Structural and base materials · Metals
Metal Lath (Render and Plaster Background)
Describes what metal lath does as a render and plaster background, why its own corrosion protection is critical once buried, and where it bridges difficult junctions.
Educational reference entry. Suitability depends on the complete assembly, substrate, exposure, installation method, manufacturer documentation, local requirements and qualified professional review.
Overview
What metal lath is
Metal lath is thin metal sheet that has been slit and stretched into a mesh, or pressed into a ribbed and perforated profile, then fixed to a wall or frame as a background for render or plaster. The openings let the wet coat pass through and form a key behind the mesh, so the finished coat is held mechanically rather than relying on suction from the material behind it.
It is used where the background cannot provide a key of its own: over timber or light gauge steel framing, across the junction between two different materials, over older surfaces that cannot be relied upon, and on curved or shaped work where a coat has to be built out. Related products such as beads, stops and reinforcing strips do a comparable job at edges, corners and openings.
The lath's own protection matters more than it first appears. It sits inside a wet, alkaline coat, often on an external wall, and if the metal corrodes it stains through the finish and can push the coat away from the wall. That is why the coating on the lath, the fixings chosen with it, and whether the lath will stay dry in service are treated as specification questions rather than as site choices.
Terminology
Common names and aliases
These names describe the same thing. The library keeps one entry per material so that an acronym, a trade name or a regional spelling never becomes a second, thinner page.
- Expanded metal lath
- Render lath
- Rib lath
- Plaster lath
Applications
Common applications
Where this material is typically encountered. A typical application is not a statement that it suits your project.
- Render backgrounds over timber or light gauge steel framing on external walls.
- Bridging the junction where two different backgrounds meet beneath a rendered finish.
- Plaster backgrounds to curved walls, arches, soffits and shaped features.
- Repair work where a section of solid background has failed and cannot take a coat directly.
- Backgrounds for thicker or built-out coats that a plain surface would not hold.
- Ceiling and bulkhead work in refurbishment where a traditional plaster finish is reinstated.
Consideration
Where it is commonly considered
- Where a framed wall is to receive a rendered or plastered finish rather than boards.
- Where a substrate offers no suction and a mechanical key is needed in its place.
- Where a curved or irregular shape has to be formed and then finished in one plane.
- Where a junction between dissimilar backgrounds would otherwise crack through the finish.
- Where existing plasterwork is being repaired and a compatible background is wanted.
Professional review
Where additional professional review is especially important
These are the situations where the answer depends on the specific building, and where a qualified professional should be involved before anything is decided.
- Where the wall will be persistently wet or the lath cannot be kept dry within the build-up.
- Where the fixings or the lath coating have not been matched to the exposure of the wall.
- Where the lath would be in contact with a dissimilar metal in damp conditions.
- Where the supporting frame or background cannot carry the weight of the finished coats.
- Where movement in the background has not been accounted for in the render design.
Properties
Key properties to discuss
Qualitative topics worth raising. Measured values, classes and grades come from the manufacturer's technical documentation for the specific product, not from a reference page.
- Substrate dependency
- Lath converts a background that has no key into one that does, but it does not make a weak or moving background sound. What the lath is fixed to still governs how the finished coat behaves over time.
- Moisture behaviour
- The lath sits inside a damp coat while it is applied, and can be wetted again later through cracks or a failed detail above. Whether it stays dry within the finished build-up is the question that decides how it fares.
- Installation dependency
- Fixing centres, laps, the direction the mesh is fitted and the tension held across the face all affect the finished surface. This is skilled work rather than simply fastening a sheet to a wall.
- Repairability
- Once rendered over, the lath cannot be inspected. Repairs mean cutting the coat back to sound material, which usually results in a patch noticeably larger than the damage that was visible.
- Appearance
- Corrosion within the lath shows through as rust staining, fine cracking or bulging in the finished coat, often long before anyone can see what is actually causing it.
- Fire-related questions
- Behaviour in fire depends on the whole wall or ceiling assembly and its documentation, and is a matter for the project designers and for local requirements rather than for the lath considered alone.
- Documentation
- The lath type, its coating, the specified fixings and the render or plaster system it is intended to work with all belong to one specification and are worth seeing together.
Exposure
Exposure and environmental conditions
- Is the lath in an internal, sheltered external or fully exposed position on the building?
- Could water reach the lath through cracks, junctions or a defective detail above it?
- Is this elevation likely to see driving rain through much of the year?
- Is there any risk of condensation forming within the build-up behind the render?
- Is the location coastal, or otherwise likely to carry salt onto the wall surface?
Assembly
Substrate and system dependencies
What this material sits on, is fixed to or depends on. This is usually where performance is won or lost.
- The frame or wall behind the lath carries the whole weight of the finished coats.
- A separating or breather layer is often specified between the background and the lath.
- Fixings have to suit both the lath and the background, and be compatible with each.
- Beads, stops and movement joints form part of the same background arrangement.
- The render or plaster specification is chosen with the lath rather than independently of it.
Appearance
Appearance and finish considerations
- A poorly tensioned lath can telegraph as waviness through an otherwise flat rendered finish.
- Laps and fixing lines can show through if the coat is applied thinly over them.
- Where lath bridges two backgrounds, a well detailed finish should read as continuous.
- Rust staining through a finished coat is a strong visual signal of a background problem.
- Curved and shaped work depends on the lath being formed accurately before any coat goes on.
Durability
Durability questions
Questions to raise rather than a service life to expect. No material has a universal lifespan — it depends on the assembly, the exposure and the upkeep.
- What coating does the lath carry, and what exposure was it selected for?
- Which fixings are specified, and do they suit both the lath and the background?
- How does the design keep water from reaching the lath through the finished coat?
- How is movement between different backgrounds handled behind the render?
- Is the background strong enough to carry the weight of the coats over the long term?
Upkeep
Maintenance and repair considerations
- What would rust staining or fine cracking in a rendered surface indicate?
- How much render would have to be removed to inspect the lath behind it?
- Who would assess a bulging or hollow-sounding area of rendered lath?
- How are junctions and abutments kept sound so water does not get behind the coat?
- Is a record kept of where lath was used, for reference during future works?
Installation
Installation dependencies
What the installation depends on — not how to do it. Method belongs to the manufacturer's instructions and the trades carrying out the work.
- The background has to be complete, aligned and dry enough before the lath goes on.
- Fixing pattern and lap arrangement follow the specification for the system being used.
- Tension across the face stops the lath moving under the weight of the wet coat.
- Cut ends and edges need treating in line with the specified protection.
- Coordination with beads, stops and movement joints before any rendering begins.
Documentation
Documentation to request
Ask for these in writing, for the specific product being proposed, and keep them with the project record.
- The lath type and coating being supplied, with the manufacturer's guidance on exposure.
- Fixing specifications and their compatibility with the lath and the background.
- The render or plaster system that the lath is intended to be used with.
- Details at movement joints, junctions, edges and openings.
- Any guidance on separating layers between the lath and the background behind it.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- What is the lath being fixed to, and can that background carry the weight of the coats?
- What coating does the lath carry, and what exposure was that chosen for?
- Are the fixings compatible with the lath, and with the background behind it?
- How does the design keep water away from the lath once the render is complete?
- Where does the lath bridge dissimilar backgrounds, and how is movement handled there?
- Is a separating layer specified between the background and the lath?
- What render or plaster system is being used with this particular lath?
- How would a defect behind a finished coat be identified and then investigated?
Blind spots
Commonly overlooked points
- Rust stains on a rendered wall usually come from behind the surface rather than from it.
- Fixings are easily overlooked, and a mismatch with the lath causes trouble in a damp wall.
- Lath does not strengthen a weak background; it only gives the coat something to grip.
- Once rendered over, the lath is invisible, so decisions made beforehand are hard to revisit.
- The junction where lath meets a solid background is where cracks most often appear.
What this page does not do
- Whether a background can carry rendered or plastered coats is a question for a qualified professional.
- Lath coating and fixing compatibility should be confirmed with the manufacturer for the specific exposure.
- Fire and weather performance belong to the complete wall assembly and its documentation rather than to the lath.
Related entries
Related materials
Materials from the same family, an adjacent role in the assembly, or a shared application.
Inspiration
Related Ideas Library pages
Design directions where this material commonly appears.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this material comes up.
Go deeper
Related Build Design Hub guides
Planning guidance and neutral comparisons behind the decisions on this page.
Metals in Construction
Reference entries for the metals used in building — structural and light-gauge steel, galvanised and stainless steel, aluminium, copper, zinc, lead and reinforcement.
Browse all metals entries →