Walls and Blockwork: Building Up from DPC

More than a third of all residential construction defects recorded by LABC involve external walls. Not roofs. Not foundations. Walls. The part of your extension that everyone assumes is straightforward is actually where the most things go wrong, from botched DPC installations to wall ties pushed into mortar after the fact rather than set in as courses are laid.

The good news is that cavity wall construction for an extension is well-understood by any competent bricklayer. Your job isn't to lay blocks. It's to understand what's being built, check quality at each payment milestone, and know when to involve your building control officer.

What You're Actually Building

A cavity wall is two separate skins of masonry with a gap between them. The inner leaf is blockwork (typically 100mm lightweight or dense concrete blocks). The outer leaf is facing brickwork that matches your existing house. The cavity between them is partially or fully filled with insulation, and the two leaves are tied together with metal wall ties at regular intervals.

This is not a single solid wall. It's a system, and every component has a job: the inner leaf carries structural load, the outer leaf keeps out rain, the cavity prevents moisture crossing from outside to inside, the insulation meets thermal regulations, and the wall ties stop the two leaves from separating.

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Under Part L 2021 (the current energy efficiency regulations), new extension walls must achieve a U-value of 0.18 W/m²K. That's a big step up from the old 0.28 requirement. In practice, it means most extensions now need a 100-150mm cavity to accommodate enough insulation. Your architect and building control officer will specify the exact build-up.

Block Types and Mortar

Your bricklayer will use two main types of block.

Lightweight aerated blocks (brands like Thermalite and Celcon) are the standard for the inner leaf above DPC level. They're easier to cut, lighter to handle, and provide better thermal performance than dense blocks. A bricklayer can lay more of them in a day because they're less physically demanding.

Dense concrete blocks go below DPC level and in any load-bearing location your structural engineer specifies. They're heavier, stronger in compression, and more moisture-resistant. Below DPC, you may also see engineering bricks, which are virtually impervious to water.

The mortar mix changes depending on what's being laid and where. Below DPC, a stronger 3:1 sand-to-cement ratio is standard. Above DPC with lightweight blocks, the mix is weaker, 5:1 or 6:1, because a strong mortar on a weak block causes cracking. If your builder uses a 3:1 mix above DPC with aircrete blocks, that's a problem: the blocks will crack as the mortar shrinks.

Sand colour varies between quarries and between deliveries from the same quarry. If a load of sand arrives looking noticeably more yellow than you expected, don't panic. Different quarries produce sand with different mineral content. It's cosmetic, not structural. Ask your builder to confirm, but it's almost always fine.