Suspended Ceiling Grid Systems: Main Tees, Cross Tees, Hanger Wire Spacing and Levelling Procedure

Quick Answer: A standard suspended ceiling grid uses 38mm main tees at 1200mm centres, 600mm cross tees at 600mm centres, and 1.0mm hanger wire at 1200mm grid nodes — with a perimeter angle fixed at the finished ceiling height. The grid supports either mineral fibre tiles (600×600mm or 1200×600mm) or plasterboard sheets screwed to the grid. Hanger wire spacing must not exceed 1200mm in any direction, and all wires must be plumb within ±2°. Levelness tolerance is ±3mm over 3m.

Summary

Suspended ceiling grid (also called a lay-in grid or T-bar grid) is the fastest way to create a finished ceiling below an existing structure. It is ubiquitous in commercial offices, retail, and multi-unit residential. Unlike a continuous MF (metal furring) plasterboard ceiling, a grid system with lay-in tiles allows access to the service void above without removing fixed board — tiles simply lift out. This access requirement is why grid ceilings dominate in commercial buildings.

There are two main grid types used in the UK. The exposed grid (Metric Grid, sometimes called a T-bar or Armstrong grid) has visible tees below the tile face — the tee flange is exposed and forms a visual pattern. The concealed grid has tees that sit above the tile face — the tiles have slots or clips that engage the concealed tee, leaving an apparently seamless ceiling. Exposed grid is faster and cheaper; concealed grid is higher specification and more expensive.

Plasterboard can also be suspended from a grid, but this creates a fixed ceiling. In UK commercial construction, the term "grid ceiling" usually implies lay-in tiles. The MF plasterboard system (see acoustic ceiling systems) uses a similar hanger and runner structure but is fixed-board rather than lay-in tile.

Key Facts

• Main tee (MT24) — primary grid member, spans between hanger wires; 38mm wide face, 3.6m or 6m stock lengths
• Cross tee — secondary grid member; clips into main tee at 600mm or 1200mm intervals; 600mm or 1200mm length
• 600×600mm grid — standard commercial module; 38mm tee visible on both sides of tile; 600mm main tees and 600mm cross tees
• 1200×600mm grid — elongated module; 38mm main tee at 1200mm; 600mm cross tees at 600mm; tiles 1200×600mm
• Perimeter angle (PA) — L-shaped angle fixed to walls at finished ceiling level; supports the edges of the grid and tiles at the perimeter
• Hanger wire — 1.0mm or 1.6mm galvanised wire; tensioned plumb from structural soffit to main tee; maximum 1200mm apart along the tee
• Hanger fixing to concrete soffit — proprietary snap-hook or loop anchors driven or drilled into the slab; must achieve minimum pull-out load (check product data)
• Hanger fixing to steel — beam clamp or cantilever clip; clamp around the bottom flange of a steel beam
• Hanger fixing to timber — screw-in hook or eye screw into joist; minimum 40mm into solid timber
• Wire tensioning — hanger wire bent back on itself at the tee level; must be taut with no slack; slack wire allows tee movement under tile loading
• Levelness — ±3mm over any 3m run is typical tolerance; ±6mm over the full room is acceptable in standard commercial specification
• Fire requirements — mineral fibre ceiling tiles are typically Class 0 or Class A1; grid components are non-combustible steel; system may need to be specified as a fire-rated ceiling in some applications
• Tile weight — standard mineral fibre 600×600mm: approximately 2.5–3.5 kg/m²; 1200×600mm: proportionally heavier; grid must be specified for the tile weight
• Integrated services — recessed lighting, ventilation diffusers, sprinkler heads, and speakers are typically integrated into the tile module (cut or specific manufacturer tiles required)
• Acoustic tiles — NRC (Noise Reduction Coefficient) and Rw data published by tile manufacturers; grid type does not directly affect acoustic performance; tile selection determines sound absorption

Quick Reference Table

Detailed Guidance

Setting Out — Before Starting

Setting out determines whether the perimeter tiles cut symmetrically or awkwardly at the walls. A room where the grid is not set out correctly will have thin slivers of tile at one wall and over-wide tiles at the opposite wall — a visible quality failure.

Setting out procedure:

1. Measure the room width at the grid level; divide by 600mm (or 1200mm if that's the module) to find how many whole tiles fit
2. Calculate the fractional tile width: if the room is 3750mm wide and the module is 600mm, there are 6.25 tiles — the fractional tile is 0.25 × 600 = 150mm. This is too narrow and will look poor.
3. Adjust the grid centreline to make the perimeter tiles half-width larger: shift the grid by 300mm so the border tiles are 450mm wide (150/2 + 300 = 450mm). Now both sides have equal 450mm tiles — much better.
4. Repeat for both axes (length and width of the room)
5. Mark the adjusted grid centreline on the floor with a chalk line; use a laser level to transfer to the soffit

Perimeter Angle Installation

The perimeter angle is fixed to the walls at the finished ceiling level. It supports the tile edges at the wall perimeter and provides a level reference for the grid.

• Mark the ceiling height on all four walls using a laser level; the line must be perfectly level around the room
• Fix perimeter angle (typically 25×25mm L-angle, 0.5mm galvanised) with appropriate fixings for wall substrate at 400mm maximum centres
• Angle face down (the leg that tiles rest on is the downward-facing horizontal flange)
• Mitred at internal corners for a clean visual; overlapped at external corners

Hanger Wire Installation

Hanger wires are fixed to the structural soffit first, then connected to the main tees during grid installation.

• Mark hanger positions on the soffit at the main tee crossing points: every 1200mm along each main tee line
• Fix hanger anchors: powder-actuated nail (Hilti DX type) for concrete; expansion anchor for precast soffit; screw-in hook for timber joist; beam clamp for steel beam
• Drop hanger wire from each anchor; wire length = void depth + 200mm for bending
• Hanger wire minimum diameter: 1.0mm galvanised for standard commercial tiles; 1.6mm for heavier loads (acoustic board, plasterboard)

Grid Assembly and Levelling

1. Install main tees: slot first main tee through hanger wires at the offset position from one wall; support the tee on the perimeter angles at each end; the tee must reach from one perimeter angle to the opposite, with ends resting on the perimeter angle flange
2. Level first main tee: using a laser level or taut string line, adjust hanger wires so the tee is at the correct height; wrap wire back on itself around the tee flange and twist to lock — minimum 3 wraps; wire must be taut (not loose, not bent)
3. Install remaining main tees: at 600mm (or 1200mm) intervals parallel to the first; level each against the laser or string line
4. Install cross tees: clip cross tees into the main tees at 600mm centres; listen for the click as the cross tee lug engages the main tee slot; check level across the cross tees
5. Perimeter tiles: where the grid meets the perimeter angle, cut tiles to fit; cut mineral fibre with a sharp Stanley knife and straight edge; support cut tiles on the perimeter angle flange — they will not be supported by a cross tee at the wall side

Service Integration

Integrated recessed luminaires, sprinkler heads, ventilation diffusers, and other services require careful coordination with the grid:

• Recessed luminaires (LED lay-in panels or recessed downlights): position within a 600×600mm module; the fitting size must fit within the tile cutout; confirm weight with grid supplier — heavy fittings (>5kg) may need their own hanger wires independent of the ceiling grid
• Sprinkler heads: position according to the sprinkler design layout; may require the tile to be cut and a proprietary surround fitted; sprinkler pipe hangers must be independent of ceiling grid hangers
• Linear slot diffusers: replace a row of tiles with a linear diffuser module; the diffuser module must be the same width as the grid module
• Smoke detectors: typically sit within a tile module; the detector requires a cut-out in the tile and a proprietary mounting surround

Fire-Rated Grid Ceiling

A standard exposed grid ceiling is not fire-rated. If the ceiling must contribute to a fire compartment (e.g. providing 30-minute protection to a floor above in a commercial building), a tested fire-rated ceiling assembly is required.

Fire-rated grid ceilings use:

• Standard or proprietary grid components
• Specialist mineral fibre tiles tested to EN 13501 (Class A1 for non-combustibility)
• Perimeter seals and barrier details that prevent fire entering the ceiling void
• The complete assembly is tested as a unit — cannot mix and match components from different manufacturers

Examples: Armstrong Ultima+ dB (tested fire ceiling assembly); Knauf AMF Thermatex (tested). Check current product data for specific EI rating and installation requirements.

Frequently Asked Questions

What is the maximum void depth for a suspended ceiling grid?

Grid systems are typically designed for voids from 150mm to 1000mm. Below 150mm, it becomes difficult to install and adjust hanger wires and still maintain tile access. Above 1000mm, hanger wire stability becomes a concern — use bracing or thicker wire (2.0mm) for deep voids. Very deep voids (>1500mm) may require intermediate bracing to prevent grid sway.

Can I hang heavy items from a suspended ceiling grid?

Standard grid components have a load capacity of approximately 5kg per 600×600mm module. Fittings heavier than this — large LED panels, projectors, speakers — require independent hanger wires fixed to the structural soffit and passing through the ceiling void, with no load transferred to the grid. The fitting hangs from the structure, not the grid; the grid tiles are cut or adapted around it.

Do I need to pin the tiles in the grid?

In most commercial applications, tiles sit in the grid by gravity and are not pinned. This allows easy access. Pinning (using proprietary grid clips or adhesive pads) is used when the tiles are at risk of displacement — from air pressure (supply air vents nearby), from vibration (near plant rooms), or from upward pressure (positive pressure HVAC systems). Some grid systems are designed for concealed-grid clip-in access — the tile must be clipped in for the concealed appearance, but unclips for access.

Regulations & Standards

• BS EN 13964:2014 — suspended ceilings; requirements and test methods

• BS EN 13501-1:2018 — fire classification: reaction to fire for products and assemblies

• Building Regulations Approved Document B — where fire-rated ceiling assemblies are required

• Building Regulations Approved Document E — acoustic performance of ceiling assemblies in residential applications

• CIBSE Guide F — energy efficiency in buildings; reflective ceilings and daylighting contribution

• Armstrong Ceilings UK — grid system technical data, installation guides, fire-rated ceiling assemblies

• British Gypsum MF Ceiling Guide — MF plasterboard ceiling system data

• Knauf AMF — mineral fibre ceiling tile and grid data

• acoustic ceiling systems — resilient and independent plasterboard ceilings for acoustic performance

• building regs part b fire lining — fire-rated ceiling requirements

• building regs part e acoustic — acoustic performance for ceilings between dwellings

• drylining around services — integrating services into ceiling voids