Mass timber is fabricated far more precisely than concrete and steel are built, so the drawings must state where tight tolerances apply and give each interface a way to absorb the difference. Survey what the timber will bear on before fabrication or installation, and compare it with the shop drawings.
How tight mass timber tolerances are
WoodWorks notes in its Index of Mass Timber Connections (updated March 2025) that mass timber fabrication tolerances are extremely precise and might be as low as ±1/16 inch, while tolerances for steel, concrete, and masonry can be larger. Its guide to vertical additions with mass timber says the building must sit on a plumb, level, and square base, or the connections must have tolerances built in.
The WoodWorks BC Standard Connections paper (December 2025) adds that dissimilar materials are often installed to different tolerances, so the variation must be accommodated at member interfaces, and that pre-engineered hangers can have tight installation tolerances compared to typical erection allowances. Sources verified October 22, 2026. Long-term movement after installation is covered in differential movement in tall mass timber. On existing buildings, see mass timber vertical additions and adaptive reuse.
Where the drawings set tolerances
| Location | What to show in the documents |
|---|---|
| Concrete and steel specifications | Any tighter tolerance required at timber bearing points, and which trade must meet it. |
| Interface details | How the connection absorbs variation: grout beds, shims, oversized or slotted holes, or adjustable base plates. |
| Hanger selection | Whether the specified hanger allows enough placement tolerance at the interface where it is used. |
| Survey requirements | When the supporting structure is surveyed and who compares the results with the shop drawings. |
| Tolerance zones | Which locations need tight tolerances and which can accept normal ones, so each trade knows where it matters. |
Seven tolerance checks
- Stated requirement. Check whether the concrete and steel specifications state tighter tolerances at mass timber interfaces. The WoodWorks BC and Seagate checklist asks whether these were communicated to those trades.
- Adjustability at every bearing. Confirm each column base and beam bearing on concrete or steel has a detail that absorbs variation. A PCL presentation hosted by WoodWorks recommends building tolerance into the interface detail.
- Grouted bases. Where column bases are grouted, confirm the details show the grout and base plate. WoodWorks describes grout packs and oversized steel base plates used to set column elevations on one project.
- Hanger fit. Compare the specified hangers with the interface they sit on. A custom hanger may allow more placement tolerance at a concrete interface than a dovetail hanger.
- Clear zones. Check that the drawings show where tight and normal tolerances apply. The BC checklist asks whether these locations are clear to the trades responsible.
- Survey timing. Confirm the documents require a survey of the supporting structure before installation, and say who compares it with the shop drawings.
- Movement after installation. Check that connections let the timber shrink and swell without restraint. The Standard Connections paper warns that restraining these changes can induce stresses and surface checking.
Survey before fabrication or installation
For vertical additions, WoodWorks describes using a 3D survey or laser scan of the existing structure, and advises getting the critical dimensions early so the details can be given to the manufacturer. The PCL presentation lists base plate layout and verification and overlaying a field scan with the 3D model as quality-control steps.
On new buildings, the WoodWorks BC and Seagate Mass Timber checklist asks about three weeks before work on site whether the existing structure is within tolerance, for example whether embeds are out of place. Compare survey results with the shop drawings while there is time to adjust. See coordinating mass timber with concrete and steel for the interface details themselves.
Where Groundbook fits
Upload the architectural, structural, and MEP drawings with the relevant specification sections, and run a cross-discipline review. Groundbook flags potential disagreements, such as an opening shown on an MEP plan but missing from the structural panel layout, with links to each source sheet. Your team decides what each finding means for the fabrication package. Groundbook does not check structural capacity, fire ratings, or the fabricator’s model.