Introduction
Finding studs behind plaster and lath means locating the wall framing behind the plaster so you know where to anchor. In practice, DIY folks use magnets to find fasteners and then verify with a probing method to confirm stud depth. This approach helps you avoid guessing and reduces the risk of drilling into wires or hollow spots.
Choose either magnetic stud finders or a probing tool, and use them together to compare results. Be mindful that plaster and lath walls can conceal different construction materials, so verify with the manufacturer instructions and local rules before any cutting. If you’re unsure, stick to safe probing practices, check for hidden wires, and favor slow, shallow tests until you confirm solid wood or metal studs behind the surface.
Key takeaways
- Magnetic finds help locate studs behind plaster, but verify with probing.
- Probe lightly with a slender tool to confirm stud edges without excessive damage.
- Pause before drilling, identify electrical paths; wear eye protection and avoid hits.
- Different wall types require adjusted technique, plaster-lath vs drywall matters.
- Use multiple methods: magnets, probes, and scanner checks increase accuracy.
- Mark studs clearly and plan mounting to avoid repeated probing damage.
Table of Contents
- Introduction
- Key takeaways
- Why Plaster-and-Lath Walls Are Different
- Tools and Materials Checklist
- Using Magnets to Find Studs
- Safe Probing Techniques (Minimally Invasive)
- Electronic and Alternative Scanning Options
- Avoiding Electrical and Plumbing Hits
- Common Mistakes and Troubleshooting
- Best Practices for Marking, Planning, and Mounting
- Conclusion
- FAQ
Why Plaster-and-Lath Walls Are Different
Plaster-on-lath walls add thickness through multiple plaster layers that can obscure the actual stud edges. This often causes conventional stud-finders to misread or miss studs entirely. Lath strips also distribute load differently and can shift where a stud seems to be located.
Wall thickness varies with lath material and plaster thickness, which can recess or conceal edges and affect edge-dinding accuracy. Magnetic tools may grab onto lath metal or nails instead of the stud, so you’ll need to shift from static searching to careful probing and cross-checking. A practical approach combines magnet checks, true-edge probing at multiple heights, and a dry-run behind the surface before cutting or drilling.
Wall construction basics
Lath-and-plaster walls are built on a framework of vertical studs, typically spaced 16″ or 24″ apart. Horizontal backing, called lath strips, run perpendicular to the studs and provide a surface for the plaster. The plaster is applied in multiple layers, with keys – small ridges – added between each layer to improve adhesion.
The finished wall’s thickness varies based on lath material (wood or metal) and plaster buildup. Stud edges may be recessed or concealed behind the lath and plaster, making them hard to locate with common methods.
How plaster/lath affects signals and sound
Plaster-on-lath walls are dense and layered. This can weaken or mislead feedback from electronic stud finders, magnets, and tapping.
Electronic stud finders may struggle to penetrate the wall’s depth, leading to false readings or missed studs. Magnets might grab onto lath metal or nails instead of the stud itself. Tapping sounds may not travel clearly through the dense material, making it hard to pinpoint stud locations.
The layered nature of plaster-on-lath walls also means that stud edges are often obscured, further complicating detection methods.
Tools and Materials Checklist
A tools checklist matters for avoiding wire hits, and PPE and disciplined drilling or probing near plaster and lath are essential. Start with the basics and add specialty items as needed. Plan for safe handling and repeatable results.
Essential tools include magnets for quick metal detection, push probes or awls for pinpointing edges, a calibrated stud finder suitable for plaster/lath, and small drill bits for controlled test holes. Optional items can improve accuracy, such as a flashlight, laser level, or inspection mirror. Verify device calibration on known targets and consider renting higher-quality gear if you don’t own it.
Magnetic tools (types and selection)
Before you start using magnetic tools on your project, consult this checklist to ensure you’ve got the right ones for the job. This will save you time, money, and potential rework.
- Understand magnet types: Rare-earth magnets (like neodymium) are stronger than standard magnets but can be more expensive. Commercial magnetic stud finders use these strong magnets to locate metal studs behind walls.
- Check magnetic strength: Look for magnets with a pull force of at least 50 lbs. This is usually sufficient for most DIY tasks. For heavier duty work, consider magnets with a pull force up to 100 lbs or more.
- Verify magnet size: Ensure the magnet’s width and length are suitable for your project. A larger magnet will cover more area but may be harder to control.
- Check magnet thickness: Thicker magnets have a stronger grip but can be bulkier. Consider the thickness that fits best with your project needs.
- Portability matters: For larger projects, consider magnetic tools with handles or wands for easier maneuvering. For smaller tasks, handheld magnets may suffice.
- Avoid cheap magnets: While they might seem like a bargain, low-quality magnets can lose their strength quickly and may not be strong enough for your project. What goes wrong if you skip this: You’ll waste time and money on frequent replacements.
- Test before use: Before relying on your magnetic tool, test it on a similar surface to ensure it works as expected. This can help avoid costly mistakes.
- Safety first: Always wear safety glasses when using strong magnets to protect your eyes from any debris or projectiles.
Quick rule: When in doubt, opt for a magnet with a pull force of at least 50 lbs and a size that fits your project’s needs. This will give you the best balance between strength and maneuverability.
Probing and piercing tools
Before you start any probing or drilling, use this checklist to ensure you have the right tools and know how to use them safely. This will help you avoid damaging your surfaces and structures.
- Low-profile probes: Check they’re thin enough (around 1/8″ to 3/16″) for minimal damage. Tap gently to locate voids or check for hollow spots.
- Thin awls: Ensure the tip is sharp and narrow (around 1/16″ to 1/8″). Use for precise piercing into wood or soft materials.
- Short drill bits: Confirm they’re short enough (under 2″) to minimize damage if you hit a hard surface. Use for small holes in various materials.
- Patching materials: Have drywall compound, spackling paste, or wood filler ready to repair any test holes made during probing.
- Magnetic tool holder: If using magnetic tools (as mentioned earlier), ensure the holder is securely attached and can withstand vibrations.
- Drill bit depth stop: Set a depth stop on your drill to prevent over-drilling. Check it regularly as drilling progresses.
- Safety glasses: Ensure they provide adequate protection from debris and are comfortable for extended use.
- Clamp or vise: Have one ready to secure materials while probing or drilling, preventing slippage and accidents.
Quick rule: Always probe first, then drill. This helps locate hidden obstacles and prevents costly mistakes.
Using Magnets to Find Studs
Magnets on plaster-and-lath walls help locate nails or screws that indicate stud positions, but lath nails can create false positives. Understand the limits of magnets for these walls and be prepared to verify with probing. Use magnets as a first-pass cue, not as a final locator.
Choose a magnet with appropriate strength and surface area for plaster, and plan a grid or pattern of passes to cover the wall. Mark hits clearly and translate those marks into likely stud centers. Cross-check suspicious spots with a gentle probe and consider nearby electrical boxes for orientation.
Sweep patterns and marking
To find studs behind plaster-and-lath walls using a magnet, you’ll need a systematic sweep pattern. Start at one corner of the wall and hold your magnet flat against the surface.
Move it in overlapping strokes, either horizontally or vertically, about 16 inches apart – that’s the standard spacing for studs. If you’re sweeping horizontally, make sure to overlap each pass by a few inches to ensure you don’t miss any nails or screws.
When your magnet sticks to something, mark it with a pencil. Since there are usually two nails or screws per stud, mark both hits and draw a line between them. This will help you locate the center of the stud for hanging heavy items.
Interpreting magnetic hits and avoiding false positives
While using a magnet is an effective way to find studs, it’s not foolproof. Lath nails, metal lath, or plaster embeds can sometimes cause false positives.
- Lath Nails: These are often spaced irregularly and won’t line up with your sweep pattern. If you see a single nail hit that doesn’t align with others, it’s likely a lath nail.
- Metal Lath: Some walls have metal lath under the plaster. This can cause widespread magnetic hits. If this happens, try using a smaller magnet or switch to probing.
- Plaster Embeds: Sometimes, small metal objects get embedded in the plaster during construction. These will show up as isolated magnetic hits.
- Electrical Boxes: These are usually anchored to studs. If you find a magnetic hit near an electrical box, it’s likely a true stud hit.
To confirm your findings, use a pry bar or probe gently around the magnet hits. If it feels solid, you’ve likely found a stud. Always remember that plaster-and-lath walls can hide wires, so be cautious and considerate when probing.
Safe Probing Techniques (Minimally Invasive)
Use push probes, awls, or short drill bits to verify stud location with minimal hole size. Define minimally invasive depth targets to avoid plaster damage and hidden wiring. Start with non-penetrating checks first where possible.
Follow a step-by-step probing protocol: begin with magnet checks, mark candidate lines, then perform shallow probe passes at the marked intervals. Advance only millimeters per pass and pause to listen or feel for obstructions. Patch every hole properly afterward for a clean repair.
Where and how deep to probe
Probing plaster-and-lath walls requires finesse. You’re aiming for the wood studs, not wires or pipes.
Start at the top, near the ceiling line. Work downwards in small sections. This way, if you hit a wire or pipe, it’s less likely to be live.
Probe gently. Limit penetration to 1-2 inches. If you feel resistance, stop. You might have hit metal (wire) or void (empty space). Use your magnet to check if it’s a wire. If not, try another spot nearby.
Small test holes and repair
Making tiny test holes helps confirm stud presence without causing major damage. Use your push probe or awl for this.
Insert the tool at a 45-degree angle. This creates a smaller entry point. Twist as you advance, then withdraw and inspect the hole.
If you see wood shavings, you’ve hit a stud. If not, try again nearby. To repair, use a small amount of joint compound or spackling for plaster lath. For drywall, use a drywall patch kit. Sand smooth and touch up with paint.
Electronic and Alternative Scanning Options
Electronic stud finders, multi-mode wall scanners, and acoustic/tap methods each have strengths and limits on plaster-and-lath walls. Know what each device detects and what it may miss behind multiple plaster layers. Use them as part of a broader verification plan.
Consider how lath thickness and plaster mass affect sensor depth readings and note typical dead zones behind plaster. Calibrate devices on known targets, perform multiple passes, and triangulate findings with magnets or probing to reduce wall damage while avoiding live wires.
When to rent a wall scanner
A wall scanner with multiple detection modes can be a game-changer for complex jobs. Here’s when it’s worth renting or borrowing one:
Complex layouts: If you’re dealing with irregular framing, deeper studs, or multiple layers of plaster, a scanner can help pinpoint them more accurately.
Load-bearing locations: When precision is crucial, like when installing heavy shelving or reinforcing walls for safety reasons, a scanner can provide the confidence you need.
Remember, renting isn’t always cheaper than buying. If you’re planning to use a scanner frequently, consider purchasing one.
Acoustic and visual alternatives
While electronic devices are handy, sometimes old-school methods or visual clues can help locate studs behind plaster and lath. Here’s how:
Tapping/listening: Tap the wall with a knuckle or light hammer. Listen for a dull thud that indicates a solid stud. This method works best on drywall but can be used on plaster-and-lath walls too.
Infrared/thermal cameras: These devices can detect heat signatures, which might help locate plumbing or electrical wires behind the wall. However, they’re not as effective at finding studs in plaster-and-lath construction due to the material’s thermal properties.
Visual clues: Baseboard nails, trim seams, and outlet locations can sometimes indicate where studs are. However, these aren’t foolproof methods, so use them alongside other techniques for better accuracy.
Avoiding Electrical and Plumbing Hits
Treat every plaster-and-lath wall as potentially harboring live electrical lines or pipes. Plan probing only after verifying safety with the proper tools. If in doubt, stop and reassess before proceeding.
Use a non-contact tester to scan around studs, outlets, and suspected routes, and shut off relevant circuits if needed. A wall scanner that can detect live wires helps, but understand its limitations behind plaster and lath. Document hazards clearly and proceed with shallow probing only when you’re confident it’s safe.
Detecting wires and pipes before you probe
Before you start probing, assume there are hidden utilities. Use a non-contact voltage tester to scan the area around studs, outlets, switches, and suspected utility routes.
Check every outlet and switch in the room. If they’re live, so might be wires running through walls nearby.
Trace likely cable runs. Follow along baseboards or ceiling lines where wires often hide. Mark these areas to avoid probing them.
If you encounter a live wire or plumbing
Stop work immediately if your probe hits a live wire or pipe. Do not attempt to continue.
Avoid contact. Even if the wire seems dead, it could still be live.
If safe to do so, de-energize the circuit at the panel/breaker box. Re-check with your tester to confirm no power is present.
Consult a licensed electrician or plumber. They have specialized tools and training to locate and isolate utilities safely.
Common Mistakes and Troubleshooting
Avoid relying on a single method; combine magnet sweeps, electronic reading, and careful probing for reliability. Plaster and lath can offset studs, making signals appear inconsistent. Map centers carefully and cross-check often.
Be aware of misreads from metal lath, nails, or pipes, and distinguish hollow from solid signals. Probe shallowly, stop if you encounter resistance, and re-check with a fresh approach. When in doubt, reassess strategy or consult a pro for mounting plans.
Dealing with inconsistent results
So, you’re getting mixed signals from your leveling tools? Don’t worry, it happens. Here’s how to sort it out:
First, double-check your base compaction. Inconsistent readings often point back to a poorly prepared base. Use a plate compactor if needed.
Now, let’s triangulate. Combine these methods to build confidence in your marks:
– Magnet sweeps: Use your magnetic level for quick, overhead checks. It’s great for spotting major errors.
– Probing: Grab a 2-foot level and check the high and low points. This helps pinpoint minor variations.
– Visual cues: Look at your string line or laser level. If it’s wavy, you’ve got a problem. Recheck your base and marks.
If all else fails, re-zero your tools and start over. It might be user error. Keep at it, and you’ll get consistent results.
When plaster, lath, or trim confuses readings
Ever get a false positive on your stud finder? It’s not always the wall’s fault. Trim, corner beads, metal lath, or thick plaster can trick your tool into thinking it’s found a stud.
Don’t rely on just one reading. Move your stud finder around. If you’re getting consistent readings in the same spot, it might be a real stud. But if it’s all over the place, you’re likely dealing with a false positive.
If you’re using a magnetic stud finder, try switching to an electronic one. They work differently and can give you a more accurate reading. And always check base compaction. If it’s loose or uneven, your readings will be wonky no matter what tool you use.
Remember, every wall is different. Don’t get frustrated if you have to try a few methods before you find the one that works. And always trust your instincts. If something feels off, it probably is.
Best Practices for Marking, Planning, and Mounting
Document a precise stud map, noting centers, edges, irregular spacing, and potential false positives from lath. Plan to verify with at least two methods, then mark clearly and consistently. Have a plan for rechecking marks before drilling.
Choose anchors rated for plaster-on-lath and consider load requirements; select fasteners that distribute load and specify appropriate drill bit sizes. Perform wire-detection checks and annotate results, then verify with house plans or dimensions when available to avoid surprises behind the surface.
Creating and using a stud map
Start by sketching out your wall, dividing it into manageable sections. Use a simple grid system with clear labels for each section.
Mark stud centers as you find them, noting both the centerline and edge offsets. This helps plan accurate anchor placement later on.
Record any irregularities or potential false positives, like areas with metal lath or varying plaster thickness. These can throw off your readings.
Use consistent labeling for each stud, noting its location and any special considerations. This map will guide your anchor planning and mounting.
Anchoring heavy loads and layout tips
For heavy loads, spread the weight across multiple studs. This distributes stress evenly and prevents overloading a single anchor point.
Choose anchors or backing that can handle your load requirements. Hollow-wall toggles and plaster anchors are good choices for plaster-on-lath walls.
Plan your layout around verified studs. For shelves, cabinets, or TV mounts, align them with stud centers to maximize support.
Consider using a laser level or chalk lines to ensure consistent alignment across multiple anchor points. This helps maintain a straight, even installation.
Conclusion
Finding studs behind plaster and lath is about being deliberate, not guessing. Do the work with care and you’ll protect the wall, the finish, and your people from shocks or big repairs later.
First, confirm your wall type, then use magnets to locate likely studs, probe in a controlled curve to confirm spacing, scan with any electronic options you have, and always check for wires or plumbing before you drill. Mark your plan clearly, then double-check your mounting point against the actual stud lines before you commit. Keep the process slow, test in a small area, and stick to a simple mounting plan with proper fasteners.
Avoid these mistakes: assuming one method will always work, drilling or nailing without confirming a stud, or ignoring hidden wires and pipes. Always test tools on a scrap area, keep the power off when probing near outlets, and use the correct fasteners for plaster and lath. Safety first, and follow your marking and mounting plan rather than improvising in the moment.
If you hit a snag you can’t safely resolve—like uncertain wiring, damaged plaster, or a wall that won’t take fasteners—call a pro rather than forcing it. Stay deliberate, stay safe, and you’ll finish with a solid mount and a clean wall that looks right for years to come.
FAQ
Which methods reliably locate studs behind plaster and lath?
Start with a magnet to flag likely stud centers, then probe lightly in that area to confirm depth. If you’re unsure, measure from known corners or corners of doors and windows to triangulate stud spacing. For stubborn walls, use a combination of magnet tests and shallow probing until wood shows up solid, not hollow.
Magnetic stud finders vs. mechanical probes — which should I use?
Magnet sticks tend to be quick and cheap for locating screws in lath, but they aren’t foolproof. Mechanical probes are more precise about actual stud edges but take a careful hand and a shallow cut. Use magnets to guide you, then confirm with a probe before drilling or cutting.
What wall construction clues help me identify materials behind plaster and lath?
Look for wooden lath strips or metal lath patterns under the plaster, and listen for a hollow sound when tapping. If you see or feel wire or metal behind the plaster, pause and reassess. When in doubt, inspect adjacent walls or remove a small, non-critical area to verify materials.
How can I avoid hitting electrical wires while probing or cutting?
Always start with a cautious approach: probe lightly and never push hard near outlets, switches, or boxed wiring. If you suspect wiring, pause and check with a non-contact voltage tester. If you must cut, reroute or protect wires and follow safe cutting practices.