Which type of concrete block do I need — dense, medium or aerated?

Blocks all look similar on a pallet, and they behave very differently. The property that matters most in practice is density: it drives the strength, the weight, the thermal conductivity and, crucially for a householder, whether an ordinary wall plug will hold a shelf in it. That last point is the one people discover after the wall is built and plastered.

Step by step

1. Establish whether it is load bearing.A block in a load-bearing wall has a required compressive strength, and that is a designed figure. If it carries a floor, a roof or a wall above, get the strength from the design and buy to it, not to what the merchant has in stock.
2. Decide what you need to hang on it afterwards.This is the question people forget. Kitchen units, wall-hung basins, radiators, TVs and boilers all impose real loads. Aerated block will not hold an ordinary plug, and though purpose-made fixings and resin work well, they have to be planned for.
3. Weigh thermal performance against strength.The lighter and more aerated a block is, the better it insulates and the weaker and softer it is. In a modern cavity wall the inner leaf is often a thermal block precisely for this reason, and the fixing problem is accepted and designed around.
4. Think about the lift.A dense solid block is a heavy repeated lift and the source of a great many back injuries. There are weight thresholds above which blocks should be handled by two people or with mechanical aid. Aerated blocks are dramatically lighter for the same face size.
5. Use dense blocks below ground.Below the damp proof course, in trench fill, and in anything retaining, use a block specified for it. Aerated blocks are not generally suitable below ground unless specifically rated.
6. Match the block thickness to the wall, not the face size.Face dimensions are standard, so the count per square metre is the same whatever the thickness. What changes is strength, weight, thermal performance and price.
7. Cut aerated blocks with a saw.One of their real advantages. A coarse hand saw or a purpose-made block saw cuts them cleanly with no dust cloud and no grinder. Dense blocks need a bolster or a wet-cut disc.
8. Let blockwork dry before plastering.Blocks arrive wet and shrink as they dry. Blockwork plastered too soon shrinks behind the plaster and cracks it, usually along the joints.

Alternatives

• Dense aggregate block: Made with dense aggregate, heavy, strong, and the choice for load-bearing work, below ground, retaining structures and anywhere a lot of load or a lot of fixings are involved. Poor thermal performance and hard on the back — a solid dense block is a serious lift.
• Medium-density / lightweight aggregate block: Made with a lighter aggregate. A compromise: reasonable strength, moderate weight, better thermal performance than dense, and it takes ordinary fixings reasonably well. A very common inner-leaf and partition block.
• Aerated / autoclaved aerated concrete block: Very light, easily cut with a hand saw, good thermal performance, and available in high-strength versions for load-bearing use. Soft: ordinary expansion plugs crush their own hole, so it needs purpose-made fixings, and it does not take a heavy load on a plug at all.
• Hollow and cellular blocks: Dense blocks with voids to reduce weight, sometimes filled with concrete and reinforcement in retaining or reinforced work. Used where a designed solution calls for them.
• Foundation and trench blocks: Wide blocks used to fill a trench foundation in one thickness rather than building two leaves below ground. Heavy, and a two-person lift.

Questions people ask