how to find joists beneath plaster lath and plaster

3 min read 20-01-2025

how to find joists beneath plaster lath and plaster

Finding joists hidden beneath plaster lath and plaster can be tricky, but it's crucial before hanging anything heavy or cutting into the wall. This article outlines several methods to locate these hidden structural supports, ensuring a safe and successful project. Incorrectly locating joists can lead to structural damage and costly repairs. This guide will help you locate them accurately and safely.

Why Finding Joists Matters

Before undertaking any task involving your walls – whether it's hanging shelves, installing cabinets, or running electrical wiring – accurately locating joists is paramount. Knowing the joist locations prevents damage to your home's structure, saving you time, money, and potential injury. Hanging heavy items on an unsupported section of wall can lead to drywall damage, plaster cracking, or even the collapse of the entire section.

Methods for Locating Joists

Several methods can help you pinpoint the location of joists hidden beneath plaster lath and plaster. Some are more invasive than others, so choose the approach best suited to your comfort level and the scope of your project.

1. Using a Stud Finder

A stud finder is a readily available tool at most hardware stores. Many modern stud finders utilize magnetic sensors or electronic sensors to detect the wooden joists. However, these methods may not work reliably with plaster walls. Plaster walls often contain metal lath, which can interfere with the magnetic sensors, resulting in inaccurate readings. While electronic sensors might detect the wood, the results are less precise than with drywall.

Limitations: The accuracy of a stud finder on plaster walls is often less reliable than on drywall. Always use multiple methods to confirm the location of your joists.

2. Tapping and Listening

This traditional method involves lightly tapping the wall's surface with your knuckles. Listen for a difference in sound. Solid, thudding sounds usually indicate a joist, while a hollow sound suggests an empty space. This method requires a bit of practice, and the results may be inconclusive, particularly with thick plaster.

Limitations: This method is subjective and depends on your ability to discern subtle sound differences. It is best used in combination with other methods.

3. Visual Inspection (If Possible)

If you have access to an adjacent room or attic, visually inspecting the joists can offer a clear advantage. This could provide a starting point for more accurate measurements in the plaster wall.

Limitations: This method is only possible if you have convenient access to other parts of your house.

4. Using a Magnetic Stud Finder (with Caution)

Some magnetic stud finders can work on plaster walls, but they are less reliable than on drywall. The metal lath can interfere with the results. It's always best to combine this method with other techniques.

Limitations: Metal lath within the plaster can cause inaccurate readings.

5. Employing a Powerful Magnet

A powerful neodymium magnet can sometimes attract the metal lath, hinting at the position of the joists. However, the metal lath isn't always consistently spaced along the joists, making this method less precise.

Limitations: Only helpful if your plaster contains metal lath and its placement directly corresponds with the wood joists.

6. Employing a moisture meter or thermal imaging camera

A moisture meter or thermal imaging camera can, in some cases, provide some data. These methods rely on slight variations in temperature or moisture content that might exist between solid wood joists and the surrounding plaster. The effectiveness of this method can depend on the insulation and other environmental conditions, however. These methods can be unreliable with older homes due to inconsistencies in material and construction.

Limitations: This method is highly dependent on environmental conditions and building materials. It's not a primary method and should be utilized with others to confirm the presence of joists.

Confirmation and Safety Precautions

Regardless of the method you use, always double-check your findings. It is better to err on the side of caution and avoid drilling into the wall until you're absolutely certain you've accurately located a joist. Always use multiple methods to confirm your results and protect yourself by wearing safety glasses when using tools.

Conclusion

Locating joists beneath plaster lath and plaster requires patience and the use of multiple methods for confirmation. By combining the approaches outlined in this guide, you can safely and accurately determine joist positions, ensuring the success and safety of any home improvement project involving your walls. Remember to always prioritize safety and double-check your work. If you are unsure, consult a qualified professional.