Acro Props
How to hire and use Acro props safely for steel beam installation: BS EN 1065 classes, Strongboy vs needles, spreader boards, 48-hour mortar cure, CDM 2015 duties.
A 4m steel beam goes in next Tuesday. Your builder turns up with two props from his van and starts knocking out brickwork. By the afternoon the ceiling has dropped 15mm at the centre, the plaster has cracked across the room above, and the structural engineer is on his way back to assess the damage. The fix is a five-figure retrospective insurance claim and a six-week schedule slip. Acro props are cheap. Using too few of them, or the wrong size, is the single most expensive mistake on a domestic structural job.
What an Acro prop actually is
An Acro prop is an adjustable telescopic steel column used to take the load of a wall, floor, or roof while you cut an opening below it. The brand name "Acrow" became generic in the same way Hoover did for vacuum cleaners. Hire depots use the names interchangeably. The proper trade term is adjustable telescopic steel prop.
The mechanism is dead simple. An outer steel tube has a threaded collar near the top. An inner tube slides inside it with a row of pin holes drilled along its length. You set the rough length by passing a steel pin through matching holes in both tubes, then fine-tune the final height by turning the threaded collar. A welded base plate at one end and a top plate at the other provide bearing surfaces against the timber spreader boards.
You'll need props on any job that opens up a load-bearing wall: installing a steel beam for a knock-through, fitting a window or door lintel into an existing wall, removing a chimney breast, or temporarily supporting a floor while joists are sistered or replaced. Anything where structure above needs to keep standing while the wall below it gets cut.
The five sizes and what they mean
Acro props come in five standard sizes, numbered 0 through 4. The number refers to the closed length and the maximum extension, not the load capacity. As you extend a prop, its safe working load (SWL) drops, often substantially.
| Size | Closed length | Extended length | Weight | Typical use |
|---|---|---|---|---|
| Size 0 | 1.07m | 1.83m | 13-20 kg | Low ceilings, cellars, under stairs |
| Size 1 | 1.75m | 3.12m | 18-21 kg | Standard residential ceilings (2.4m) |
| Size 2 | 1.98m | 3.35m | 20-22 kg | Standard ceilings with packing or sloped floors |
| Size 3 | 2.59m | 3.96m | 22-25 kg | Tall Victorian/Edwardian ceilings, double-height spaces |
| Size 4 | 3.20m | 4.88m | 26-30 kg | Full double-height halls, stairwells |
For a typical UK residential ceiling height of 2.4m, you want Size 1 or Size 2. A Size 1 at full extension reaches 3.12m, which is enough headroom for the spreader boards top and bottom plus a small adjustment margin. Don't hire larger props than you need. The bigger the prop, the lower its SWL when extended near its limit, and the harder it is to handle on a domestic site.
BS EN 1065 classes: why hire stock is rated Class D
The current British Standard for adjustable steel props is BS EN 1065:1999, which superseded the older BS 4074:1982. Many UK suppliers still print BS 4074 on product pages out of habit. The standard you actually care about is BS EN 1065.
The standard sorts props into classes by load capacity. Two of those classes matter for domestic work:
- Class B has a variable load capacity. The SWL is calculated from a formula (68.0 / length squared) and decreases as you extend the prop. At minimum extension, a Class B prop carries well over 35kN. At full extension on a Size 2 or larger, it can fall below 17kN.
- Class D has a fixed characteristic load of 34kN regardless of extension. You get the same load capacity at any height within the prop's range.
Hire stock is almost always Class D. It's the safer specification for general use because the SWL doesn't change as the prop is set up at different heights. A 34kN capacity (about 3,400 kg per prop) is well in excess of anything a domestic residential wall imposes per propping point. For context, a typical 3m steel beam in a residential rear extension carries about 8-15kN of dead and imposed load on each end, which is comfortably within the SWL of a single Class D prop. You use multiple props because of spacing requirements and load distribution, not because each prop is anywhere near its limit.
Refuse any prop that is not stamped or labelled with a class marking. Unmarked stock is end-of-life equipment that someone has kept hiring because it still functions mechanically.
Acro vs Strongboy: the decision that costs people money
The Acro prop on its own is just a vertical column. To support a wall, you need something for the prop to push against horizontally, otherwise it would just lift the floor above. Two systems handle this: needles and Strongboys.
A needle is a horizontal timber beam (typically a hefty piece of structural timber, often 100x100mm or larger) threaded right through the wall above the opening, with an Acro prop on each end. The needle takes the wall's weight; the props transfer that weight to the floor. Needles can carry serious load. SWL per timber needle is around 1,700kg, and for full-thickness Victorian solid walls or wide-span openings, this is the only sensible option.
A Strongboy is a steel attachment that clips onto the top of an Acro prop and slides a thin (10mm) steel blade into a single mortar joint above the opening. The blade rests on the brickwork course rather than passing through the wall. This is much less invasive (you only need to rake out one mortar joint per Strongboy rather than knocking a hole through the wall) but the SWL is far lower: 340kg per Strongboy, and a maximum safe working height of 3 metres.
| Feature | Strongboy | Timber needle |
|---|---|---|
| SWL per support point | 340 kg | 1,700 kg (5x higher) |
| Max working height | 3 m | Limited only by prop size |
| Wall preparation | Rake out one mortar joint per Strongboy | Cut a hole right through the wall |
| Spacing limit | 900 mm centres maximum | Set per structural engineer's spec |
| Best for | Standard residential lintels, single-storey loadings, openings up to 3m | Wider spans, double-storey loadings, walls in poor condition |
| Eccentricity limit | Max 215mm prop centreline to cavity centreline | Set by needle length and prop spacing |
For a standard kitchen knock-through with a 2.4-3.6m steel and a single-storey roof or floor above, Strongboys at 900mm centres are usually fine and your structural engineer will say so on the drawing. For anything carrying two storeys, anything spanning more than about 3m, or any wall in old or fragile condition (lime mortar, cracked brickwork, rubble core), you want needles. Don't decide this yourself. The structural engineer's drawings should specify which approach to use, often as a sketch showing the temporary works arrangement.
How to set props up properly
The mistakes that wreck walls happen at setup, not under load.
Start with the floor. Concrete slabs are usually fine on their own. Suspended timber floors are not. The point load of a single Acro prop on a floorboard will punch straight through it, and even on top of a single joist a heavily loaded prop will crush the timber or topple sideways. Always run a softwood scaffold board (typically 100x50mm, 600mm minimum length, ideally 225mm wide and longer than that) beneath the prop's base plate, spanning at least three joists. The same applies at the top: a scaffold board between the prop's top plate and the wall or needle spreads the load and prevents the bearing surfaces from local crushing. Both top and bottom spreaders are non-negotiable.
Set the rough length first. Pull the inner tube up so the pin holes line up at roughly the right height, slot the steel pin through, and check the chain is intact (the pin should be chained to the prop so it can't get lost). Then set the prop in position and tighten the threaded collar by hand. Hand-tight is enough. Over-tightening with a bar is one of the most common errors and bends the threaded collar permanently. The prop should take the load, not push the ceiling up.
Verify vertical with a spirit level on the outside of the outer tube. The maximum allowable deviation is 1.5° from vertical. Beyond that and the prop is operating outside its designed load path; the SWL no longer applies. If you cannot get the prop vertical because of a sloped or uneven floor, pack underneath the spreader board with thin slate or hardwood shims. Never pack with timber offcuts that can compress under load.
Place the steel beam on the floor inside the proposed opening before you set up any props. If you put the props in first, you have nowhere to swing the beam into position when the time comes. Standard sequence: deliver the steel, position it loosely on the floor, then prop the wall, then cut the opening, then lift the steel onto its padstones.
For multiple props on the same opening, brace them together with horizontal timbers or scaffold tube and clamps. A single prop in isolation can twist or kick out under uneven loading. Two or more props laced together are an order of magnitude more stable.
Spacing: the 900mm rule
Industry guidance and the Strongboy SWL together set a maximum spacing of 900mm centre-to-centre between propping points along the wall. For a 3.6m steel beam, that means four pairs of props (one pair near each end of the steel and two pairs distributed across the middle). For a 4m beam, five pairs.
This is a maximum, not a target. On older walls with soft lime mortar or rubble cores, drop the spacing to 600mm. Your structural engineer should specify the actual count in the drawings. The 900mm rule is what the engineer uses as a starting point, not a substitute for engineering judgement.
For openings above about 4m, or full rear-wall removal, expect Building Control to ask for lateral stability calculations on top of the temporary works arrangement. Props alone are not the whole story when you remove a full structural wall.
The 48-hour padstone cure rule
The point of installing a steel beam is to transfer the load above onto solid bearing surfaces called padstones (concrete or engineering brick blocks bedded into the wall at each end of the steel, sized to spread the beam's point load across the masonry below). Padstones are bedded in mortar. That mortar must reach adequate strength before the beam is loaded onto it, or the padstone crushes the brickwork beneath and the beam settles.
The minimum cure time before removing props is 48 hours, and this is the single most ignored rule on residential beam installations. The convention aligns with NHBC guidance, which requires padstone mortar to achieve adequate strength before the load transfer. In cold weather (below 5°C), Portland cement mortar cures dramatically more slowly. A 48-hour cure in summer can take 96 hours or more in February. If the padstones were laid on a Friday afternoon and the temperature dropped over the weekend, leave the props in place until Tuesday.
Removing props before the padstone mortar has cured is the most common cause of structural defects on domestic beam installations. The visible symptom is fresh cracking in the wall above the beam within days of completion, and a beam that sits 5-15mm lower than it should. The fix involves jacking the beam back up, replacing the padstones, and reinstating the wall above. Budget five-figure costs for a problem that costs nothing to avoid by waiting two days.
Hire cost and what to refuse at the depot
Acro prop hire is cheap. Expect £4 – £10 at most UK hire depots. Online specialists (Lakeside Hire, SAB Supply, Acro-Props.co.uk) sit at the lower end of the range with delivery included on multi-week hires. National chains (HSS, Brandon Hire, Speedy) sit at the upper end. Strongboy attachments hire separately at a few pounds per week each.
For a typical kitchen knock-through with a 3.6m steel, you'll need 4 pairs of props (8 total) plus 4 Strongboys for two weeks. Total hire cost works out at around one to two hundred pounds. Set against the structural engineer fee and the four to five-figure builder cost for the full knock-through, prop hire is a rounding error.
When you collect or take delivery, inspect every prop:
- Base plate and top plate: not bent, not cracked. A bent plate will not bear properly against a spreader board and is a failure waiting to happen.
- Inner tube: visibly straight, no bow along its length. Sight down the tube; any visible curve means it's been overloaded in the past and the SWL is unreliable.
- Threads on the outer tube collar: not stripped, not flattened. The collar must turn freely along the full length of the thread.
- Pin holes on the inner tube: round and crisp, not deformed or oval. A deformed pin hole means the pin can slip under load.
- Pin and chain: pin present, chain intact. Lost pins get charged on return.
- Class marking: stamped or labelled clearly. Refuse unmarked stock.
Refusing damaged stock at the depot is your right and the depot will swap it without argument. Refusing it on site after delivery is harder. Inspect before signing the delivery note.
CDM 2015: who is responsible
The Construction (Design and Management) Regulations 2015 apply to every construction project in the UK, including domestic extensions. Temporary works (which includes propping) are explicitly construction work under CDM 2015. Both the contractor doing the work and the designer specifying it have legal duties.
In practice, this means three things on a domestic job:
- The structural engineer (acting as the designer) specifies the prop sizes, count, and arrangement on their drawings or in a separate temporary works sketch. This becomes the propping schedule.
- The contractor (or you, if you're managing the project directly) is responsible for setting the props up correctly per that schedule and for inspecting them daily during use.
- For larger or more complex jobs, the engineer signs off the temporary works arrangement before the wall is cut. For a standard residential beam, this sign-off is implicit in the structural drawings.
Day-to-day inspection during use is your job. Check the props each morning before work starts: collars still tight, no visible movement, spreader boards still in full contact, no cracking around the bearing points. After heavy rain or any plant movement on the floor above, check again. Re-tighten the collar by hand if it has loosened, but do not over-torque.
Always wear a hard hat when setting props or working under a propped wall. Mortar dust, brick fragments, and small pieces of plaster fall as the load transfers and as the wall above flexes. Steel-toe boots and gloves are also non-negotiable. Props can topple if knocked, and a 25kg steel column falling onto an unprotected foot is a hospital visit.
Where you'll need this
- Steels and lintels - props support the wall above the opening while the steel beam is fitted onto its padstones, then stay in place for the 48-hour cure before being struck.
These tools appear on any extension, renovation, or alteration project that involves opening up a load-bearing wall, fitting a lintel, or temporarily supporting structure during demolition. The setup principles, sizing logic, and CDM duties are identical whether you're knocking through into a kitchen, replacing a window with French doors, or removing a chimney breast on an upstairs landing.