Key takeaways
- Autoclaved aerated concrete is porous, so it loses strength and gains weight when it takes on water, and keeping the blocks dry from the yard to the finished render is the first job on the wall.
- The thin-bed adhesive the manufacturer specifies is part of the wall system, and a general mortar pushed into the joints changes how the wall behaves and how the render bonds to it.
- Accuracy in the first courses decides how much grinding and filling the rest of the wall will need, because the thin joint can only correct a bed that was laid level in the first place.
- Render and movement details have to suit aerated concrete, which moves and dries differently from brick, and most cracking appears where that difference was ignored at the junctions.
- The real cost of a wet wall is a re-render, a cracked head and a dispute about who caused it, which is why the moisture decisions belong in the programme from the start.
Autoclaved aerated concrete is one of the easiest masonry products to lay. The blocks are light, they cut with an ordinary saw, and a competent crew can build a wall faster than in brick or standard block. That speed is the reason the material is popular, and it is also the reason the mistakes are made. The blocks are forgiving of the cutting and the handling, but they are not forgiving of water, and the decisions that matter most are made before the first block is set.
The problem is that most of what goes wrong on an aerated concrete wall is not visible when the wall goes up. A pack left in the rain, an adhesive swapped for a general mortar, a first course that is a little out of level, or a render put on before the wall has dried will all look fine on the day. The failure shows months later as a hairline crack at a window head, a patch of render that lifts or a damp mark on the inside of the wall. By then the cost is a re-render and an argument about who caused it.
This article covers the handling of the blocks, the laying with thin-bed adhesive, the protection of the wall from water, and the render and moisture details that are specific to aerated concrete. It is not a general guide to brickwork, and the principles of masonry movement and ties in brick are covered elsewhere. The differences here are the ones that a builder who knows brick is most likely to miss.
Why aerated concrete behaves differently from brick
Autoclaved aerated concrete is a cellular material. Its structure is full of small air pockets, which is what makes it light and gives it its insulating properties. The same porosity that makes it light makes it absorbent. A block that takes on water gains weight, its strength drops, and it dries slowly from the inside. A wall built from wet blocks is therefore weaker and heavier than the designer assumed, and it will continue to change as it dries.
That drying matters for the render. Aerated concrete shrinks as it loses moisture and expands with the temperature and with the moisture it picks up. Brick moves much less in the same conditions. A render that bonds well to a stable brick face can be pulled apart by an aerated concrete wall that is still drying, particularly at the points where the wall meets a different material. The mechanism is simple. The wall is moving, the render is not, and the join between them takes the strain.
Use the manufacturer’s data for the specific block, because the figures for strength, absorption and movement differ between products.
Handling and storing the blocks before they go up
The blocks arrive on pallets in packs, usually wrapped, and the wrapping is the first line of protection. It is also the thing most often torn off on site and left in the yard. Keep the packs wrapped, keep them off the ground and keep them out of the reach of runoff. A pack that sits on a wet slab for a week will take on water through the bottom course, and the water will not leave it quickly.
Handle the blocks as if they were fragile, because the corners and edges chip easily. A chipped corner is not a cosmetic problem. It is a weak point in the wall and an uneven joint at the next course, and a block with a broken face should be cut out and set aside. Use the lifting method the manufacturer recommends, and do not stack packs higher than the pallets are designed to carry. Our article on moisture in new construction materials covers the wider drying question, and the same discipline applies to the blocks on site.
Protecting the blocks before and after they go up
- Packs are kept off the ground on timber bearers and covered at every stage between delivery and laying.
- No pack is stood in water, left in a puddle line or stored where runoff from the slab can reach it.
- Blocks are not laid when they are wet, and any block that has taken on water is set aside to dry.
- The roof and the wall wrap or cladding are closed in as soon as the masonry reaches the top of the wall.
- Openings and the top of each lift are covered at the end of every day and before any rain.
- Chipped and damaged blocks are cut out and not laid, because a damaged face weakens the bond.
Cutting aerated concrete without making a mess of the wall
Aerated concrete cuts easily, and that makes it tempting to cut every block to size on the wall with whatever tool is nearest. A fine-toothed saw or a rasp gives a clean edge. A coarse saw or a hammer gives a ragged one, and a ragged edge takes more adhesive, leaves a thicker joint and is harder to keep plumb. Cut the blocks on the ground where you can, and cut them to a shape you can lay without forcing.
The dust from cutting is the other consideration. It settles on the work, gets into the adhesive and gets into the lungs of whoever is standing downwind. Control it at the source, with the methods your work health and safety plan calls for, and keep the cutting area clear of the people who do not need to be in it.
Laying with thin-bed adhesive, not general mortar
The most common mistake on aerated concrete walls is laying the blocks in a general mortar. The crew knows how to lay with mortar, the mortar is on the truck, and the wall goes up quickly. The problem is that the joint is designed to be thin, and a thick mortar joint changes the way the wall carries load, the way it moves and the way the render bonds to the face. It also adds water to the wall at the joints, which is exactly the thing the wall cannot take.
The thin-bed adhesive is part of the wall system. It is mixed to the manufacturer’s instructions, spread with the tool the manufacturer specifies, and applied in a thin, even bed. Use the adhesive the system was designed for, and follow the data sheet on working time and on how long the adhesive should be left before the next course goes on. Do not thin the adhesive with water to make it spread more easily, because that changes its strength and its bond.
Laying the wall in thin-bed adhesive
- 1Set and check the first courseBed the first course on a level, flat base and check it with a straightedge before anything goes on top.
- 2Use the adhesive the system is designed forMix the manufacturer’s adhesive exactly as the data sheet describes, and do not substitute a general mortar.
- 3Spread the bed on each blockSpread the adhesive with the tool the manufacturer specifies and keep the joint as thin as the system allows.
- 4Check plumb and level as you goCheck each course against the line, the corners and the openings before the adhesive begins to set.
- 5Let the wall settle before fixingLeave the wall long enough to settle before any heavy fixing or lateral load goes on it.
The first course is the one that decides the rest of the wall. The adhesive joint is thin enough that it can only correct small variations, so a bed that is out of level will carry that error up the whole wall. Set the first course on a flat, level base, check it with a straightedge, and correct it before anything goes on top. A wall that is accurate at the base is much easier to keep plumb, and much less work to grind and fill later.
Protecting the wall from water while it goes up
An aerated concrete wall is exposed to the weather for as long as it takes to build, and the exposure is longer than many programmes allow for. The safest approach is to get the roof on and the wall wrapped or clad as soon as the masonry reaches the top of the wall. Water that runs down an open top course or into an unfinished opening will be absorbed by the blocks and carried into the wall, and it will stay there.
Cover the top of each lift at the end of every day, and cover any opening that is not yet framed. A blocked gutter, a wind-blown tarp or a gap at the top of the wall can put a great deal of water into the blocks in a single storm. The moisture then has to leave the wall through the render, and that takes time the programme may not have allowed for.
The moisture decisions belong in the programme from the start. Put the roof, the wall wrap and the cover in the sequence before the masonry begins, and make the sequence part of the crew’s instructions. A decision to leave the top open for a few days to suit the carpenters is the decision that produces the wet wall, and the cost arrives long after the carpenters have moved on.
Fixings, services and the junctions that move
Aerated concrete takes fixings differently from brick. The block is light and porous, and a fixing that holds well in a dense material can pull out of the cellular structure under load. Use the fixing system the manufacturer specifies for the load and the material, and do not assume a standard plug will do the job. Heavy items such as wall-mounted cabinets, water heaters and balustrades need particular care, and they need the right fixing rated for the block, not the one in the toolbox.
Chases and penetrations are the other place where the wall loses its integrity. Cutting a chase for a pipe or cable weakens the block and exposes its porous face, and the cut is a path for water if it is not sealed and finished. Make the chase as small as the service allows, and seal and render it before the cavity or the internal finish goes on.
The junctions where aerated concrete meets other materials are where the movement shows. A wall that meets a concrete slab, a brick wall or a timber frame will move differently at that junction, and the render will crack along the line of difference if the movement has not been provided for. Control joints and movement details belong in the design, and they are covered more fully in our article on brickwork movement joints and ties. The principle carries over to aerated concrete: the wall moves, so the joint has to allow for it.
Render and finish: the details that decide the callbacks
The render on an aerated concrete wall is a system, not a single coat of material. A bonding coat, a reinforcing mesh and a base coat are usually specified together, and they are designed for the substrate and for the way the substrate moves. Substituting a different render because it is cheaper or because the rendering crew prefers it is one of the quickest ways to create a bond failure. Check the system with the render manufacturer before it is applied, and keep the record of that check.
Timing matters as much as the product. The render should go on only when the wall has dried to the condition the render manufacturer specifies. A render applied to a wet wall traps moisture behind it, and the moisture cannot escape. The result is blistering, lifting and a finish that looks sound until the first hot dry week. Measure the wall, do not guess, and follow the manufacturer’s stated condition before the render is applied.
Mesh and the joints in the render need the same attention as the wall beneath. Reinforcing has to be lapped and embedded as the system requires, and the render has to be carried over the movement joints rather than bridging them. A render that bridges a joint will crack at the joint, and the crack will show through the finish as a straight line that the client sees every day.
The mistake that costs the most. Rendering or painting a wall that is still wet, because the programme needed the wall finished before the next trade. The moisture that is sealed in comes out later through the render, the paint or the plasterboard, and the re-work costs more than the time that was saved.
A worked example: the rear addition in wet weather
Consider a builder in western Sydney who was building a rear addition from aerated concrete blocks over a wet autumn. The blocks were delivered in advance and stacked on the slab during a run of rain. The packs were uncovered for several days, and the bottom courses were laid from the wet packs because the crew was behind. The adhesive was a general mortar that the labourer had brought from another job, and the first course was set with a small fall that nobody checked.
The wall went up quickly, and the roof was closed in only after the render crew had been booked. The render was applied in the next dry spell, while the wall was still drying. Within the first summer, a hairline crack opened at one window head, where the render met the lintel, and a patch of render lifted near a downpipe. An inspection found the bottom courses holding moisture, and the first course was visibly out of level in the corner.
The builder had to strip the affected render, grind and fill the low course, and re-render the wall after it dried to the condition the manufacturer specified. The render manufacturer declined a warranty claim because the wall had been rendered wet, and the builder absorbed the cost. Every decision that caused the problem was made to keep the programme moving. None of them looked dangerous on the day.
What to check before the render and paint go on
Before the render crew arrives, walk the wall with the render manufacturer’s condition in hand. Check the wall for moisture by the method the manufacturer specifies, check that the first course and the corners are accurate, and check that the movement joints are in place at the junctions. Confirm the mesh and the bonding coat are the products the system specifies, and that the crew has been told the sequence. The check takes less time than the rework.
Keep the record of the check. A dated note with the moisture readings, the condition of the wall and the names of the people who signed it off is the best evidence the builder has if a render or finish defect appears later. Where the work is insured through the Home Building Compensation Fund, the record also matters for any defect claim, so keep it with the building file and not in someone’s phone.
Where the responsibility sits
The builder who controls the job usually owns the sequence, the handling and the choice of system, even when the masonry is subcontracted. Put the adhesive, the protection, the render system and the moisture check into the subcontract, and make the subcontractor sign that they have been seen. A subcontractor who lays in a general mortar because it is what they have always used should be told before the wall goes up, not after the crack appears.
Waterproofing problems in wet areas are more serious than most cosmetic defects, and a waterproofing problem could be classified as a major defect. Treat any moisture ingress in a wet area as serious from the first sign, and do not assume that a cosmetic fix will do.
Closing the wall properly
Aerated concrete rewards a builder who respects the material. Keep the blocks dry from the truck to the wall, lay them in the adhesive the system was designed for, get the first course right, close the wall in before the weather finds it, and render only when the wall has dried to the condition the manufacturer requires. Each of those steps is simple. Skipping any of them is how the callbacks start.
Handling and moisture are the parts of the job that most often decide whether a wall comes back to the builder for repair. The PointCPD home page lists the NSW builder courses we offer for builders who want to keep their own learning current.
Questions builders ask
Can I lay autoclaved aerated concrete blocks in general mortar?
Not as a substitute for the adhesive the manufacturer specifies. The thin-bed system is designed as a whole, and a general mortar changes how the wall carries load, moves and takes the render. Check the data sheet and use the adhesive the system calls for.
What happens if aerated concrete blocks get wet before they are laid?
A wet block gains weight, loses strength and dries slowly from the inside, so it can move as it dries and carry moisture into the wall. Keep the packs covered and off the ground, and set aside any block that has taken on water until it has dried.
When can I render an aerated concrete wall?
Only when the wall has dried to the condition the render manufacturer specifies, and the render has to be the system the manufacturer designed for the substrate. Measure the moisture by the method the manufacturer sets out rather than guessing from the weather.
Do I need movement joints in an aerated concrete wall?
Yes, at the places where the wall meets a different material or where movement is expected. The render has to be carried over the movement joint rather than bridging it, or it will crack along the line of the joint.
Can I cut aerated concrete blocks on the wall with any saw?
A fine-toothed saw or a rasp gives the clean edge the thin joint needs, and a coarse tool gives a ragged edge that is harder to keep plumb. Cut as much as you can on the ground, and control the dust with the methods your work health and safety plan calls for.
Who is responsible if the render cracks on an aerated concrete wall?
The builder who controls the job usually answers for the sequence, the handling and the choice of system, even where the masonry or render is subcontracted. Keep a dated record of the moisture checks, the products used and the sign-offs, because that record is the evidence if a defect claim follows.
Sources
- Continuing professional development for builders and swimming pool builders · NSW Government
- General building work licensing · NSW Government
- Home Building Act 1989 (NSW) · NSW Legislation
Published 25 September 2026. General information for builders, not legal, financial or engineering advice for a specific job. Check current requirements with the relevant regulator or standard before relying on them.
