Structural openings and RSJ lintels for bifold doors
Knocking through a rear wall and fitting an RSJ structural steel lintel for a 3.0m to 5.0m bifold door costs €1,800 to €3,500 in Ireland. Because folding doors glide inside a precision aluminium head track, the steel beam above must be sized by a structural engineer for near-zero deflection (maximum 1.5mm to 2mm under full load).
What your builder and structural engineer need to know about steel deflection, fitting tolerances, and Building Regulations Part L and Part K.
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Calculate your bifold door supply & fit cost
Pick the number of folding panels, your frame and glazing specification, and whether the structural opening is ready or needs an RSJ knock-through.
Estimated supply & fit guide
€6,600 to €11,100
Bifold door: €4,800–€7,600 · RSJ knock-through: €1,800 to €3,500
Combines a 4-panel bifold door (3.2m–4.0m opening) in thermally broken double-glazed aluminium (€4,800 to €7,600) with knocking out a wider rear wall opening, fitting a structural RSJ steel lintel on concrete padstones, and plastering the internal reveals (€1,800 to €3,500).
Send this for a written quoteWhy a standard concrete lintel will jam a bifold door
A standard prestressed concrete window lintel or light steel box lintel can sag by 5mm to 10mm in the middle of a 4-metre span once the roof and first-floor blockwork settle on it. Even a 4mm dip in the centre of a bifold head track will pinch the top guide rollers and stop the doors folding past the midpoint.
- Bottom-rolling vs top-hung systems: Always specify a bottom-rolling bifold system where 100% of the dead weight of the glass panels sits on the concrete floor foundation, leaving the overhead RSJ steel beam to carry only the masonry wall above.
- Deflection limit (L/500 or max 2mm): Ask your engineer to size the Universal Beam (UB) or Parallel Flange Channel (PFC) so live and dead load deflection after installation does not exceed 1.5mm to 2.0mm across the entire clear span.
- Fitting tolerance: Builders should form the structural masonry opening 10mm to 15mm wider and higher than the outer frame dimension so the fitters can pack the jambs dead plumb and level with a laser.
What a rear-wall knock-through costs step by step
| Stage of structural knock-through | What is involved | Typical cost range |
|---|---|---|
| 1. Structural engineer calc & sign-off | Sizing the RSJ steel beam & padstones for Opinion on Compliance | €350 to €650 |
| 2. Acrow propping & needle beams | Supporting first-floor joists and outer/inner leaf blockwork | Included in builder fee |
| 3. Supply & lift RSJ steel + padstones | Galvanized or primed steel beam bedded on engineering brick/concrete padstones | €850 to €1,700 |
| 4. Cut out wall below, DPC & plaster reveals | Cavity closer, insulated plasterboard reveals, skim & rubble skip | €600 to €1,150 |
| Total knock-through work (before door) | Ready for the bifold surveyor's final laser measure | €1,800 to €3,500 |
Part L, Part K and planning permission
- Part L (Conservation of Fuel and Energy): Replacement external doors in existing dwellings must achieve a whole-door U-value of 1.4 W/m²K or better. If your new bifold opening pushes total glazing above 25% of the floor area in an extension, your BER assessor may ask for 0.9 W/m²K triple-glazed bifolds to balance the heat-loss calculation.
- Part K & EN 12600 Safety Glass: Because bifold panes run down to floor level, every pane inside and out must be toughened or laminated safety glass kite-marked to EN 12150 / EN 12600.
- Planning permission: Widening a ground-floor rear patio opening facing your back garden is normally exempted development under Section 4(1)(h) of the Planning and Development Act 2000, unless the house is a Protected Structure or in an Architectural Conservation Area.
Real examples and installation detail
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Questions homeowners ask
When does the bifold company take final manufacturing measurements?
Only after the RSJ steel beam is installed, the props are removed, and the masonry opening is formed (or after accurate steel-to-datum levels are signed off by your builder). Manufacturing takes 3 to 5 weeks from final sign-off.
Do I need an Opinion on Compliance when widening a back wall for bifold doors?
Yes. Any structural alteration that removes load-bearing masonry and inserts a steel beam should be designed and inspected by a Chartered Engineer who issues a Certificate / Opinion on Compliance with Building Regulations for when you ever sell the house.
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Planning bifold doors for an extension or patio knock-through?
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