How do you measure a crack in a corner with a Saugnac gauge?

Cracks do not form only on flat surfaces: a re-entrant or projecting corner between two walls, a floor-to-wall junction, or a joint between two structures are all configurations where a crack can appear and develop.

The question of how to measure a crack in a corner comes up regularly, because fitting a crack monitor on an edge is not quite the same as fitting one on a flat surface.

Our standard single-axis monitoring products – G1 gauge, E1 gauge and R1 gauge – are all designed to be fixed in a corner. This guide sets out the principles they share and the specific features of each instrument, along with recommendations for reliable measurement.

Why a specific installation for cracks in a corner?

A crack monitoring gauge measures the opening along one axis. When the crack lies on an edge between two planes, the two faces of the structure are no longer in the same plane: the gauge must therefore follow that angle to connect a measurement point on each face. Corner installation makes it possible to fix an anchor point on either side of the edge, in line with each wall surface.

Typical cases of a crack in a corner

Measuring a crack in a corner with the G1 gauge

The G1 gauge tracks the movement of a crack along one axis (0.1 mm accuracy on the vernier) by comparison with a reference reading. Its advantage in a corner: the moving arm has been designed to fold, which avoids any additional fixing accessory. In practice, the gauge follows the edge and anchors a point on each of the two faces.

The way this fold is carried out depends on the age of the gauge.

Models from mid-2025 onwards: direct folding at the score line

Since mid-2025, the pull-tab has a moulded score line: a simple fold line cast into the part. You fold the pull-tab to the desired angle at that point, then fix the gauge as you would on a flat surface (adhesives, epoxy glue or wall plugs). No part to fit and no particular adjustment of the pull-tab: this is the quickest option.

Models before mid-2025: the foldable strip

Before mid-2025, folding is done with a foldable strip that clips into the moving arm (its lug seats into the slotted hole). Once the strip has been folded to the desired angle, installation proceeds exactly as for the recent models.

The step-by-step detail of these two methods – adjusting the moving arm, choosing the range and precautions when folding in cold weather – is already covered in our dedicated guide: How do you install a G1-type crack monitor?

The special case of a projecting corner

On a re-entrant corner, the folded gauge naturally settles into the corner. On a projecting corner (external edge), there is no recess: the folded part ends up in mid-air and the moving arm has no continuous surface to rest on.

In this configuration, an aluminium angle bar is inserted to recreate the fixing surface needed on either side of the edge. Supplied with a double-sided adhesive and a self-tapping screw, it is fixed astride the projecting corner and provides a clean bearing surface for the gauge.

If the direction of movement remains undetermined

On a crack in a corner, the direction of opening is not always known in advance. It is then possible to mount two gauges head-to-tail on either side of the same point.

The idea: to cover both possible directions, so that at least one of the two gauges stays within a usable reading range whatever the actual movement.

Measuring a crack in a corner with the E1 gauge

The E1 gauge is a removable digital crack monitor offering a resolution of 0.01 mm. The principle relies on two P1E1 mounting plates fixed on either side of the crack (for single-axis monitoring), which serve as a positioning reference: the gauge is placed into the mounting plates to take the reading, then only the mounting plates remain on the support.

For corner installation, the E1 has a single-axis fitting template that makes it easier to fix the mounting plates on a 90° corner. The procedure has four steps, the template being used to position each mounting plate precisely on either side of the edge.

1. Position the single-axis template on the first wall, hard against the corner, and mark the fixing point using the hole furthest from the corner. The movement is measured along the axis of the template.
2. Position the template on the second wall (perpendicular to the first), hard against the corner, and mark the fixing point using the hole closest to the corner. The two can be reversed depending on the configuration.
3. Drill the two Ø4 mm holes to a depth of 30 mm at the marks.
4. Fix each mounting plate with the self-tapping screw (or by fast-curing epoxy bonding), following the procedure in the E1 instructions.

As with the G1, if there is any doubt about the direction of the movement to be monitored, it is possible to fit two pairs of mounting plates in order to monitor both movements.

Measuring a crack in a corner with the R1 gauge

The R1 gauge is a wireless crack monitor (linear sensor with a resolution of 0.01 mm, temperature and humidity sensors, LTE-M / NB-IoT transmission) that enables remote monitoring of a crack opening. It is fitted with an articulated mount allowing both a standard fixing and a corner fixing.

The full corner-fixing procedure (drilling, wall plugs, range adjustment) is described in the R1 gauge instructions provided in English, in the “corner fixing” section. The video below shows the corner-installation step visually:

Note: this video is in French. You can turn on the automatic subtitles for better understanding, but it mainly lets you see the installation steps visually.

For wireless corner monitoring where the direction of movement is uncertain, the same principle as for the G1 and E1 applies: you can multiply the measurement points to cover the different movements.

Going beyond corner opening : differential settlement and shear

The corner installation described above concerns the monitoring of the opening of a crack (along one axis). When a crack in a corner is accompanied by an out-of-plane movement – the two lips of the crack shift relative to each other perpendicular to the surface – this is referred to as differential settlement (or step). This movement, as well as shear, calls for dedicated products or accessories:

• G3 gauge: measures the evolution of horizontal or vertical differential settlement between two structures.
• E1 misalignment kit: fixing kit for the E1 mounting plates to monitor differential settlement with the digital gauge.
• R1 misalignment kit: to remotely measure the evolution of differential settlement with the wireless gauge.

The choice between a simple corner installation (opening) and a dedicated product depends on the nature of the movement to be monitored and the configuration of the point to be instrumented. If you are unsure which instrument is best suited, contact us.

The key points to remember

• The G1, E1 and R1 gauges are all designed for corner installation.
• The G1 folds at the score line (from mid-2025) or via the foldable strip (before mid-2025), observing the temperature precautions when folding. On a projecting corner, an aluminium angle bar recreates the bearing surface.
• The E1 uses its single-axis fitting template to fix the mounting plates at 90° in the corner.
• The R1 uses its articulated mount, connected to an anchor point on each face of the corner.
• If the direction of movement is unknown, fit two gauges in opposition (or two pairs of mounting plates).
• For an out-of-plane movement (differential settlement), turn to the G3 or the dedicated misalignment kits.

Frequently asked questions

Can you fit a crack monitoring gauge in a corner?

Yes. The Saugnac single-axis monitoring gauges – G1, E1 and R1 – are designed to be fixed in a corner. The G1 gauge folds at its folding zone, the E1 gauge uses its single-axis fitting template to fix the mounting plates at 90°, and the R1 gauge has an articulated mount with a rotation-locking nut.

How do you fold a G1 gauge for corner installation?

It depends on the model: the recent gauges (from mid-2025) fold directly at the score line of the pull-tab, while the earlier models use the foldable strip supplied. In both cases, folding is done cold, with a few precautions in low temperatures. The detailed procedure is described in our guide How do you install a G1-type crack monitor?

How do you fit a G1 gauge on a projecting corner?

On a projecting corner (external edge), there is no recess to house the folded gauge. You then use an aluminium angle bar, fixed astride the edge with its double-sided adhesive and its self-tapping screw, which recreates a flat bearing surface on each side to fix the gauge.

How far from the corner should you drill to fix an R1 gauge?

For a corner installation of the R1 gauge, the first hole is drilled about 30 mm from the corner (piston side), and the second about 216 mm from the corner. The shortest grub screw is used on the piston side, the longest on the ball-joint side.

What should you do if you do not know the direction of movement of a crack in a corner?

When the direction of movement is uncertain, it is advisable to fit two gauges in opposition (or two pairs of mounting plates for the E1). That way, whatever direction the crack works in, at least one gauge will stay within its usable measurement range.

Does a corner installation allow differential settlement to be measured?

No: a corner installation measures the opening of the crack along one axis. To monitor an out-of-plane movement (differential settlement or step), you need to turn to a dedicated product such as the G3 gauge, the E1 misalignment kit or the R1 misalignment kit.