Cutting an Opening in a Masonry Wall

The Extra Metre and a Half Is the Whole Job

A rear elevation with a 2.1 m patio door in it, and a drawing that wants 3.6 m. On site that is one line moved 750 mm each way, which sounds like a morning with a disc cutter, and the quotes written on that basis are the ones that end with a prop foot through a ceiling. Above the line sit two and a half metres of cavity wall, a floor landing on the inner leaf, and an existing lintel with brickwork that has been arching quietly over it since the house was built, thrusting its weight sideways into abutments that stop holding the instant they are cut into.

The work splits into two load questions with two different answers. While the wall is open, everything above the line is standing on a row of needles with a prop each side of each one. Once the permanent support is bedded, the inner leaf and the floor stand on a beam, and the outer leaf stands on a steel angle bolted to that beam — a separate member, sized against a separate load, and the item most often missing from a steel schedule that otherwise looks complete.

Answer them in that order and only in that order. The temporary case is not a smaller version of the permanent one. It has its own load path, its own bearings, its own factor of safety and a design life measured in days. Sizing the beam first and then assuming the props will be comfortable because the beam is comfortable is the exact reasoning that puts a needle through a bed joint that cannot hold it.

Read the Wall Before Drawing the Prop Line

Start by establishing what the wall actually is, because every number downstream depends on it and the elevation drawing will not tell you. Take a brick out at the proposed head, or drill a 12 mm hole and put a borescope through, and settle three things by looking: how many leaves there are, whether the cavity is clear or filled, and what the inner leaf is made of. Dense concrete blockwork, aerated blockwork and common brick sit within a factor of two of each other on unit weight, and that factor lands straight on the prop load with nothing to soften it.

Joist direction is worth more than any other single fact and cannot be read off a floorplan. Lift a board on the floor above and look. Joists landing on the inner leaf put a floor line load on top of the masonry line load, and on a domestic wall with a reasonable span the floor share often exceeds the brickwork share — which surprises people who expect masonry to dominate because masonry is what they can see. Joists running parallel are not an exemption either: a trimmer at a stair or a chimney breast frequently lands on the wall anyway, and hangers rotted out of a wall plate have a habit of turning up in the same bay.

Then find the existing lintel and work out what it has been doing. Rake a perp joint above each end of the old opening and look for the bearing. Masonry over a long-standing opening usually arches, shedding load sideways into the abutments, and the first needle hole cut into an abutment removes exactly the material the arch was thrusting against. That is why the first needle position is a structural decision rather than a setting-out convenience, and why the temporary works arithmetic on this site takes the full rectangle above the opening instead of the triangle an arch would allow.

The paperwork is not separate from the survey; it comes out of it. In the UK, needling and propping is temporary works under BS 5975, Code of Practice for Temporary Works Procedures and the Permissible Stress Design of Falsework, which means a brief, a design, a check and a named person before load is transferred. Where the stock predates the ban, disturbing the fabric triggers the Control of Asbestos Regulations 2012 and a refurbishment and demolition survey to the approach in HSE HSG264, Asbestos: The Survey Guide. In the United States the demolition standard, OSHA 29 CFR 1926 Subpart T, requires a written engineering survey by a competent person before structural removal begins, and an occupied house is precisely the situation it was written for.

The Prop Is the Easy Half

What the prop stands on decides more jobs than what the prop is. A prop delivers several tonnes through a foot plate barely wider than a hand, and almost no floor takes that undiluted. On a ground-bearing slab the sole plate spreads it into something with a defensible allowable pressure; on a suspended timber floor it has to spread across more than one joist, and the joists themselves then need checking for a point load nobody designed them for. The calculator above returns a required bearing area, but the allowable pressure that produces it has to come from the temporary works designer looking at what is under that specific prop, not from a figure someone remembered.

A prop's safe working load is a function of its extension, and the fall is steep. BS EN 1065, Adjustable Telescopic Steel Props — Product Specifications, Design and Assessment by Calculation and Tests, classifies props precisely because a prop near the top of its range is a different member from the same prop half-closed. Nothing on this page settles it and no rule of thumb does either: read the figure off the chart for the make and model in front of you, at the extension it is actually set to. Then keep the prop plumb, pin it with the proper pin rather than a nail or a bolt found in the van, and re-check plumb after the first cut — a prop that has gone out of true has lost capacity nobody has recalculated and nobody can see.

The failure that hurts people is not vertical. A needled wall with its bottom taken out is a free-standing panel of masonry restrained only by whatever is left at its ends and by the needles themselves, and it has no more tensile strength than the mortar in its beds. Wind through a newly open rear elevation, a scaffold board leaned against it, or a lorry passing on a suspended floor is all it takes. Restraint out of plane is a separate item in the scheme with its own raking props or its own bracing, and it is the part that gets left off when the vertical numbers look reassuring.

Cutting It Out Without Shaking It Down

Nothing about this cut is a demolition cut. Percussion loosens the beds either side of where you are working, and mortar in a wall old enough to be worth widening has very little to give up before it stops holding the courses above onto the needles. Stitch-drill the corners, run a diamond blade between them, and take the last of the material out by hand so that the course you are supporting is left bearing on brickwork you have not disturbed. Where the outer leaf is fair-faced and going to be seen, the same discipline is also the only way to leave a reveal worth pointing.

Silica is the other reason the method matters. Dry cutting brick and block with a handheld saw produces respirable crystalline silica far above any exposure limit within minutes, and OSHA's construction standard, 29 CFR 1926.1153, sets out the controls: for a handheld power saw, an integrated water delivery system running continuously, with the respiratory protection the standard's Table 1 attaches to the task duration. In the UK the same exposure is controlled under the Control of Substances Hazardous to Health Regulations 2002, and the practical answer is identical — water or on-tool extraction, every cut, indoors most of all, where there is no wind to help and the dust settles into the room the client is going to live in.

1. Set out needle positions on a course line rather than on an even division of the opening — each hole has to land in a bed joint the wall can spare.
2. Cut the needle holes instead of driving them: drill the corners, cut between, hand-clean the last of it, and leave the course above bearing on sound work.
3. Thread the needles, wedge them hard up to the underside of that course, then bring the props up on their sole plates until the wall is standing on the scheme and not on the material about to be removed.
4. Check every prop plumb once tightened, and again after the first course comes out.
5. Take the opening down in courses from the top, working outward from the middle rather than starting at a reveal.
6. Cut the two leaves as separate operations, each supported in its own right before the other is disturbed.
7. Form the bearing pockets last, and cut them generous enough that the beam can be offered up rather than driven in — a beam hammered into a tight pocket unloads the needles above it on the way through.
8. Clean the bearing surfaces down to sound material before anything is bedded on them; a padstone set on cutting dust is a padstone that settles.

Handing the Load Back

The transfer is where schemes fail, not the peak load. Once the beam is offered up and the padstones are in, there is a gap between the top of the beam and the underside of the masonry, and everything depends on that gap being filled solid across the whole bearing before anything is struck. Dry pack worked in hard, slate and mortar, or a non-shrink grout — full width, full length, no dabs and no timber packers left in as a convenience. Leave it to gain strength for the period the material's own data sheet states rather than the period the programme would prefer.

Strike in stages and never one prop at a time to zero. Ease each prop a little, work along the row, come back and ease again, so the load transfers gradually across the whole line rather than dumping onto whichever bay released first. Between stages, look at the packing: a joint that is closing up is telling you the pack was not solid, and the moment to find that out is while there are still props holding most of the weight. On a two-leaf scheme the inner and outer supports get struck separately, in whatever order the temporary works design says, which is not always the order that suits the scaffold.

Then measure something and write it down. A level along the beam soffit and a reading at midspan against a fixed datum before striking, after part-striking and after full release costs twenty minutes and settles an argument that is otherwise unwinnable. Telltales across any crack that already existed, dated, do the same job for the wall. If a crack turns up over the door in month four, that short record is the difference between a structure that has moved exactly as predicted and one where a bearing gave way — and between those two answers sits the cost of taking the opening apart again.

The Openings That Should Not Get Wider

Some of these jobs should be priced as a refusal. Where the pier left beside the opening cannot be argued adequate and there is nowhere to widen it, the honest answer is a narrower opening or a post — and a post in the middle of a kitchen island is a conversation held before the steel is ordered, never after. Where the footing under the pier is a shallow strip on clay, a concentrated reaction lands somewhere that has been happily carrying a distributed one for a century, and the underpinning that follows costs several times the opening.

Party walls are their own gate and have nothing to do with structural capacity. In England and Wales, cutting into a party structure — which includes needling into it and bearing a new beam on it — is notifiable under the Party Wall etc. Act 1996, and the notice on the adjoining owner goes out well ahead of the start date, not the week before. Discovering this after the props are up is expensive in a way no calculator on this site can help with.

The last one is the wall that is not behaving as a wall. Bulging on a face, beds that have washed out, a cavity packed with rubble from a 1960s repair, or a leaf that rings hollow across a whole panel all mean the masonry above the opening cannot be relied on to span between needles at any spacing. That is a case for a full-height support scheme designed for the wall you actually have, not a case for closer needles — and the load figures on this page are the input to that design rather than a substitute for it.