# Construction Tolerances: Why Nothing Is Built Exactly to Drawing

> A construction tolerance is the permitted difference between the size or position shown on a drawing and what is actually built. Every trade works to its own tolerance: in-situ concrete is less precise than factory-made steel, aluminium or glass. Where trades meet, the design must provide enough adjustment to absorb the difference.

## Why it matters on a project

Many disputes on site start with "it doesn't fit". A facade bracket cannot reach the
slab edge, a partition head meets a beam lower than drawn, or a raised floor cannot
be levelled on an uneven slab. Most of these trace back to tolerances that were
never compared between trades.

## Different trades, different accuracy

| Trade | Made where | Relative accuracy |
| --- | --- | --- |
| In-situ concrete frame | On site | Lowest |
| Structural steel | Factory, erected on site | Medium |
| Masonry and blockwork | On site | Low to medium |
| Aluminium and glass facade | Factory | High |
| Joinery and fit-out elements | Factory or workshop | High |

The actual values for a project come from its specification and the codes it
references; concrete tolerances are addressed in **IS 456**, and the principles of
expressing and managing accuracy are set out in **ISO 1803** and **BS 5606**.

## How good projects manage it

- **One control grid and benchmark set** shared by every trade, set out to
  **ISO 4463-1** principles.
- **Interface details that state adjustment**, such as slotted bracket holes, packers
  and shims, sized for the combined deviation.
- **A survey of the as-built structure** before fabrication of facade or joinery
  starts.
- **Hold points** where the next trade checks and accepts the surface it builds on.
- **As-built records** so later trades work from real dimensions.

## Common mistakes

- **Designing a factory-made element to drawing dimensions** with no allowance for the structure it fixes to.
- **Fabricating before surveying** the structure.
- **Each trade setting out from its own reference**.
- **Using up adjustment in the first installation step**, leaving none for the next.
- **Treating out-of-tolerance work as a site problem** instead of raising it at the hold point.

## Standards referenced

Principles of accuracy in **ISO 1803** and **BS 5606**; setting out under
**ISO 4463-1**; concrete tolerances in **IS 456**. Tolerance values and interface
adjustments for a specific project must be set out in its specification by the
design team.

## Frequently asked questions

### Why can't everything be built exactly to drawing?

Because every process has variation. Formwork moves, concrete shrinks, steel is fabricated and erected within limits, and setting out has measurement error. Tolerances accept that and define how much variation is acceptable. Demanding zero deviation is neither achievable nor measurable.

### Why do tolerances matter most at interfaces?

Because each trade can be within its own tolerance and the two still fail to meet. A slab edge built within concrete tolerance can be far enough out for a curtain wall bracket, made to a much tighter factory tolerance, to run out of adjustment. Interface problems are rarely one trade's fault; they come from designs that did not allow for the combined deviation.

### What is the difference between tolerance and clearance?

Tolerance is the permitted variation in size or position. Clearance is the designed gap between components that allows them to fit together despite that variation, and to move in service. A good interface detail states both: the tolerances each trade will work to and the clearance and adjustment provided to absorb them.

### What is setting out?

Transferring the positions and levels on the drawings to the site, using grid lines, control points and benchmarks. Errors in setting out propagate into every trade that follows, which is why a single verified grid shared by all trades, and checked surveys at key stages, are basic disciplines on any project.

### When should tolerances be agreed?

At design stage, before packages are tendered. Each package should state the tolerances it works to and the tolerances it assumes from the trades it sits on. A survey of the structure before a facade or fit-out package starts fabrication confirms whether those assumptions hold, while there is still time to adjust.

## Sources

- [ISO 1803](https://www.iso.org/standards.html) — Building construction — tolerances — expression of dimensional accuracy — principles and terminology
- [ISO 4463-1](https://www.iso.org/standards.html) — Measurement methods for building — setting-out and measurement
- BS 5606 — Guide to accuracy in building
- [IS 456](https://standards.bis.gov.in/website/published-standards/department-wise) — Plain and reinforced concrete — code of practice

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Source: https://hagerstone.com/glossary/construction-tolerances