Metal inlay specification: strips and movement

At a glance

Inches are the sources’ units and the millimetre figures are conversions, rounded. The US sources describe US practice, not UK standards.

The six decisions to write down

A metal inlay is a strip let into a groove. Whatever the metal, six decisions fix whether it comes out flush, stays put and survives finishing. Each belongs on the drawing or the schedule, not in the maker’s head.

Our metal inlay pages describe the channel as cut fractionally tight, the metal pressed home and bonded, then levelled and finished flush with the surface, and say strips are typically 1 to 3 mm wide. They give no groove depth, strip thickness or tolerance, and no such figure was found in any trade document read for this sheet. The bracketed values in the wording below are examples for the partner workshop to replace.

Three trades, three sets of sizes

Strips are published by trades that use them differently, and their numbers do not transfer. Do not borrow one trade’s size for another.

• Furniture and panelling stringing: 1 to 3 mm wide, flush with the surface (our metal inlay pages). A fine line for definition, set into a stable ground.
• A metal strip routed into a timber floor on site: the National Wood Flooring Association’s publication B300 says it is preferable to use ¼ in (about 6.4 mm) thick nonferrous metal, to use a router bit the same width as the strip, and to leave the metal about 1/32 in (0.8 mm) below the surface so the wood can be sanded down to it. It also says to flatten the floor first so the router cuts a consistent depth.
• Divider strips in terrazzo: the National Terrazzo & Mosaic Association guide gives widths from 18 gauge to ½ in (12.7 mm), with 16 gauge or 1/8 in (3.2 mm) standard. Dividers set over joints in the base follow the base; elsewhere in epoxy terrazzo they are strictly decorative and do not work as levelling devices or crack preventers. Gauge sizes are not converted because the thickness of a gauge depends on the metal and the gauge system.

Which metal, and how much it moves

Metal. Brass is the default on our pages, with bronze, nickel silver and stainless steel offered for other colours. For a timber floor that will be sanded, NWFA says not to use ferrous metals such as steel and stainless steel, because of the risk of sparks during sanding, and names brass, copper and aluminium as the nonferrous options, preferably with nails of the same metal. For terrazzo, the NTMA guide lists zinc and brass for cement systems, and zinc, brass, plastic and aluminium for epoxy. Where brass meets another metal, or sits in stone that is washed or damp, see galvanic corrosion. For polish and wear-through on a plated strip, see plating and coating thickness: a strip that will be sanded or re-polished should be solid metal, not a thin plated layer.

Movement. NWFA puts the principle simply: wood expands and contracts with humidity, while stone and metal react only to temperature changes. A brass line moves little, so the risk is the timber around it. Using the coefficient for brass from the thermal expansion sheet and a 20 K swing (that sheet’s assumption, not a rule):

Over 300 mm, plain-sawn oak across the grain can move about 47 times as much with humidity as that brass line does with temperature (thermal expansion sheet), which is why a metal line across a wide solid timber top is broken into short lengths or laid along the grain. The timber arithmetic and the rule for which way a line may run are in inlays in joinery.

Flush, sanded and finished

Metal and wood sand differently, so the order of work matters. NWFA’s guidance for floors is a useful checklist even where the product is furniture:

• Test the bond. Do a test installation and sanding to be sure the adhesive keeps the metal and the wood together through sanding. Two-part epoxy and moisture-cure urethane are the adhesives it names as commonly used.
• Heat. Metal dulls abrasive and can leave uneven sanding patterns, and overheating metal can burn the wood next to it or expand the strip and break the bond. Sand in short bursts in different areas and let the metal cool.
• Level. A strip left slightly below the surface (about 0.8 mm) is better than one proud of it; if metal stands proud it is cut down with a router rather than sanded flush.
• Clean up. Vacuum the filings and clean the surface with a compatible solvent before finishing, and check the finish maker’s advice on solvents.
• Surface of the metal. Bright, brushed, darkened or patinated: decide before finishing. See lacquered or unlacquered and patination.

For a floor, see also borders and inlays in wood floors and floor transitions and movement joints.

Metal inlay strip [brass, CZ[ ]] [2] mm wide by [ ] mm thick, solid through its thickness, let flush into a groove [ ] mm deep cut into [solid timber / the core of a veneered board], bonded with [an adhesive approved in writing by the workshop] [and pinned at [ ] mm centres]. Finished [flush with the finished surface / [ ] mm below the surface before finishing]. Width tolerance plus or minus [ ] mm; level tolerance plus or minus [ ] mm. Runs across the grain of solid timber not longer than [ ] mm, or broken by [ ] mm joints. Method statement and an approved sample in the project timber and metal before production.

Nonferrous metal only [brass / copper / aluminium], no steel or stainless steel. Strip [6.4] mm thick, set in a groove cut with a router bit the width of the strip, bonded with [two-part epoxy] and pinned with nails of the same metal at [230 to 255] mm centres. Strip set [0.8] mm below the wood surface before sanding. Bond to be proved on a sample through sanding before the floor is laid. Metal to be sanded in short bursts to avoid overheating.

Divider strips [brass / zinc] [3.2] mm wide, of a metal approved by the terrazzo system maker for the [cement / epoxy] matrix, set over joints in the base as shown on drawing [ ] and elsewhere as the pattern drawing. Strip supplier to confirm the metal is suitable for the matrix and the cleaning regime.

Standards and sources

• NWFA Technical Publication No. B300, Ornamental Floors: Design & Installation, 3rd edition (2015): pages 17 to 18 and 21; US practice.
• National Terrazzo & Mosaic Association, Terrazzo Systems Reference Guide (2026): system table, dividers; US practice.
• Library sheet: thermal expansion of metals (TL-MET-23): brass coefficient.
• Our metal inlay and brass inlay strips pages: 1 to 3 mm stringing.

Questions we are asked

What size are brass inlay strips?

Our pages describe decorative stringing as typically 1 to 3 mm wide, set flush. For strips routed into a sanded timber floor, the US National Wood Flooring Association prefers nonferrous metal about 1/4 in (6.4 mm) thick. US terrazzo divider strips are standard at 1/8 in (3.2 mm). Each trade's sizes belong to that trade.

How deep should the groove be?

No trade document read for this sheet gives a groove depth for decorative inlay. For floor strips NWFA gives a strip thickness and says to leave the metal about 0.8 mm below the surface before sanding. Ask the partner workshop to state the groove depth and approve a sample.

Can I use stainless steel for an inlay in a timber floor?

NWFA says not to use steel or stainless steel in a floor that will be sanded, because of the risk of sparks. Brass, copper and aluminium are the nonferrous metals it names. For a piece of furniture that is not sanded in place, ask the workshop.

Does a brass inlay move?

Slightly, with temperature: about 0.12 mm over 300 mm in a 20 K swing. The bigger risk is the timber, which moves with humidity, so long metal lines across a wide solid timber top are broken into short lengths or run along the grain.

Should the strip be solid or plated?

If it will be sanded, polished or re-polished, solid metal gives something to work with. A thin plated layer can wear or polish through: see the plating and coating thickness sheet.