Fibre Reinforced Facade Encyclopedia: Design and detailing
An undercut anchor is a concealed mechanical fixing set into a hole drilled part way into the back face of a cladding panel, where the hole is widened into a cone at depth and a sleeve expands into that cone so the anchor locks by form rather than by friction. It is the standard concealed fixing for dimension stone and is used on thicker cementitious and engineered panels where nothing may be visible on the face. For the wider family of methods it sits within, see panel fixing systems.
A friction expansion plug pushes outwards on the hole wall and can split a thin brittle panel. An undercut anchor bears on a shoulder of material below the surface, so the load path is compression into the body of the panel and the panel is not pre-stressed at all.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Hole geometry | Cylindrical shaft with a conical enlargement at depth | Cut by a two-stage or single-pass undercut bit |
| Hole depth | 60% to 75% of panel thickness typically | Never through the panel; residual thickness governs |
| Minimum panel thickness, stone | 20 to 30 mm | Below about 20 mm the residual section is too thin |
| Drilling depth tolerance | plus or minus 0.1 to 0.3 mm | Achieved by a depth stop on a rig, not by hand |
| Edge distance | 60 to 100 mm minimum, typical | Manufacturer data governs; the failure cone must develop |
| Anchor spacing | 150 to 200 mm minimum, typical | Overlapping failure cones reduce capacity |
| Anchors per panel | 4 as standard | Two carry dead load, all four carry wind |
| Characteristic pull-out | 2 to 8 kN per anchor | Substrate dependent; must be established by test on the actual panel |
| Factor of safety on test result | 3 to 5 on the mean failure load | Brittle substrate, so a high factor and project-specific testing |
How the anchor carries load
The mechanics are the reason the fixing exists. Drill a plain hole in a brittle panel and fit an expansion plug, and the plug transfers load by friction while forcing the hole walls apart. In stone or a cement composite that radial force is a splitting force, and the panel is weakest in exactly that direction. Cut a conical enlargement at the bottom of the hole instead, let a sleeve open into it, and the anchor bears against a shoulder of material. Load is carried in bearing and shear on a cone of material around the anchor, and the panel is not put into tension by the act of fixing it.
There are four failure modes to check, and the governing one changes with geometry. Cone pull-out, where a conical plug of panel material is pulled out around the anchor, governs when edge distance and depth are adequate. Edge breakout governs when the anchor is too close to a free edge. Panel bending between anchors governs when anchors are too widely spaced for the wind load. Anchor shear or sleeve slip governs only when the anchor is undersized or the hole was cut badly.
The hole is the engineering
Performance is decided by the hole, which is why this is a factory operation. The hole must be perpendicular to the panel face within a close angular tolerance, at the correct depth from the back face, with the undercut fully formed and concentric and all dust and slurry removed. Depth is critical: too shallow and the undercut is not fully formed; too deep and the residual thickness between the anchor cone and the visible face is too small, producing a blemish now or a spall later.
Drilling is a rig operation with a depth stop, usually water cooled, using either a two-stage process, cylindrical bit followed by an undercutting bit, or a single-pass bit that cuts both profiles at once. Hand drilling is not acceptable for a structural anchor in a brittle panel: neither depth nor perpendicularity can be held.
Edge distance, spacing and layout
The anchor develops capacity through a cone of material around it, so anything that truncates that cone reduces it: a free edge, a neighbouring anchor, a rebate, a chase or a return on the back of the panel. Manufacturer data gives minimum edge distances and spacings for full capacity with reduction factors for closer positions. Typical minimums are in the order of 60 to 100 mm to an edge and 150 to 200 mm between anchors, but the product data governs against the specific substrate.
Layout is a four-anchor arrangement in most cases, with the two upper anchors carrying dead load and all four carrying wind load. Anchors sit inboard of the edges by roughly a fifth of the panel dimension, where the moment at the anchor and the moment at the edge come closest to balancing. Non-rectangular panels need the layout worked out case by case.
Setting tools and installation
The anchor is set with a dedicated tool matched to the anchor type, either a calibrated tool that expands the sleeve by a fixed displacement or a torque-controlled one. Both have to be verified, because setting tools drift and an under-set anchor looks identical to a correctly set one from outside. Record the tool, the setting parameter and the operator against the production batch. The anchor body is stainless steel in almost all facade work, commonly grade A4 or 316 in exposed and marine conditions, with matching bolt, washer and bracket components to avoid galvanic action. Adjustment is taken up in the bracket, never in the anchor hole; see tolerances and substructure.
Pull-out testing and quality control
Capacity cannot be taken from a catalogue alone, because it depends on the substrate as much as on the anchor. The accepted approach is project-specific pull-out testing on production panels of the actual material, thickness and finish, using the actual anchor, drilled by the actual rig. ASTM C1354 is the standard test method for individual anchorages in dimension stone; ASTM E488 covers anchors in concrete elements.
Report individual failure loads rather than only the mean, calculate the characteristic value at the specified fractile, and apply the project factor of safety. Factors of 3 to 5 are usual for brittle substrates because the scatter is wide and failure is sudden. Testing continues into production as a batch check, not only as a one-off qualification.
When an undercut anchor is the wrong fixing
Undercut anchors need material depth. A glass fibre reinforced concrete facade skin is typically 10 to 20 mm thick, which does not leave enough residual section for a reliable undercut, so glass fibre reinforced concrete panels are normally fixed with cast-in anchors instead: gravity anchors carrying dead load at a fixed point and flex anchors carrying wind load while allowing the skin to move independently of the frame behind. Fibre reinforced polymer panels are thinner again and use bonded or laminated-in inserts, where the load is spread into the laminate over an area rather than concentrated on a cone.
The general rule is that undercut anchors suit thick, dense, brittle panels, and cast-in or bonded fixings suit thin composite panels. Specifying one into a thin composite skin is the common misapplication, and it usually surfaces when the first pull-out test fails.
Standards and references
- ASTM C1354 Standard Test Method for Strength of Individual Stone Anchorages in Dimension Stone
- ASTM E488 Standard Test Methods for Strength of Anchors in Concrete Elements
- AS 5216 Design of post-installed and cast-in fastenings in concrete
- AS/NZS 1170.2 Structural design actions: wind actions, for the design load case
- AS 4284 Testing of building facades, where the anchorage forms part of a tested assembly
- EOTA ETAG 034 Kits for external wall claddings, European Technical Assessment route
- GRCA Practical Design Guide for GRC, anchor and fixing provisions
- Manufacturer European Technical Assessment (ETA) or equivalent third party assessment for the specific anchor and substrate
Related terms
Questions specifiers ask
How does an undercut anchor differ from an expansion anchor?
An expansion anchor grips by friction and forces the hole walls apart, which can split a brittle panel. An undercut anchor locks into a conical enlargement below the surface and bears on a shoulder of material, so the panel is not pre-stressed by the fixing.
How deep is the hole?
Typically 60 to 75 per cent of the panel thickness, never through the panel. The residual thickness between the top of the undercut and the visible face is the dimension that decides whether the anchor will telegraph through the finish.
Can undercut anchors be used in GRC?
Generally not in a facade skin of 10 to 20 mm, because there is not enough residual section. Glass fibre reinforced concrete panels normally use cast-in gravity and flex anchors instead, and fibre reinforced polymer panels use bonded or laminated-in inserts.
Is pull-out testing required?
Yes, project-specific. Capacity depends on the substrate as much as on the anchor, so test production panels of the actual material, thickness and finish with the actual anchor and drilling rig, and apply a factor of safety of 3 to 5 on the mean failure load.
Fixing thin composite panels? ShapeShell-RC uses cast-in gravity and flex anchors and ShapeShell-RT uses bonded inserts, both engineered for the panel thickness. Talk to our engineering team
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.