Laying AAC blocks with thin-bed mortar
AAC blocks leave the factory cut to millimetre accuracy — and that accuracy is what allows a 2–3 mm adhesive bed instead of 10–20 mm of conventional mortar. It cuts both ways: an AAC wall is never truer than its first course, and a thin bed has no thickness to hide a mistake in. The whole wall comes out of how the first course is set.
The gain is not only speed: thin joints mean fewer thermal bridges (conventional mortar is a conductor slicing the block's insulation every 20 cm), and consumption around one fifth of conventional mortar for the same wall.
The first course: conventional bed, merciless levelling
The first course alone is laid on a 1–3 cm conventional cement mortar bed over the damp-proof course — that bed is what swallows the slab or beam's level errors so they never travel upward. Set the end blocks with a level in both axes, stretch a line between them, fill the course to it. Let the first course set 12 hours before building on it — laying onto a soft bed shifts the hour of levelling you just did.
The thin bed: a scoop the width of the block
From the second course the thin-bed takes over: spread it with a notched scoop the exact width of the block (4–6 mm notch) for a fully covered, evenly thick bed. Perpend joints get mortar too, buttered on the block's end before placing — except with tongue-and-groove blocks, where they are omitted by design. Each block is placed and tapped down with a rubber mallet to the line, and every block is levelled, not every course.
Brush every course free of dust before spreading the next — sawing dust is the same bonding enemy as in the preparation guide. AAC is never soaked like clay brick; dusting off is enough, with a light mist on the bedding face only in severe heat.
Bond and cutting
Half bond like brickwork: an offset of at least a third of a block length (half is better) between the joints of successive courses. Cut with the AAC hand saw or a band saw — a straight cut keeps the joint thin; splitting with a chisel leaves a ragged face that needs thick mortar and defeats the whole system. Avoid pieces shorter than 10 cm by sharing the cut between both ends of the course.
Tying to the frame, and the head-of-wall gap
Partition walls tie to concrete columns with flexible metal ties every two to three courses (~60 cm), fixed to the column and bedded into the joint — a rigid mortar-and-scabbling connection cracks with the frame's first movement. And the wall head is never wedged tight to the slab: leave a 15–20 mm gap filled with PU foam or a compressible strip, because beams deflect over time (creep) and will bear on your wall if you leave them nowhere else to go.
Local reinforcement: under every window opening, bed joint reinforcement (two bars or ladder mesh) into the course carrying the sill, extending at least 50 cm past each side — sill corners are the first place AAC walls crack. Openings take proprietary or cast lintels bearing 20–25 cm each end.
After laying: the render
AAC is highly absorbent — before any render it needs a suction-regulating primer or the wall drinks the render's water and the face crumbles (the substrate guide details the test). Indoors, gypsum plaster goes straight over the primer; wherever two materials meet (block against a concrete column), bed a 20 cm strip of fibre mesh into the render across the junction line.
The AAC mistakes
- An unlevelled first course — the whole wall inherits it.
- Building on the first course before its bed has set.
- Empty perpends in plain-ended blocks: sound paths, air paths, crack paths.
- Soaking the blocks like clay brick.
- Wedging the wall head tight under the beam with no compression gap.
- No sill reinforcement — the diagonal crack off the window corner.
- Rendering straight onto the raw, thirsty face with no primer.
Frequently asked
Does thin-bed mortar work with clay brick or concrete block?
No — the system depends on AAC's cutting accuracy (±1–2 mm). Clay brick and concrete block, with their tolerances, need conventional mortar thick enough to swallow the variation.
How soon can I render a new AAC wall?
Thin-bed joints set fast, but give the wall two to three days to settle and shed its construction moisture, then primer and render. A rained-on wall waits until its face is fully dry.
How do I chase services into AAC?
With a hand router or chasing tool — AAC cuts easily. The rule: chase depth never exceeds a third of the wall thickness, and long horizontal chases in thin partitions are avoided because they sever the wall's cross-section.
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Last updated: 2026-08-09