How to design an anchor connection in a natural stone wall

anchoring in natural stone vs concrete - The full guide to the selection of anchors

Anchoring in natural stone versus concrete — how do you get it right?

Concrete is a uniform material with clearly defined properties, which lets us design connections fairly precisely to suit the requirements of the application. A connection into a wall built of natural stone is far more complex. Natural stone is generally not a uniform material: it contains cracks, voids both small and large, moisture, and more. On a single wall there may be several types of stone, or zones where the stone is more or less cracked.

The uncertainty of natural stone

You must be aware that in a wall built of natural stone, even five separate tests with good results are no guarantee that the sixth anchor will carry the same load. The uncertainty of the raw material has to be built into the design, and you should avoid concentrating too much load on any single anchor. As a rule it is also better to go deeper, particularly with chemical anchoring — because the more the stress is distributed (the adhesive bonds along the whole length of the anchor, so a longer anchor spreads the load over a greater length), the stronger the anchor will be.

The critical stages before installing into natural stone

1.Mapping the wall. The first step is to map the zones of the wall and define them clearly.

2.Assessment. For each zone, estimate the anchor's load capacity according to the type of natural stone and the condition of the wall.

3.Testing on site. Carry out pull-out tests on site to confirm that the load assessment made at the outset was correct.

Which anchor suits natural stone best?

What anchoring solutions exist, and how well do they suit a connection into natural stone? Here are the three types of anchor used in construction and cladding projects:

Anchor type	Behaviour in natural stone	How the stresses are distributed	Suitability for natural stone
Chemical anchor	Works in every type of stone (sometimes with a reduction in load compared with concrete). Requires skill.	Excellent distribution of stress along the whole anchor and borehole.	★ The best option
Concrete screw	Surprisingly good: pull-out loads similar to the capacity in concrete, in hard stone, limestone and Jerusalem stone.	The thread engages and spreads the load over a significant length, roughly 100 mm.	✓ A genuinely good option
Wedge anchor	Results in pull-out tests on natural stone ranged from disappointing to very disappointing.	Stresses concentrated around the expansion clips.	✗ Not recommended

1. Chemical anchor

In many years of experience in the field, I have not seen a stone in which a chemical anchor failed to work — provided the type of adhesive is matched to the application (occasionally with a significant load reduction compared with an installation in concrete). This is in practice the best option. That said, chemical anchoring demands a degree of skill, it is not easy when working at height, and where there are voids in the stone the installation becomes considerably more complicated and is not recommended without training and experience.

2. Concrete screw

In every test I have carried out on concrete screws in hard stone, limestone or Jerusalem stone, the screw carried pull-out loads similar to its capacity in concrete. The strength comes from the thread cutting into the walls of the borehole and distributing the load evenly over a length of roughly 100 mm. Naturally you should try to place the screw in zones where the stone appears harder and less cracked — but in many cases the concrete screw is a genuinely good option: very easy to install, with a load capacity that is a pleasant surprise.

3. Wedge anchor

The wedge anchor is a fairly standard solution if you read articles about rock climbing and abseiling in the United States or Europe. Despite that, in most of the pull-out tests I have carried out on wedge anchors in stone walls, the results ranged from disappointing to very disappointing. The likely reason is that in a wedge anchor all the stresses are heavily concentrated around the expansion clips — as against, for example, the concrete screw, where the thread engages over some 100 mm of length. This concentrated local pressure leads to rapid failure and splitting of the stone.

Additional guidance and common questions

What do you do if you find hidden voids inside the stone while drilling?

When you hit internal voids in the stone, a mechanical anchor will fail, because there is no continuous surface for it to expand against. Even ordinary chemical anchoring becomes complicated, because the adhesive can flow away into the void instead of enveloping the threaded rod. The professional solution in this case is to use a mesh sleeve designed for chemical anchoring in hollow materials. The sleeve is inserted into the borehole and holds the adhesive in place, while allowing it to squeeze out through the holes in the mesh to form a strong mechanical key.

Why does distributing the load matter so much in natural stone walls?

Unlike reinforced concrete, natural stone may contain natural voids, microscopic cracks and weaknesses that cannot be seen. Concentrating a high load on a single anchor can cause the stone to break locally around the anchoring point. Spreading the load across several anchors, spaced apart from one another, together with increasing the embedment depth to create a larger bond area, is what gives the system its stability.