and the center.
How to find floor joists without a stud finder?
In case you don’t own a stud finder of any kind, or you’d simply like to use more traditional methods, you will need a hammer. First, you will have to locate joists using magnets, such as the magnetism present in a magnetic stud finder.
Different types of floor joists can be located using simple magnetism. Once you’ve located them, you will need to use a hammer to tap the floor and detect any hollow points. Before starting this test, it is necessary to protect your flooring with a cloth.
A hollow space will produce a deeper, louder sound, while a firm surface will produce a muffled sound. This sound distinction will help you locate the floor joists, after which you’ll want to mark the target area using painter’s sticky tape.
Another method that doesn’t require stud finders is measuring the distance, but this will only work if you know the distance between the joists. Keeping this distance in mind, start measuring from the edge of the floor, and use a hammer and the sound test to locate joists.
David Kessler is a CSLB licensed contractor with over 25 years of experience. He is specialized in home building, construction, and electrical works; he also has a keen eye for interior design and, over the years, has collaborated with various architects.
