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A deflection head (also called a movement head, or slotted head track) is the detail at the top of an infill SFS wall that allows the primary structure to move without cracking the wall. It's one of the most important details in SFS construction, and one of the most commonly misunderstood. This guide explains what deflection heads do, how much deflection allowance you need, and what happens if you get it wrong.

What is a deflection head?

A deflection head is the detail at the top of an infill SFS wall that allows the primary structure (the floor above) to deflect vertically under load, without transferring that movement into the SFS wall. It's achieved using a slotted head track, which allows the track to slide vertically relative to the fixed structural element above.

Without a deflection head, the infill wall would be rigidly connected at both top and bottom. When the primary structure deflected, the wall would be either crushed (if the deflection were downward) or pulled apart (if the deflection were upward). Either scenario would cause cracking, failure of board joints, and potential loss of fire and acoustic performance.

The deflection head allows the wall to accommodate the movement without damage.

Why infill walls need them

Infill SFS walls are built between structural elements — typically the floor slab above and the floor slab below. The primary structure (slabs, beams, columns) deflects under load. Deflection values are typically:

• Short-span domestic floors — 5–10mm
• Commercial floors — 10–20mm
• Long-span floors — 20–30mm or more
• Post-tensioned slabs — 20–40mm or more

If the infill wall were rigidly connected to the structure above and below, the wall would have to compress or stretch to accommodate that deflection. Board-lined walls can't accommodate more than a few millimetres of movement before cracks appear. So the deflection head is required to allow the movement without damage.

How a slotted head track works

A slotted head track is a standard SFS track with vertical slots cut in the web (the part of the track that fixes to the soffit above). The slots allow the track to slide vertically as the structure deflects.

The track is fixed to the slab above using anchors through the slots. The studs are fixed to the track using screws. As the slab deflects downward, the track slides down through the anchors, and the studs move with it. The wall compresses by the amount of the deflection, but the boards don't crack because the movement is distributed through the slotted track.

Inside the track, there's usually a deflection gap — an empty space of 20–30mm between the top of the studs and the soffit above. This gap allows further movement after the track has slid to its limit.

Components of a deflection head

• Slotted head track — standard track with slots cut in the web
• Fixed to slab above — anchors through the slots
• Studs fixed to track — but not fixed to the slab above
• Deflection gap — 20–30mm empty space above the studs
• Flexible sealant — often applied between the track and the soffit to maintain fire and acoustic performance

How much deflection allowance?

Deflection allowance is the amount of movement the deflection head is designed to accommodate. It's typically specified by the structural engineer and depends on:

• The primary structure — concrete slabs deflect differently from steel beams
• The span — longer spans deflect more
• The load — heavier loads cause more deflection
• The building type — commercial floors typically have higher deflection than domestic
• Building Regulations — often specify minimum deflection values

The deflection allowance is specified in the project specification or by the structural engineer. It's not something the SFS contractor should guess at.

Installation best practice

Correct track selection

Use a slotted head track, not a plain head track. Some manufacturers supply pre-slotted tracks; others require you to cut slots on site. Follow the manufacturer's instructions.

Correct fixing pattern

Fix the track through the slots, not through solid parts of the web. The anchors should be positioned so the track can slide through them. Don't over-tighten — the track should be able to move.

Correct deflection gap

Leave the specified deflection gap between the top of the studs and the soffit above. Don't fill this gap with rigid material (board, plaster, etc) — it must be free to close.

Flexible sealant

Apply a flexible fire-resistant sealant between the track and the soffit. This maintains fire and acoustic performance while allowing movement. Use an intumescent or acoustic sealant specified for the application.

Board detail at head

Boards should not be fixed to the head track — they should be free to slide with the track. Typically the board finishes just below the track, with the gap sealed with tape or sealant.

Corner detail

At wall junctions with other elements (columns, perpendicular walls), the deflection head must be continuous. Don't leave a gap at corners.

Why loadbearing walls don't use them

Loadbearing SFS walls don't use deflection heads because the wall is part of the load path. The wall carries load from above and must be rigidly connected to the structure above and below to transfer that load safely.

If a loadbearing wall had a slotted head track, the wall would sink under load, transferring loads elsewhere or collapsing. The deflection of the primary structure is accommodated within the wall's own structural design, not by allowing movement at the head.

Loadbearing walls use a plain head track, fixed rigidly to the structure above, with the load transferred through the wall to the base track and into the structure below.

Common mistakes

Using plain head track on infill

This is the most common deflection head mistake. Without the slots, the wall can't accommodate movement and will crack as the structure deflects.

Insufficient deflection allowance

Using 10mm allowance where 20mm is needed. The wall will crack when the structure deflects more than 10mm.

Filling the deflection gap

Some contractors fill the gap with plaster, board, or rigid sealant to "finish" the detail. This defeats the purpose — the gap must remain free to close.

Over-tightening anchors

Over-tightening anchors through the slots can compress the track and prevent it from sliding. Torque to the manufacturer's specification, not tighter.

Boards fixed to head track

Boards should not be fixed to the slotted head track — they should finish below it, free to move. Boards fixed to the track will crack as the track slides.

Wrong sealant

Rigid sealant (cement-based, hard-setting fillers) will crack as the track moves. Use flexible fire-resistant sealant specified for the application.

Missing at corners

The deflection head must run continuously around corners, not stop at the corner. Interrupted deflection heads at corners crack.

Summary

Deflection heads are essential for infill SFS walls. They allow the primary structure to deflect under load without cracking the wall. They're achieved using a slotted head track with a deflection gap and flexible sealant. The deflection allowance is specified by the structural engineer and depends on the project.

Loadbearing SFS walls don't use deflection heads because the wall is part of the load path. Never use a slotted head track on a loadbearing wall, and never use a plain head track on an infill wall.