Fibre Reinforced Facade Encyclopedia: Design and detailing
Construction tolerances are the permitted deviations between what was drawn and what is built. Every element in a facade carries them: the concrete frame, the cast-in anchors, the substructure brackets, the panel itself, and the survey that ties them all together. Tolerance is not an allowance for poor workmanship. It is a designed quantity, and the facade that does not fit is almost always the one where nobody added the deviations up.
A frame at plus or minus 20 mm, a cast-in anchor at plus or minus 10 mm, a bracket at plus or minus 5 mm and a panel at plus or minus 3 mm is a worst case stack of 38 mm. A 15 mm joint cannot absorb it. The adjustment has to be designed into the bracket.
At a glance
| Property | Typical value | Note |
|---|---|---|
| In situ concrete frame, position | plus or minus 15 to 25 mm | Typical Australian practice; confirm against the structural specification |
| Slab edge line | plus or minus 15 to 25 mm | Usually the largest single deviation on a facade |
| Slab level | plus or minus 10 to 15 mm per floor | Accumulates up the building unless re-surveyed |
| Cast-in anchor position | plus or minus 10 mm | Before any bracket adjustment is considered |
| Bracket adjustment required | 30 to 50 mm in three axes | What the substructure must actually provide |
| GRC panel dimensions | plus or minus 3 mm up to 3 m; plus or minus 5 mm over 3 m | Typical GRCA-based project requirement |
| FRP panel dimensions | plus or minus 2 to 3 mm | Mould-derived; the mould sets the achievable figure |
| Panel bow and twist | length/360, or 3 mm, whichever is greater | Typical facade panel limit; state it in the specification |
| Joint width variation | plus or minus 3 to 5 mm on a nominal joint | Variation above about 3 mm in a run is visible |
| Installed panel face alignment | plus or minus 3 mm between adjacent panels | Step between neighbours, measured after installation |
Four families of deviation
- Manufacturing tolerance. How closely the panel matches its drawing. For a moulded panel this is really a mould tolerance: the panel can be no more accurate than the surface it was cast against, and a well-made CNC-cut mould will hold a tighter figure than a hand-built one.
- Structural tolerance. How closely the frame matches its drawing. This is the largest and the least controllable of the four, and the facade designer does not get to choose it.
- Setting out tolerance. How accurately the facade grid is transferred from the survey control onto the structure, and how accurately the brackets are positioned from it.
- Erection tolerance. How accurately the panel is installed relative to its neighbours and to the setting out, once every earlier deviation is already present.
The first is controlled by the fabricator, the second by the head contractor and the structural engineer, the third and fourth by the installer. They are separately specified, separately measured and separately argued about, which is precisely why they need to be considered together.
The accumulation problem
Deviations stack. If the slab edge is 20 mm out at one level, the cast-in is 10 mm out within that, the bracket is set 5 mm out of position and the panel is 3 mm long, the worst case is not 20 mm but 38 mm. That figure has to be absorbed somewhere, and there are only three places it can go: the bracket adjustment, the joint width, or the panel face alignment. Only the first of those is designed to move.
Worst case addition is conservative, because all four deviations reaching their maximum in the same direction at the same location is unlikely. Statistical addition, taking the square root of the sum of the squares, gives a figure closer to what is observed and is appropriate over a run of panels. Worst case addition is still the right method at critical singular locations: corners, mitres, abutments with other trades, and anywhere a single panel is captured between two fixed elements.
Where the adjustment lives
Adjustment belongs in the bracket, and it must be available in all three axes: in and out from the structure, up and down, and along the facade. In and out adjustment is typically taken up by shims or packers behind a base plate, vertical adjustment by a slotted bracket or a serrated channel, and horizontal adjustment by slotted holes or a sliding channel nut. Thirty to fifty millimetres of range in each axis is a working target on an in situ concrete frame; less than that, and the installer will start fixing brackets off-position and taking the difference out of the joints.
Adjustment does not belong in the panel and it does not belong in the joint alone. A panel that has to be trimmed on site loses its factory edge and its finish. A joint that absorbs setting out deviation has no capacity left for the movement it was designed for, which is how tolerance failures turn into sealant failures a year later. See movement joints for the movement the joint was supposed to be reserving, and joint design for how much the joint can reasonably take.
Survey before fabrication
The single most useful intervention on a panelised facade costs a few days and is regularly skipped: survey the structure as built, before panels are released for manufacture. A total station or laser scan survey of slab edges, column faces and cast-in positions gives the real geometry, and the setting out can then be re-cut against it rather than against the structural drawings.
On repeating flat panels this lets joint widths be adjusted level by level so deviation is distributed rather than accumulated. On a freeform or double curved facade it is not optional, because there is no orthogonal reference to correct against on site and a panel that does not fit cannot be trimmed. Feed the survey back into the model; see bim and shop drawings.
Writing tolerances into a specification
A facade worksection that is silent on tolerance transfers the risk to whoever is last on site. A usable clause states five things: the structural tolerance the facade design has assumed, the dimensional and flatness tolerance required of the panel, the installed tolerance required between adjacent panels and over a defined length, the method and instrument by which each will be measured, and who carries out and pays for the survey of the structure.
State the measurement method explicitly, because tolerance figures mean different things measured different ways. Face alignment between adjacent panels, deviation from a 3 metre straightedge, and deviation from a theoretical plane over a whole elevation are three different requirements. See specification and natspec.
Standards and references
- AS 3600 Concrete structures, including construction tolerances for in situ elements
- AS 3610 Formwork for concrete, surface finish and tolerance classes
- AS 4100 Steel structures, erection tolerances
- AS 4284 Testing of building facades
- GRCA Specification for the Manufacture, Curing and Testing of GRC Products, dimensional tolerances
- BS 5606 Guide to accuracy in building
- ISO 4463 Measurement methods for building: setting out and measurement
- PCI MNL-135 Tolerance Manual for Precast and Prestressed Concrete Construction
Where this matters on a ShapeShift project
One Sydney Harbour, Barangaroo: A high rise where panel setting out had to be reconciled against the structure as built rather than as drawn.
Related terms
Questions specifiers ask
What tolerance should I assume for a concrete frame?
Typical Australian practice is plus or minus 15 to 25 mm on position, with slab edge line usually the worst case. Do not assume it; state the assumed figure in the facade specification and check it against the structural specification.
How much adjustment does a cladding bracket need?
Around 30 to 50 mm in each of three axes on an in situ concrete frame. Less than that and installers begin fixing brackets off-position and absorbing the difference in joint widths, which removes the capacity the joints were designed to reserve for movement.
Should tolerances be added worst case or statistically?
Use worst case at singular critical locations such as corners, mitres and abutments where one panel is captured between fixed elements. Statistical addition, root sum of squares, is reasonable over a long run of repeating panels.
What tolerance can a moulded panel hold?
It is a mould tolerance rather than a material one. A CNC-cut mould will typically hold plus or minus 2 to 3 mm on a fibre reinforced polymer panel and plus or minus 3 to 5 mm on a glass fibre reinforced concrete panel depending on size.
Why survey the structure before manufacture?
Because it converts an assumption into a measurement. Re-cutting the setting out against the structure as built lets deviation be distributed across joints rather than accumulated, and on freeform geometry it is the only way to guarantee the panels will fit.
Fitting moulded panels to a frame that is already built? We can work from a survey of the structure as built rather than from the structural drawings. Talk to our engineering team
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.