Which blocks are better for anchoring stone cladding?

Block types and stone cladding anchors — what you need to know before you build

On many sites, the developer or the contractor chooses the type of block on the basis of thermal insulation, acoustics or cost — without any thought for what it will mean for anchoring the stone cladding later. Then, at the cladding design stage, it often emerges that the blocks chosen are too soft to anchor into, or that expensive adjustments are needed — more fixings than standard, for instance. What was saved at the block stage gets paid back at the cladding stage.

Below is a survey of the block types common in Israel, their technical characteristics, and what each one means for anchoring stone cladding.

Quick comparison

Type of block	Compressive strength	Typical pull-out load (10 mm plug)	Suitability for anchoring
Ytong / aerated block, 4 MPa	4 MPa	Over 600 kg	Excellent
Ytong / aerated block, 2–3 MPa	2–3 MPa	Up to 150 kg	Not recommended
Hollow concrete block	5–10 MPa	160–300 kg	Good
Pumice block	Varies widely	100–340 kg	Varies — must be tested
Gypsum block	—	160–250 kg	Good

Ytong block (aerated block — AAC)

Ytong (autoclaved aerated concrete, AAC) is made from steam-cured aerated concrete. In Israel it is also sold as Ytong and Siporex. It comes in various densities — 400, 500 and 600 kg/m³ — corresponding to compressive strengths of roughly 2, 3 and 4 MPa respectively.

For thermal insulation, Ytong is outstanding: its lambda value (thermal conductivity) is around 0.09–0.14 W/(m·K) depending on density, against ordinary concrete block, whose conductivity is several times higher. The lighter blocks (400 kg/m³) insulate better — but they are weaker to anchor into.

Other advantages: easy to work (sawing, milling), light in weight, high fire resistance (up to 4 hours), and relatively low moisture absorption compared with other lightweight blocks. Main drawback: at low density, anchoring performance is poor, and ordinary mechanical anchors cannot be used — anchors made specifically for Ytong are required.

Hollow concrete block (the standard building block)

The most common block in Israel. Made from compacted concrete, usually with internal voids (hollow block), in thicknesses of 150, 200 or 250 mm. Typical compressive strength: 5–10 MPa; density around 1,100–1,400 kg/m³ for a hollow block.

For thermal insulation, hollow concrete block is significantly inferior to Ytong — lambda around 0.7–1.0 W/(m·K). An external insulation layer is therefore sometimes needed, and that has to be taken into account when designing the stone cladding fixing system.

For anchoring: a quality plug of 10 mm diameter and suitable length, installed 60–80 mm into the solid web of the block, will hold 160–300 kg. Important: make sure the plug is driven into the solid web of the block, and not into the void inside it.

Pumice block

Pumice block (scoria stone) is made from lightweight aggregates of volcanic stone. It comes in different grades — light pumice (density 700–900 kg/m³) and heavy pumice (1,000–1,200 kg/m³). The central problem is that the quality of pumice varies enormously from supplier to supplier, and even from batch to batch.

In site tests using the same quality of plug: one pumice block held no more than 100 kg, while a pumice block that looked identical came back with 340 kg in tension. The difference came down entirely to the density and quality of the pumice.

Gypsum block

A gypsum block is made of solid gypsum — typically 70, 100 or 120 mm thick — and is used mainly for internal walls and partitions. There is also a moisture-resistant version (usually coloured green) suitable for bathrooms and kitchens.

For anchoring: there is no difficulty fixing into gypsum block. A quality plug of 10 mm diameter, installed 60–80 mm into the block, will hold 160–250 kg in tension — good results for anchoring stone cladding on internal walls.

An important limitation: ordinary gypsum block is not suitable for prolonged exposure to moisture. For external use, or in damp conditions, use only the moisture-resistant (hydrophobic) version.

Termolite and other special blocks

The Israeli market also offers Termolite blocks — lightweight concrete blocks with thermal aggregates that improve the insulation. Their anchoring performance is similar to hollow concrete block, but you should check the specific compressive strength from each supplier. There are also insulated concrete blocks (sandwich blocks) with a layer of polystyrene embedded inside them — here the drilling must pass through the insulation layer and grip the concrete beyond it.

The right material for the plug matters just as much

Once you have chosen the right block, make sure the plug you use for the stone cladding is made from polyamide (PA6) — not ordinary plastic (PP), which breaks down over the years when exposed to sun, heat and cold. A European approval for plugs guarantees quality and a service life of over 20 years. Plugs with no European approval can disintegrate within just a few years outdoors.

In summary — what to do before you build

The key principle: cheap, low-density blocks save money at the construction stage — but cost more at the stone cladding stage, both in the extra fixings required and in problems later on. Getting the decision right up front saves money and avoids trouble.