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Doors and windows interrupt the continuous run of an SFS wall — and every opening needs extra framing to keep the wall structurally sound, fire-rated, acoustic, and finish-ready. Getting the opening framing right is one of the most important skills in SFS construction. This guide explains how to frame every opening correctly.

Why openings need extra framing

Every opening in an SFS wall creates a discontinuity in the normal stud layout. Instead of a continuous run of vertical studs at 600mm centres, you have:

• A gap in the framing where the opening is
• Edge studs at each side of the opening (which take the concentrated loads from above)
• A lintel across the top (which carries the load from above the opening)
• A sill across the bottom (for windows)

Without this extra framing, the wall would fail at the opening. The load from above would deflect the lintel excessively, causing cracks and board failures. The edge of the opening would be unsupported, leading to board cracking and possible failure.

Openings also interrupt the fire separation and the acoustic seal of the wall. The framing and detailing around openings must maintain these performance characteristics.

Trimmer studs

Trimmer studs (also called jamb studs or edge studs) are the vertical studs at the sides of every opening. Their purpose is to:

• Carry the load from the lintel above the opening down to the base track and floor
• Provide a rigid edge for the opening (for door frames, window frames, or reveal boards)
• Provide a fixing point for the opening trim or frame

Single vs double trimmer studs

The number of trimmer studs depends on the opening size and the load above:

• Small openings (up to 1m wide, non-loadbearing) — single trimmer stud each side
• Standard openings (doors, windows up to 1.5m) — double trimmer studs each side is typical
• Large openings (over 1.5m) — double trimmer studs, sometimes triple, depending on load
• Loadbearing walls — engineer specifies the trimmer configuration, usually double or larger

How to install trimmer studs

1. Cut the trimmer studs to length — same as the wall studs (full height, from base track to head track)
2. Position them at the edge of the opening
3. Fix them together using tek screws through the flanges
4. Fix them to the base track and head track using 2 tek screws each end
5. Check that they're plumb and that the opening size is correct

Lintels (head detail)

The lintel is the horizontal member across the top of the opening. Its purpose is to:

• Carry the load from above the opening (the wall, cladding, or loads from upper floors) down to the trimmer studs
• Provide a rigid head for the opening (for door frames, window frames, or reveal boards)

Lintel options

• Track section — the same track as the wall base/head tracks, cut to the opening width plus returns to the trimmer studs. Adequate for small openings and non-loadbearing walls.
• Double track section — two track sections, back to back, for stronger lintels. Used for standard openings in non-loadbearing walls.
• Stud section — a stud laid horizontally, fixed to the trimmer studs. Stronger than track, used for larger openings.
• Structural lintel — a steel or concrete lintel, specified by the engineer for loadbearing walls or large spans.

How to install a lintel

1. Cut the lintel to length — the opening width plus the width of the trimmer studs on each side
2. Position the lintel across the top of the opening, with its flanges fitting over the trimmer studs
3. Fix the lintel to the trimmer studs with tek screws — at least 4 fixings per end for standard openings
4. Check the lintel is level and at the correct height
5. Continue the head track above the lintel to maintain the continuous head detail (for the wall)

Sills

The sill is the horizontal member across the bottom of a window opening. For doors, there's no sill (the door goes to floor level).

Sill options

• Track section — same track as base track, cut to the sill width plus returns
• Stud section — for wider sills or larger window loads
• Structural sill — for very wide windows or heavy windows

How to install a sill

1. Cut the sill to length — opening width plus trimmer stud widths
2. Position the sill at the correct height (the sill height specified for the window)
3. Fix the sill to the trimmer studs with tek screws
4. Check the sill is level

Door openings

Door openings are the most common type of opening. Standard details:

Internal doors (non-loadbearing)

• Opening width: door width + 10mm each side (for frame and clearance)
• Opening height: door height + 40mm (for frame and clearance)
• Single or double trimmer studs each side
• Single track section lintel across the top
• No sill — the door goes to floor level
• Base track runs through the door opening (no gap)

External doors (in SFS external walls)

• Opening width: door width + 10mm each side
• Opening height: door height + 40mm
• Double trimmer studs each side (for weatherproofing and structural rigidity)
• Lintel appropriate for the load above (usually double track section)
• Threshold detail to ensure weatherproofing
• Cavity barrier detail around the door frame (fire-rated walls)

Window openings

Window openings are similar to doors but with a sill. Standard details:

Standard windows

• Opening width: window width + 10mm each side
• Opening height: window height + 10mm top and bottom
• Single or double trimmer studs each side
• Track section lintel at the top
• Track section sill at the bottom (at the sill height)
• Base track runs continuously below the window (the sill is separate)
• Head track runs continuously above the window (the lintel is in addition)

Window details

• Sill height — typically 900mm above floor for a standard window
• Head height — typically 2100mm above floor for a standard window (matching the door head)
• Cavity tray — for external windows, a cavity tray may be required above the window to divert water
• Fire-stop — for fire-rated walls, fire-stop the perimeter of the window frame

Large openings

Large openings (over 1.5m wide, or over 2.4m high) need extra framing:

• Triple trimmer studs each side, or double trimmer studs with an additional structural post
• Structural lintel — steel beam or concrete lintel specified by the engineer
• Deflection heads may be needed above the lintel, depending on the load above
• Cavity barriers at the top and bottom of the opening for fire-rated walls
• Bearing details — the lintel must bear onto the trimmer studs correctly, with adequate bearing length

Fire and acoustic detailing

Fire

Openings in fire-rated walls interrupt the fire separation. To maintain the rating:

• Fire-rated board around the opening, matching the wall specification
• Fire-rated sealant between the window/door frame and the SFS framing
• Cavity barriers around the opening if the cavity is continuous (see Cavity Barriers article)
• Fire-rated window and door frames (or a certified fire-rated assembly)
• Tested opening detail for the wall system

Acoustic

Openings in acoustic-rated walls interrupt the acoustic seal. To maintain performance:

• Acoustic sealant around the window/door frame
• Acoustic-rated windows and doors (or acoustic-rated frames and seals)
• Acoustic insulation packed into any gaps between frame and SFS framing
• Tested opening detail for the wall system

Common mistakes

Single trimmer studs on standard openings

Single trimmer studs are only adequate for very small openings. Standard doors and windows need double trimmer studs.

Trimmer studs not full height

Trimmer studs must run from base track to head track, not just from the opening to the base. Otherwise, the load from above the opening isn't transferred correctly.

Lintel not supported properly

The lintel must bear onto the trimmer studs with adequate bearing length and fixing. Lintels floating above or bearing only partially fail under load.

No sill (or wrong sill detail)

Windows need a sill at the correct height. Doors don't. Confusing the two leads to wrong opening sizes and poor finish.

No cavity barriers around fire-rated openings

Fire can bypass the wall through the cavity around the opening. Cavity barriers are required at openings in fire-rated walls.

No fire or acoustic sealant around frames

Gaps around window and door frames allow fire and sound through. Always seal with the appropriate sealant.

Opening cut too small or too large

Openings must be the right size for the window or door frame plus clearance. Cutting the SFS opening too small requires rework; too large leaves gaps that are hard to seal.

No allowance for frame fitting

The opening should be slightly larger than the frame to allow for fitting, adjustment, and packers. Standard is 10mm each side.

Summary

Opening framing in SFS walls requires trimmer studs (edge studs) on each side, a lintel (head detail) at the top, and (for windows) a sill at the bottom. Trimmer studs should be full height and usually double on standard openings. Lintels carry the load from above the opening. Sills support the window and provide a level base.

Fire-rated and acoustic-rated walls require additional detailing: fire-rated board, cavity barriers, and appropriate sealants around the openings. Large openings require engineering input for lintel size and detail.

Get the framing right and every opening performs as it should. Get it wrong and you have structural problems, fire failures, acoustic failures, and finish issues — usually after the project is finished.