Thin joint blockwork

Thin joint blockwork (thin joint masonry) is a fast, clean, accurate system for construction using autoclaved aerated concrete blocks of close dimensional tolerance with 2mm-3mm mortar joints.

The increasing demands of the UK construction industry for high build quality, greater productivity, improved thermal performance, airtightness and waste reduction, means that the benefits offered by thin layer mortar systems are becoming increasingly relevant.

Thin layer mortar is a pre-mixed cement-based product that only requires the addition of water to make an easily-applied mortar. It differs from general use mortar in that it sets more rapidly, thus giving early stability to the construction. It provides an alternative to traditional sand/cement mortar and allows the depth of the mortar to be reduced from at least 10mm to 3mm or less.


The benefits offered by thin layer mortars are provided by a system with many of the characteristics of traditional blockwork construction. This means that familiarity with the build process and flexibility are also inherent in the system. The nature of autoclaved aerated concrete blocks and the strict control over the manufacturing process ensures that the blocks are produced to a high degree of dimensional accuracy, ±1.5mm on bed height, making them ideal for use with thin layer mortar.

Faster build speed

Thin layer mortar is applied to autoclaved aerated concrete blocks by the use of a serrated scoop, which allows mortar to be applied quickly and accurately to the bed joints and perpends. The benefits are fully realised on long runs of walling.

Increased productivity

The system ensures high levels of productivity. Autoclaved aerated block walls built with thin layer mortar will typically be stable after 60 minutes. This will allow whole walls to be built in less time and enables earlier installation of other components, such as floors and roof timbers. The completion of a weather tight envelope allows external brickwork to be taken off the critical path and internal work, such as plastering, services installation and partitioning to progress ahead of a more traditional schedule.

Improved thermal performance

The Building Regulations requires attention to be given to the effects of cold bridging. Consequently, when calculating U-values for walls, heat loss through mortar joints must be taken into account. The construction of thin joint blockwork minimises this effect by reducing the amount of mortar in any given area of wall by at least 70 per cent compared with general use mortar construction.

Improved airtightness

Excellent airtightness can be achieved using thin joint technology, thereby improving the overall performance of the wall in terms of heat loss.

Reduced site wastage

A key benefit of autoclaved aerated concrete blocks is that they can be easily and accurately cut, sawn and worked on site. The precision cutting of blocks for use with thin layer mortar allows greater utilisation of the blocks, which can substantially reduce site wastage. Mortar wastage can also be minimised with the batch mixing of small quantities of thin layer mortar.

Improved construction quality

The combination of high tolerance blocks and thin layer mortar allows greater accuracy to be achieved in the internal face of a completed wall, which can provide a suitable substrate for the application of thin coat spray plaster. This has the added advantage of fast application and quick drying time, resulting in further productivity gains.

The Barratt Green House and Hanson Ecohouse, two of the demonstration houses at the BRE Innovation Park, were constructed using thin joint systems.