There is a conversation we have on site more often than we should. The brickwork won’t line through. The masonry support is at the end of its adjustment. Someone checks the frame against the frame tolerances, and it’s within them. Then nobody is quite sure what to do next.
Part of the problem is that “tolerance” gets used to mean three different things. The bigger problem is that “is the frame within tolerance?” is the wrong question. The brickwork doesn’t care where the frame is on its own. It cares where the frame is relative to the line the façade has to be built to. What actually matters is whether that relationship, at each support position, falls within the adjustment the masonry support system has available.
Three different things called tolerance
Tolerance is what the specification allows. It’s agreed in advance, it’s a design decision, and it belongs in the execution specification.
Permitted deviation is how far the built frame actually ends up from the drawn geometry, within those limits. A frame with a 20mm deviation isn’t defective if the specification allows it. It’s compliant, and nobody can reasonably be asked to correct it.
Movement is what the frame and the masonry do after they’re built: deflection under load, thermal movement, shrinkage and moisture movement over time. It’s a design allowance, not a construction one, and it comes on top of whatever deviation the frame was built with. We cover it separately in part four.
These get mixed up all the time, which is why the site conversation goes round in circles. If the frame is within its permitted deviation and the brickwork still won’t fit, nobody has necessarily done anything wrong. What usually hasn’t happened is anyone checking how the frame tolerances and the façade setting-out combine at the support.
It’s also a scope problem. As we said in part one, on who is actually responsible for the masonry support design, frame tolerances are among the most common items that sit outside everyone’s scope while affecting all of them. The engineer specifies the frame tolerance and the architect sets the façade line, but the relationship between the two often isn’t anyone’s to check.
What the frame specifications actually allow
On the façade side, expectations are usually set by the drawings, and the drawings show nominal geometry. The frame isn’t built to the drawings. It’s built to the drawings plus whatever the execution specification allows.
For concrete frames, that’s BS EN 13670, with UK practice set out in the National Structural Concrete Specification, now in its fifth edition (July 2025). If a project specification still references the fourth edition, it’s worth checking.
For steel frames, it’s the National Structural Steelwork Specification (7th edition, 1st revision, in force since October 2023) and BS EN 1090-2. Permitted deviations in the NSSS are mostly defined at fixed points, such as where beams connect to columns. That suits the frame, but the façade line runs continuously along the edge of the building, so for us it’s the position all along the edge that matters.
The steel sector is also clear about what tolerances are for. Part of the point is that whatever gets built on and around the frame can actually be built. BCSA guidance adds that tightening frame tolerances is expensive and often impractical, and that it’s generally better to provide adjustment between the frame and whatever needs to be more accurate. On a brick-clad building, most of that adjustment sits in the masonry support.
None of this means permitted deviations are unreasonable. They’re published and realistic, and they’re usually larger than the façade team assumes. The frame is entitled to be anywhere the specification allows.
Who sets the tolerance, element by element
| Element | Usually governed by | Measured from | Written to protect |
|---|---|---|---|
| Concrete frame | NSCS 5th edition, BS EN 13670 | Grid and datum, in a hierarchy from whole structure down to individual element | Structural behaviour, cover, fit of the frame |
| Steel frame | NSSS 7th edition, BS EN 1090-2 | Fixed points such as beam-to-column nodes | Assembly of the frame and its design assumptions |
| Masonry outer leaf | BS EN 1996-2, PD 6697, BS 8000-3:2020 | Its own setting-out lines, per storey and overall | Stability and appearance of the wall |
| Masonry support | No dedicated tolerance standard. BS EN 845-1 excludes shelf angles from its scope | Wherever the frame turned out to be | Whatever has been agreed, if anything has |
Masonry support is the only element in that table that has to work with all the others, and the only one without a tolerance standard of its own. What it has to accommodate is set by the project, whether or not anyone has written it down.
The adjustment envelope
Every masonry support detail is designed around a nominal relationship between the frame and the façade: how far the slab edge or edge beam sits behind the face of the brickwork, and where the support level falls relative to the frame and the brick coursing. The system then has a range of adjustment either side of that nominal position, vertically in the bracket fixing and horizontally through packing and the bracket and angle arrangement. That range is the adjustment envelope.
The check that matters is whether the actual relationship at each support position falls inside that envelope. Not whether the frame is within its own tolerance, and not whether the façade setting-out is right on its own, but the two together.
Individual deviations also add up. At a single support position, the edge beam can be within tolerance on line and on level, the column next to it within tolerance on plumb, and the façade setting-out established to its own accuracy. Each one is compliant. Together they can still push that position outside the envelope, without anyone having done anything wrong.
“The frame is within tolerance” and “the support has run out of adjustment” can both be true statements about the same wall.
These positions usually can’t be predicted from the drawings. They turn up wherever several acceptable deviations happen to go the same way.
When a position falls outside the envelope, every option is slower and more expensive than designing for it in the first place:
- Bespoke brackets for the affected locations
- Remanufacture, with the lead time that comes with it
- Revised fixing design and possibly more anchor testing. Where cast-in channels were placed at the pour, the fixing position can’t move far, if at all
- Delay to the brickwork, which is often on the critical path
- Occasionally, local work to the frame itself
Adjustment is also one of the first things to come under pressure when a package is value engineered. Less of it looks like a saving at tender. Whether it still looks like one once the frame is up is something we come back to in part five.
A repetitive façade isn’t always a repetitive support condition
A recent steel-frame project of ours shows how this plays out.
At tender and through design, it looked like a very straightforward masonry support package. The elevations were repetitive, and on the drawings the relationship between the steel edge beams and the brickwork line was the same almost everywhere. Most of the project was one standard MSA detail, repeated.
Before manufacture, the steel frame was checked against the required façade line, and the relationship wasn’t what the nominal drawings showed. We’re not saying the steelwork was out of tolerance. The individual members may well have been exactly where the specification allowed them to be. But the position of the frame relative to the line the brickwork had to be built to was different from what the drawings showed, and the standard detail couldn’t accommodate it.
So what should have been one repeated detail became a number of bespoke conditions. Because it was picked up before anything was made, those conditions could be designed into the support, and the support was made to suit the frame as it had actually been built.
We’ve now seen essentially the same issue on our last four steel-frame projects. On the previous three, it was found later, when the brickwork couldn’t be built to the required façade line. By then the support had already been manufactured, so each one became redesign, remanufacture and variation orders.
The issue was the same each time. What changed the outcome on this project was when it was found.
Check what you can before manufacture
Ideally the frame would be complete and surveyed before any support was made. In practice that rarely happens. Support design and manufacture usually overlap with frame construction, and waiting for a full as-built survey isn’t always an option.
So the realistic aim is narrower:
- Where the programme allows, check the critical frame-to-façade dimensions before manufacture. That means the slab edge or edge beam position relative to the façade line, and levels relative to the brick coursing, rather than the frame on its own. Start where the envelope is tightest, or where one detail repeats across a lot of the building.
- Check the frame in sections as it goes up. The first floors or the first elevation can show whether there’s a pattern before the rest of the support is made.
- Where a check isn’t possible, be clear about what the design has assumed. That means the nominal frame position, the façade setting-out the support is working to, and the adjustment available. Then everyone knows what the support can take, and what should trigger a check on site.
Point cloud and total station surveys have made this much quicker than it used to be. On a repetitive elevation, measuring a handful of levels can confirm or rule out the design assumptions for a large part of the building.
Agreeing frame tolerances and the façade line
None of this needs much paperwork. It needs a few things agreed and written down, rather than left to be sorted out on site.
- The frame tolerance referenceA named specification and class, not “in accordance with good practice”.
- One set of numbersThe frame, masonry and support specifications working to the same tolerances and the same façade setting-out. That doesn’t always happen when they’re written by different people at different stages.
- When the frame gets checked, and by whomA point in the programme, before manufacture where possible, with a named party responsible.
- What happens outside the envelopeA technical route to a decision, agreed before it’s needed.
What this means for you
If you are designing
The frame tolerances you specify affect the façade as much as the frame. A tighter class costs money on the frame; a looser one moves the cost to the support and the brickwork. Either can be the right call. Tell the support designer which you’ve chosen, and what façade line they’re working to.
If you are building
Try to protect a point in the programme where the critical frame dimensions can be checked before the support is manufactured, even if it only covers part of the frame. If that isn’t possible, make sure you know what the support design has assumed.
If you are pricing
A support package priced on nominal geometry assumes the as-built relationship will fall within the system’s adjustment. Ask what happens if it doesn’t, and who carries it.
On our side, this is now part of how we approach design. We look at the likely frame variation, the façade setting-out and the adjustment available in the system together, flag where the relationship looks tight, and ask for the critical frame dimensions as early as the programme allows. It isn’t always possible to check everything before manufacture. But if everyone knows what the support has been designed to accommodate, a problem on site is a known risk to be managed, not a surprise.
This article discusses general technical principles. It is not a substitute for project-specific design or specification advice. For advice on a particular masonry support package, talk to the FIRMA technical team.
References and further reading
- National Structural Concrete Specification (NSCS), 5th Edition — developed by CONSTRUCT, July 2025. Compliant with BS EN 13670, with guidance on tolerances and finishes. The Concrete Centre
- National Structural Steelwork Specification (NSSS), 7th Edition, 1st Revision — BCSA, in force October 2023. Includes erection tolerances. bcsa.org.uk
- SteelConstruction.info — Steelwork specification. Why erection tolerances exist and how permitted deviations are expressed. steelconstruction.info
- BCSA — Steel Industry Guidance Note SN02: Tolerances in structural steelwork. bcsa.org.uk
- BS EN 13670:2009 — Execution of concrete structures. BSI.
- BS EN 1090-2:2018 — Execution of steel structures and aluminium structures. Technical requirements for steel structures. BSI.
- BS 5606:2022 — Accuracy and tolerance in design and construction — Guide. BSI.
- BS EN 1996-2:2006 and PD 6697:2019 — masonry execution and the UK recommendations that accompany it. BSI. PD 6697 on BSI Knowledge
- BS 8000-3:2020 — Workmanship on construction sites. Part 3: Masonry. BSI.
- BS EN 845-1:2013+A1:2016 — Specification for ancillary components for masonry. Scope excludes shelf angles. BSI Knowledge
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