100mm Aerated Concrete Blocks: The Inner Leaf Workhorse Explained

Use the wrong block below your damp-proof course and your building control inspector will ask you to cut open completed work. Use dense blocks above it for the inner leaf and your wall's thermal performance will fail Part L by a factor of ten. Aerated blocks are the right choice for the inner leaf above DPC -but only if you understand what that means, and what happens the moment you stray outside that brief.

What they are and what they're for

Aerated concrete blocks go by several names on UK building sites and in merchants' catalogues: aircrete, Thermalite (Forterra's brand), Celcon (H+H), Toplite (Tarmac), and occasionally just "lightweight blocks." They're all the same product: autoclaved aerated concrete (AAC), made from cement, lime, water, pulverised fuel ash (PFA), and a tiny amount of aluminium powder. The aluminium reacts with the other ingredients to produce microscopic hydrogen bubbles before the block is autoclaved (steam-cured at high pressure), leaving a rigid cellular matrix full of tiny air pockets.

Those air pockets are the whole point. They drop the density from around 2000 kg/m3 (dense aggregate concrete) to approximately 600 kg/m3, which cuts thermal conductivity by almost ten times and reduces block weight from 19-20 kg (dense) to around 7 kg at typical delivered moisture content. A full shift of aerated blocks weighs roughly half what a shift of dense blocks weighs. Bricklayers work faster, their backs survive longer, and cutting at corners and openings takes seconds rather than minutes.

The UK governing standard is BS EN 771-4:2011+A1:2015, which covers autoclaved aerated concrete masonry units. Any block sold through a reputable UK merchant will carry a UKCA/CE marking and a Declaration of Performance showing the tested values for that specific product.

Specifications and variants

Standard dimensions are 440 x 215 x 100mm. With a 10mm mortar joint the working module is 450 x 225mm, giving exactly 10 blocks per square metre of wall face.

Two strength grades matter for domestic work:

For a standard single or two-storey domestic extension, standard 3.6N blocks are the correct product for the inner leaf. Building Regulations Approved Document A confirms that 3.6 N/mm2 aerated blocks are acceptable for inner leaf construction in domestic buildings up to two storeys. If your structural engineer's drawings specify "3.6N aerated" you want the standard grade. If they say "7.3N" anywhere, that's Hi-Strength 7 -a different block.

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Why thermal conductivity matters

The design thermal conductivity of a standard aerated block is 0.15 W/mK. A dense block runs at 0.95-1.33 W/mK. That difference is not marginal. An inner leaf of aerated blocks contributes meaningfully to meeting the Part L U-value requirement for extension walls (typically 0.18 W/m2K or better). An inner leaf of dense blocks contributes almost nothing thermally, meaning you'd need much thicker insulation board in the cavity to compensate. In practice, dense blocks on the inner leaf make Part L compliance difficult without adding at least 100mm of PIR insulation board. Your thermal calculations will have been done assuming aerated inner leaf blocks. Don't substitute dense without asking your structural engineer and building control first.

How to work with aerated blocks

Cutting

This is the biggest practical advantage of aerated blocks. You can cut them with a standard large-tooth hardpoint handsaw. Not a power saw, not an angle grinder, just a saw. The block cuts cleanly and quickly. A closing cut at a window reveal that would take two minutes with a disc cutter on a dense block takes 20 seconds with a handsaw on an aerated block.

For rough cuts where precision isn't critical, a bolster chisel and lump hammer will split a block cleanly along a marked line. Score around all four faces, then strike along the line. Useful for coursing adjustments where you just need a bit trimmed.

Dust from cutting aerated blocks is still fine silica dust. Wear a P3 dust mask. Outdoors where possible.

Mortar mix

Get this wrong and you'll see cracks within months. Aerated blocks expand and contract with temperature changes more than the mortar does. If you use a strong cement-rich mortar (4:1 sand:cement), the rigid mortar can't flex with the blocks, and the blocks crack at the bed joints or the plaster applied over them cracks.

The correct mix for aerated blockwork is a weaker 1:1:6 (cement:lime:sand) or 1:6 (cement:sand with plasticiser). Thermalite's own guidance recommends 5:1 to 6:1 sand:cement. Bricklayers used to laying dense blocks with a standard mix can forget this. If your block schedule shows aerated inner leaf, confirm the mortar mix with your bricklayer at the start.

Weight and handling

Each block weighs approximately 7.0-7.6 kg at typical delivered moisture content (Forterra's datasheet records 5.8 kg at 3% equilibrium moisture, but you'll handle them wetter than that). Compared to 19-20 kg for a dense block, the handling benefit is substantial. Working at height on scaffolding, this matters both for safety and laying speed.

Aerated blocks are brittle. They're not fragile, but they don't like being dropped or struck. Don't throw them from a pallet. Dropped blocks crack.

Laying at height

Aerated blocks are the standard choice for upper-course inner leaf work precisely because of the weight advantage. A bricklayer laying inner leaf aerated at first floor level is handling roughly 35% of the load of equivalent dense blocks. This makes a real difference across a week's work.

The below-DPC rule

Standard 3.6N aerated blocks must not be used below DPC in external walls. This is the most commonly confused rule in domestic blockwork.

The reason is density. PD 6697 recommends that blocks used below DPC in high-risk saturation and freeze-thaw conditions have a net density of at least 1500 kg/m3. Standard aerated blocks have a density of 600 kg/m3 -well below this threshold. In wet ground conditions, the blocks can absorb water and degrade, and in freeze-thaw cycles the absorbed moisture expands and fractures the cellular structure.

Building control inspectors make a clear distinction: standard aerated wall blocks (not acceptable below DPC in external walls), versus foundation-grade or trench blocks (which are specifically engineered for below-DPC use and can be acceptable in certain soil conditions). H+H Celcon's Foundation Block and similar products have higher density and different mix characteristics for this application.

For standard extension construction, the rule of thumb is simple:

• Below DPC: dense aggregate blocks (see 100mm Dense Concrete Blocks) or engineering bricks
• Above DPC, inner leaf: standard 3.6N aerated blocks
• Above DPC, outer leaf: facing bricks or dense aggregate blocks

A building inspector in a BuildHub thread put it plainly: "I would accept 3.6N trench blocks but I would NOT accept 3.6N standard blocks below DPC." The blocks look similar. The application isn't.

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Site storage

Aerated blocks absorb moisture readily. Water absorption is 20-30% by weight, compared to 5-10% for dense aggregate blocks. Blocks delivered and left uncovered in rain will absorb several kilograms of water each. Wet blocks are heavier to handle and, more importantly, cause problems if they're laid wet and then dry out -the drying shrinkage can cause cracking at bed joints.

Store blocks on pallets, above ground level, covered with polythene sheeting. Don't stack pallets more than two high. Inspect deliveries for damaged blocks (cracked or corner-broken units should be returned or set aside for non-structural use). If blocks have been rained on, allow them to dry before laying where possible, and don't lay in freezing conditions.

How many do you need

10 blocks per square metre is the standard figure, accounting for the 450 x 225mm working module (440mm block + 10mm mortar joint on each dimension).

For a typical single-storey rear extension with cavity walls:

• Measure the total inner leaf wall area (perimeter x wall height, less window and door openings)
• Multiply by 10 to get your block count
• Add 10% for wastage, cuts at corners, and closer blocks at window reveals
• Order in whole packs -typically 60 or 120 blocks per pack

Worked example: A 4m x 6m extension with 2.4m wall height and two window openings each 1.2m x 1.0m.

• Wall perimeter (inner leaf): (4 + 6 + 4 + 6) = 20m, but three external sides = 16m (the fourth side is the house)
• Wall face area: 16m x 2.4m = 38.4 m2
• Deduct windows: 2 x (1.2 x 1.0) = 2.4 m2
• Net area: 36.0 m2
• Blocks needed: 36.0 x 10 = 360 blocks
• Add 10% waste: 360 x 1.1 = 396 blocks
• Order: 400 blocks (4 packs of 100, or similar)

For a 4m x 6m extension, expect 400-500 inner leaf aerated blocks in total depending on wall height and number of openings. The brief's range of 400-600 blocks covers typical variation well.

Cost and where to buy

Aerated blocks are priced at £1.50 – £2.50 per block (ex VAT). Trade accounts and bulk packs bring the price toward the lower end. Trade accounts and bulk pack pricing brings the cost toward the lower end of that range. Single blocks from a small independent at retail pricing sit near the top.

The brands you'll encounter:

• Thermalite Shield (Forterra) -the most widely available. 3.6 N/mm2, 600 kg/m3, 0.15 W/mK design lambda. Available through Jewson, Travis Perkins, Buildbase, and most independent merchants.
• Celcon Standard Grade (H+H) -identical specification to Thermalite Shield. The two brands are interchangeable in practice. H+H also manufactures Celcon Foundation Blocks for below-DPC applications.
• Toplite Standard (Tarmac) -same specification again. Less common but stocked at some Jewson branches.
• Mannok Aircrete and Ytong -compliant alternatives, less commonly stocked at UK builders' merchants.

You won't always get to choose which brand arrives. All compliant 3.6N aerated blocks to BS EN 771-4 perform identically for standard inner-leaf work. Don't pay a premium to specify a particular brand.

Pack sizes vary: 60, 100, or 120 blocks per pallet depending on merchant. Buying by the pack is always cheaper per block than buying individually. Delivery costs for palletised blocks add up quickly -a 120-block pallet weighs roughly 840-900 kg. Local collection or large-volume delivery from a nearby merchants will usually be cheaper than online retailers with national delivery fees.

Fixing to aerated blocks

The most common frustration with aerated blocks is fixings pulling out. Standard nylon wall plugs designed for dense concrete or brick are inadequate in aerated blocks. The material is too soft and porous -a standard plug will spin in the hole and never grip properly.

The correct approach depends on load:

Light loads (picture frames, small shelves): purpose-made AAC screw anchors such as Fischer Termoz PN 8, or Plasplugs Thermal Block Fixings. These have a longer, wider thread profile that bites into the cellular matrix. Always use a slightly undersized pilot hole -if the specification says 7mm, try 6.5mm. The snugger fit dramatically improves pull-out resistance.

Medium loads (wall cabinets, towel rails, radiator brackets): mechanical expansion anchors rated for AAC, minimum 50mm embedment depth. Longer fixings always outperform shorter ones in aerated blocks. Never use hammer drill action when drilling -the vibration shatters the soft material around the hole before the plug has a chance to grip.

Heavy loads (TV brackets above 30kg, boiler brackets, structural fixings): chemical resin anchors are the only reliable solution. Drill the hole slightly oversized, clean it thoroughly with a wire brush and compressed air (the dust and debris must be removed or the resin won't bond to the block), inject the resin, and allow to cure before loading. This is the approach experienced tradespeople use for anything above medium load in aerated blocks.

Concrete screws (specialist masonry screws such as Fischer Ultracut or Rawlplug R-LX) are a good alternative for medium loads -drill a pilot hole slightly smaller than the screw thread, no plug required. The thread cuts directly into the block material and holds well.

RAAC vs aircrete: not the same thing

Since 2023, news coverage of Reinforced Autoclaved Aerated Concrete (RAAC) roof panels failing in schools and public buildings has made many homeowners nervous about "aerated concrete" in their walls. The two products are completely different.

RAAC is a panel product manufactured between the 1950s and 1980s, used in roof decks and floor slabs. It's reinforced with steel bars that have been corroding for decades. It was never used as a walling block.

Standard aerated building blocks (Thermalite, Celcon, Toplite) are an entirely different product. They've been the standard UK housebuilding inner leaf block since the 1980s, they're load-bearing masonry units, and they have no steel reinforcement to corrode. If your walls contain Thermalite-type aerated blocks, they are not RAAC.

Alternatives

Dense concrete blocks (100mm) -the correct choice below DPC and for the outer leaf. Ten times the thermal conductivity of aerated blocks, which makes them a poor choice for inner leaf above DPC where Part L compliance matters. Full comparison at 100mm Dense Concrete Blocks.

Hi-Strength 7 aerated blocks -same material, 7.3 N/mm2 compressive strength, 9.1 kg weight. Required for buildings above 2.5 storeys or where your SE specifies higher strength. Available from the same manufacturers; slightly more expensive per block.

140mm aerated blocks -wider blocks for solid wall construction without a cavity, or where additional thermal mass is needed. Less common in cavity wall extensions.

Where you'll need this

Aerated blocks appear in the structure phase of any masonry extension project:

• Walls and blockwork -inner leaf of cavity walls above DPC, throughout the full height of the wall. Approximately 400-600 blocks for a typical 4m x 6m single-storey extension inner leaf.

Common mistakes

Using standard aerated blocks below DPC. The block looks identical to a trench block. The specification is completely different. If your drawings show aerated blocks for the inner leaf, confirm with your bricklayer that the below-DPC courses are dense blocks or foundation-grade aerated, not standard wall blocks. Building control inspectors know to look for this.

Too-strong mortar mix. The most common cause of cracking in aerated blockwork, and it's entirely preventable. Confirm 1:1:6 or 1:6 with plasticiser before any laying starts. It's a five-minute conversation that prevents a problem discovered six months after practical completion.

Skipping the wall plate check. Where roof timbers bear on the top of an inner leaf wall, the wall plate must sit on dense blocks or a concrete padstone, not directly on aerated blocks. Aerated blocks can crack under concentrated point loads. This is a common building control defect. See your structural engineer's drawings for the specification.

Wet blocks laid without drying. Blocks stored uncovered and rained on can absorb enough moisture to cause shrinkage cracking as they dry out after laying. Cover deliveries. If blocks are visibly wet, stack them to dry before use.

Using hammer drill action when fixing. This is a finishing-phase issue, not a construction issue, but it costs money. Hammer drill action destroys the fixing point. Every time.