Horizontal movement joints in external masonry walls to framed buildings

Recent investigations and reports into cracking and defects in external masonry facades on multi-storey framed buildings have highlighted inadequacies in the design and detailing of horizontal movement joints in this type of construction.

With renewed interest in masonry cladding to medium rise residential developments, it is topical to highlight the principal risk items and draw attention to steps required to achieving a satisfactory horizontal movement joint and outcome.

Guidance

Horizontal movement joints in masonry are primarily designed to accommodate thermal and moisture movement of the masonry. Their provision is typical at every other or third floor level, depending on the supporting structure and the type of masonry being supported. Occasionally, the masonry support may be provided at every floor level depending on the particular circumstances and details of the building.

The vertical spacing of horizontal movement joints on a building may also vary from elevation to elevation, depending on the building characteristics and the type of structural elements providing support for the masonry.

On some buildings, horizontal movement joints in masonry may be required to accommodate other types of movement or deflections, for example where the masonry support system is fixed to edges of floor slabs particularly those without edge beams such as in flat slab construction. In such instances, the floor slab should be adequately designed to control floor sag, and where the sag cannot be limited sufficiently, the horizontal movement joint should be appropriately designed and detailed to accommodate such deflection without imposing undue load or crushing the masonry below the support system.

Depending on the number of storeys and expanse of the elevations in a building, horizontal movement joints in masonry may need to accommodate any or a combination of the following:

• thermal movement
• moisture movement
• deflection of the structure (e.g., slab, edge beams etc.) supporting the masonry
• deflection of the masonry support angle/system
• axial shortening of structural columns and walls
• building sway.

Typcial movement that could be expected on a reinforced concrete framed structure could be in the order given below (but should always be assessed on a project specifc basis).

Occasionally, the horizontal movement joint requirements may be complicated by having varying support for the external masonry cladding such as masonry partly supported on ledger angles and partly supported on balcony structures.

This can be further compromised where the balcony structure is cantilevered off the unsupported edge of a flat slab, with additional rotation and deflection of the balcony acting in conjunction with movement due to the masonry support. Other scenarios to consider are where differential deflection occurs between masonry walls built off foundations or stiffer floors and those supported off more flexible upper floors; and when columns or wall supports are staggered or set back on elevations, which will influence deflection profiles.

Designers, builders, and regulators should carefully consider the particular details of their building and ensure all possible permutations of scenarios are considered, to ensure any horizontal movement joint provisions are adequate.

Careful consideration should also be given to the position of vertical movement joints, in order to ensure that both sets of joints can be optimised in managing and controlling movements as well as minimising the risk of differential movement in the masonry and hence reduce incidences of defects or cracking.

You need to…

• ensure all expected movements including deflection of the structure are allowed for at horizontal movement joints when provided
• ensure all movement joints (both horizontal and vertical) are adequately positioned, coordinated, detailed, and built correctly
• ensure every component of the joints are appropriately sized, fully specified and fit for purpose, with input from all designers (Architects, Engineers etc) and adequate attention paid to project specific requirements
• ensure support system installation errors are eliminated or minimized and masonry construction around the joints are undertaken by masons experienced in this type of work.
• submit floor junction designs and details, with justified horizontal movement joint details to NHBC for review at an early stage of the project