What Are Baselines?
A baseline is the initial set of reference measurements recorded before any construction works begin. Reflective targets or prisms are fixed to the adjoining structure, then a high-precision total station captures the exact position of each target — accurate to ±1–2mm.
These initial coordinates (X easting, Y northing, Z elevation) become the fixed datum against which all future readings are compared. Without baselines, there is no way to prove whether damage or deflection existed before the works started or was caused by them.
For party wall purposes under the Party Wall etc. Act 1996, baseline data — along with a Schedule of Condition — provides the evidential foundation if a dispute over damage liability arises.
Control points are also established away from the construction zone to ensure the total station can be set up in the same reference frame on every subsequent visit.
How Do Crack Gauges Work?
Two principal types are used on UK party wall projects:
Tell-Tale Gauges
- Two overlapping plastic plates (one with a measurement grid, one with a crosshair) bonded either side of the crack
- Once plastic locking screws are removed, the plates slide freely; any crack movement shifts the crosshair off its zero point
- Direction (X = horizontal, Y = vertical) and magnitude are recorded
- Accuracy: 0.25–0.5mm
- Readings taken by photographing the gauge square-on
- Cost: £10–£20 per crack
Demec Studs
- Small metal discs with a dimple centre glued either side of the crack
- A digital vernier caliper measures the distance between dimples
- Change in distance indicates crack opening or closing
- Accuracy: ~0.1mm — more precise than tell-tales
- Only measures crack width (single direction), unlike tell-tales which capture biaxial movement
- Cost: ~£0.50 per stud (requires caliper tool)
Electronic crack gauges also exist, recording data continuously with temperature compensation — useful as cracks naturally widen and narrow with seasonal thermal movement.
What Goes in a Movement Report?
Reports are typically issued within 24 hours of each site visit and sent to the client, project manager, structural engineer, and party wall surveyor. A standard movement monitoring report contains:
- Target reference schedule — all monitored points with IDs and locations, photographs of each target
- Tabulated readings — current X, Y, Z coordinates for each target vs. baseline and previous visit
- Movement graphs — trend plots per target showing movement over time
- Trigger level status — Green, Amber, or Red status per target
- Surveyor commentary — plain-language interpretation of whether movement is significant
- Site observations — any notable changes to the structure or targets
The setup report (issued after baselines are recorded) is a separate foundational document that records agreed trigger levels, equipment specifications, control point positions, and baseline coordinates.
Trigger Level Framework
| Level | Trigger | Action | | --- | --- | --- | | Green | Below amber | Continue as planned | | Amber (Warning) | ~5–10mm | Increase frequency; notify engineer | | Red (Action) | >10mm | Stop works; engineer to attend |
Trigger levels are set by the structural engineer before works start and documented in the setup report — they cannot be set retrospectively.
Frequently Asked Questions
Q: Why are baseline readings important?
Baselines prove the condition before works began. Without them, you cannot attribute any subsequent movement or damage to the construction works versus pre-existing conditions.
Q: How often are movement reports issued?
Typically within 24 hours of each site visit. Frequency is weekly during active works, reducing to monthly post-heavy phases.
Q: What happens if a Red trigger is breached?
All works must stop immediately. The structural engineer attends, assesses the cause, and specifies remedial measures before works can restart.