Specifying MCM Flexible Stone Veneer: What Actually Belongs in the Spec
Most specifications for flexible stone veneer are copied from a ceramic tile spec and signed off. Then the panel arrives at 3 mm instead of the 6 mm that was approved. The adhesive is the wrong class. The primer sealed the wall instead of profiling it. The spec is where these projects are won or lost — and that usually happens before anyone sets foot on site.
This is a field note for architects, spec writers, and the poor contractor who has to make the submittal match the drawing. No brand names. Just the clauses that keep the installed product honest.
Define the family, not a single SKU
Write the performance, not the catalogue number. The default material here is MCM — modified clay material — a thin veneer backed with fiberglass. State the thickness band and the weight, and let the supplier meet it.
For reference, the working range is 2–9 mm thick and 3.0–10.5 kg/m². Standard sheets come at 600 × 1200 mm and 1200 × 2400 mm. Pick the thickness from the substrate and the movement, not from whatever the supplier happens to stock.
Thin (2–3 mm) belongs on flat, stable interior walls. Mid (4–6 mm) covers most work. Anything curved, high-traffic, or exposed earns the thicker end. Permanence, obviously, comes from the substrate prep — not from a thicker skin.
The numbers that should sit in every spec
| Property | Value to state | Why it matters |
|---|---|---|
| Thickness | 2–9 mm (name the project band) | Drives stiffness and impact resistance |
| Panel size | 600 × 1200 / 1200 × 2400 mm | Joint count, waste, handling |
| Weight | 3.0–10.5 kg/m² | Load on old substrates, shipping |
| Fire | A2 class (GB 8624, factory tested) | Defensible; can be specified to meet A2-s1,d0 |
| Adhesive | C2 (S1 default; S2 where movement is high) | Bond and slack for thermal move |
| Substrate | Primed per manufacturer — prime to profile, not seal | Mechanical key, not just suction |
Substrate and primer — the part everyone under-specs
A primer that cures to a glossy film breaks the mechanical key. The veneer then hangs on suction alone, and suction is the first thing to let go on a hot façade. Say “prime to open the surface; verify by the water-bead test” rather than naming a product. If water beads on the primed wall, the primer failed its only job.
Old plaster, painted block, and gypsum board each need a different primer. Treat them as three specs, not one. The full prep sequence is worth a read before you write the clause: MCM flexible stone substrate preparation guide.
Adhesive class, and why the tub is a trap
C2 S1 is the default for walls. The pre-mixed adhesive from the tub is fine for dry interior only. It is not for exterior. Not for wet areas. Not for a full-wrap curve. Spec writers who leave the adhesive open let the installer reach for whatever is on the truck.
Where the substrate moves — plywood over a timber frame, a heated floor, external cladding on light steel — step up to S2. Name the standard (ISO 13007 / EN 12004 framing), not a brand, so any compliant product qualifies. Detail lives here: flexible stone veneer adhesive guide.
Movement joints and edges
Flexible stone is still a brittle skin over a flexible backing. A continuous run past 8–10 m, and every change of plane, needs a movement joint. Do not let the builder caulk it shut because “it looks cleaner.” It looks cleaner for one winter.
External corners: pre-fold the panel or use a bonded trim. Field-folding thin veneer on a cold substrate cracks it. A short note in the spec saves a redo on the fifth floor.
Fire and code wording
Write the test standard, not a marketing claim. “A2 class (GB 8624, factory tested)” is defensible and specific. Do not write “certified to A2” — certification language invites a dispute at inspection, and the inspector will win. Frame it as a performance requirement and move on.
For the compliance side across regions, this is the deeper read: MCM flexible stone A2 fire-rating and safety compliance.
The value-engineering trap
Approved submittal at 6 mm. Delivered at 3 mm. Everyone shrugs; the difference is “just thickness.” It is not just thickness. Once the thin panel is on the wall, you cannot retrofit the stiffness back in.
Lock it three ways: reference the standard, state the acceptable tolerance, and require lot verification against the approved submittal before installation starts. This is the single most common post-award failure on flexible stone jobs, and it is the easiest to prevent on paper.
Logistics line
State the origin plainly: FOB Qinzhou. Give lead time as a range, not a promise, and let procurement plan around it. The board ships; the schedule should too.
A minimal spec section (MasterFormat 09 29 00 style)
- Product: MCM flexible stone veneer
- Thickness band: [state project value within 2–9 mm]
- Sheet size: 600 × 1200 mm or 1200 × 2400 mm
- Weight: maximum 3.0–10.5 kg/m²
- Fire: A2 class (GB 8624, factory tested)
- Adhesive: C2 S1 (S2 where movement is noted)
- Substrate: prime per manufacturer — profile, not seal
- Movement joints: per drawing, every plane change and 8–10 m run
- Submittal: full verification against approved lot
Background for clients and buyers sits here: MCM flexible stone buyers guide. And if the material itself is new to the team, start with what MCM flexible stone is, then the installation guide and the certification standards guide.
Frequently Asked Questions
What thickness of MCM flexible stone should I specify?
Match thickness to the substrate and exposure, not to the easiest option to source. Flat stable interior walls take 2–3 mm. Most walls sit at 4–6 mm. Curved, high-traffic, or exterior work earns the thicker end of the 2–9 mm range. State a band in the spec rather than a single number so the supplier can meet performance.
Can I use pre-mixed tub adhesive for flexible stone veneer?
Only for dry, interior walls. Pre-mixed adhesives are not rated for exterior use, wet areas, or full-wrap curves. For walls in general, specify C2 S1; step up to C2 S2 where the substrate moves, such as over timber frames, heated floors, or light-steel cladding.
Does flexible stone veneer need movement joints?
Yes. It is a brittle skin over a flexible backing, so treat any continuous run beyond 8–10 m and every change of plane with a movement joint. Sealing those joints for appearance removes the slack the system relies on and leads to cracking.
How should fire performance be written in the spec?
State the test standard, not a marketing claim: “A2 class (GB 8624, factory tested).” Avoid wording like “certified to A2,” which creates a compliance dispute at inspection. Frame fire as a performance requirement tied to a named standard.
How do I stop a thinner panel being substituted after award?
Reference the product standard, state the acceptable thickness tolerance, and require lot verification against the approved submittal before installation begins. Once a thinner panel is fixed to the wall, the lost stiffness cannot be added back.
