This technical document provides additional guidance relating to how the Functional Requirements in the Technical Manual may be satisfied. This article covers the following:
Please note, this guidance document is in relation to low rise residential buildings only. For commercial or high rise buildings, advice should be sought from the structural engineer.
The structural engineer must provide structural calculations and drawings clearly detailing the following:
The above must be in line with the requirements set out in Section 6 of our Technical Manual, PD6697 and EN 1996-1-1: Eurocode 6.
Buildings by many, may be seen as inert or static objects, however, in actual fact they are prone to a considerable amount of movement, primarily due to environmental factors such as variations in temperature and moisture. Movement joints (sometimes referred to as expansion joints) are often introduced within the design and it consists of a gap which allows for expansion and contraction. The gap is filled with a compressible filler and a weatherproof sealant is layered on top.
Having no movement joints (or movement joints at the incorrect spacing’s) can therefore cause stress cracking in the external wall (and adjacent elements) as sufficient allowance has not been catered for expansion and contraction.
Spacing of movement joints
For masonry walls, movement joints should be spaced as per the below table (as reproduced from the Technical Manual):
Wall tie positioning adjacent to movement joints
Wall ties should be spaced as per the diagram below. A structural engineer would need to provide justification and calculations for any deviation.
For timber framed structures, maintaining a 225mm horizontal wall tie spacing can be difficult as the timber studs may be more than 225mm away from the movement joint.
Introducing additional timber studs may be impractical and therefore an alternative solution of using slip ties at 300mm vertical centres with 50mm embedment into the masonry either side of the movement joint, could be considered subject to a structural engineers design.
Positioning of movement joints
For masonry cavity walls, horizontal expansion will occur at corners which can push the corners out and this in turn can eventually cause cracking, however this can be avoided by sub-dividing walls into shorter lengths with movement joints. Movement joints should also be provided as close to corners as possible (but not less than 665mm from a corner).
The shaded area in figure 2 below shows the ideal position for the first movement joint – first movement joint should be no less than 665mm from a corner and up to half recommended length of wall without a movement joint.
Short returns in clay masonry (less than 675mm) should have movement joints as per figure 3 below. Movement joints can be specified with the use of a ‘compressible joint’ or a ‘slide by joint’.
Unrestrained masonry such as parapets and free standing walls require greater movement control due to their greater exposure, less restraint and stronger mortar mixes. Movement joint spacing for parapet walls should therefore be half then what is stated in table 1 and 1.5m from corners. For further guidance on movement control in parapet walls, the structural engineer should be consulted. For further guidance on parapets, please see our Guide to Creating Weatherproof Parapet Walls.
To meet our warranty requirements, compressible filler, such as polyurethane foam, should be used to form the joint and be sealed to prevent water penetration and achieve a service life of at least 15 years (without maintenance). Movement joints should be sealed with an appropriate sealant which can provide sufficient flexibility whilst resisting water penetration. When sealants are used in proximity with stone it is important to select a non-oil-based sealant to help prevent any staining to the stone.
Elastic sealants (Type E) are suitable as they allow for reversible movement. Where a back-up material is used to control the sealant depth, it will also provide a compressible space into which the sealant can deform.
Where a backing material is used, the following must be considered:
Please note, hemp, fibreboard, cork and similar materials should not be used for expansion joints in clay brick masonry, but may be used for contraction joints in calcium silicate and concrete masonry (PD6697).
Movement joints in concrete bricks
The rate and extent of movement in concrete bricks can be considerably different to that of clay bricks, for specific guidance on movement joints in concrete bricks, please see our article ‘Accounting for Movement in Concrete Bricks’.
Anticipated differential movement for timber frame structures
Timber frame structures will move at a different rate to their masonry cladding on the exterior face and this should be taken into account by the structural engineer. Movement joints should be provided under all openings and at eaves and verges with the size of the movement joint as per guidance provided in Section 6.2 of our Technical Manual.
Any material or component attached to the timber superstructure that overhangs the brick or blockwork (e.g. cladding attached to the timber frame, window sills, roof eaves, and verges) or projects through the masonry (e.g. balcony supports, flues, extractor fan vents, or overflow pipes) should have a clear gap beneath and at the top of the masonry cladding to allow differential movement to take place, thus avoiding damage to the components or cladding.
Gap sizes should allow for anticipated differential movement while allowing for drainage and ventilation requirements. Insect infestation should be avoided by using screens to cover gaps exceeding 4mm.
Differential movement at floor zone with cladding supported on timber frame
Horizontal cross grain timber and construction gaps are concentrated at floor zones and this is where the majority of movement occurs. Vertical timber battens or other rigid cladding support systems should not span over the floor zones of timber frame buildings.
Gaps should be provided to accommodate anticipated differential movement and the compressed size of any filler. Unlike self-supporting claddings, movement is not cumulative but should be calculated individually for each floor zone using the following formula of 1mm for every 38mm of horizontal cross grain timber.
Accounting for movement in Light Gauge Steel Frame
Light gauge steel frame (LGSF) structures will also move at a different rate compared to the external cladding and this should be taken into account by the structural engineer. Movement joints should also be located under openings, eaves and verges.
In addition, any material or component attached to the LGSF structure that overhangs the brick or blockwork (e.g. cladding attached to the LGSF, window sills, roof eaves, and verges) or projects through the masonry (e.g. balcony supports, flues, extractor fan vents, or overflow pipes) should have a clear gap beneath and at the top of the masonry cladding to allow differential movement to take place, thus avoiding damage to the components or cladding.
Rendered walls should be constructed with control measures in place to reduce the risk of damage to the render from movement in the substrate.
Rendering onto masonry walls
Any movement joints within the substrate should be carried through to the face of the render (figure 6).
Where recommended by the blockwork manufacturer, cracking of the substrate could be significantly reduced by introducing a specialist proprietary bed joint reinforcement within the mortar joints. This should be provided in accordance with the structural engineer’s specification.
Ensure that the reinforcement is continuous and joints lapped in accordance with the manufacturer’s requirements (generally 450 - 500mm laps and continued around corners). Specialist corner units are likely to be required, check with the block manufacturer.
Introducing bed joint reinforcement at weak points such as above and below window and doors openings will greatly assist in minimising cracking to these areas. Where possible, the reinforcement should project 600mm beyond the opening.
Render should not bridge across dissimilar materials, however if this cannot be avoided, the render should be stopped at an appropriately formed movement joint or austenitic stainless steel lath reinforcement should be used across the joint with a separation strip behind.
Rendering onto render board and timber/steel framed structures
Note: Render boards must be proven to meet a relevant BS EN standard and proven suitable as a direct render board e.g. BS EN 12467 to a category A weather resistance and have third party product approval by a UKAS or international equivalent.
Gaps between boards should be provided in accordance with the manufacturer’s recommendations and carried through to the face of the render. Care should be taken to ensure there are no excessive gaps between the boards and appropriate weather seals are incorporated against walls and frames.
Where renders spans across an intermediate floor zone in timber frame construction, allow for differential movement due to timber shrinkage by incorporating a movement joint as per figure 5 above.
Vertical movement joints should be provided at the required intervals. The actual spacing and position of the joints will be determined by the shape of the area to be rendered and generally vertical movement joints should be provided at maximum 5m centres.
For further guidance on render boards, please see Section 6.4 of our Technical Manual
Mesh reinforcement around openings
For rendering onto both masonry and render board, mesh reinforcement is required around openings as per figure 7 below.
In summary, prior to works starting on site, rendered walls and their anticipated movement should be considered as part of the design by the structural engineer.
Brickwork and other building materials move at different rates and therefore where differing materials abut each other there should be consideration for how the movement is to be accommodated by your structural engineer (movement joints, bed joint reinforcement etc.).
On any given wall elevation where there is a mix of masonry e.g. Brickwork external leaf lower level with a rendered block upper level, the requirement of a full height movement joint should be based on the shorter spacing requirement e.g. for the blockwork at 6m - not brickwork at 12m.
Changes in height
A movement joint should be considered where there is a significant change in height in the building elevation. This can be achieved with the use of a compressible or slide-by joint and should run the full height of the building.
Bed joint reinforcement
Bed Joint reinforcement may also be required to critical areas to accommodate stresses such as above and below window openings. The Structural engineer or render manufacturer may require this to be provided as part of the overall design specification. Where provided, they will be in addition to movement joints, not instead of. Bed joint reinforcement potentially can increase spacings of movement joints subject to the structural engineer’s specification.
Movement joints below DPC
Where the finished ground level is 600mm or greater below the horizontal DPC, the movement joint should be continued within the external leaf of the sub structure
The orientation of your building can have a substantial impact on the movement of different materials due to temperature changes. Allowance should therefore be made for south, south west, south east and the rear face of north facing parapets to move in a controlled manner with the use of movement joints and/or bed joint reinforcement. The structural engineer and render manufacturer should provide a project specific specification.
Retrofitting movement joints
Movement joints must be considered and detailed at design stage and discussed on site before any works begin. Their positions must be fully communicated to the site team and relevant trades before bricklaying begins.
In instances where they’ve been missed on site, a structural engineer should be consulted to provide a designed solution and the following should be considered:
Induced movement joints are not recommended (e.g. retrospective cuts in completed panels with a petrol Stihl saw) as these rarely achieve full depth of the cut and required width of joint) and the necessary wall tie provision may not be in place either
The provision of movement joints should be considered at an early stage by your designer and they should meet the requirements of Section 6 of our Technical Manual, PD6697 and EN 1996-1-1: Eurocode 6.
Every care was taken to ensure the information in this article was correct at the time of publication (March 2022). Guidance provided does not replace the reader’s professional judgement and any construction project should comply with the relevant Building Regulations or applicable technical standards. For the most up to date Premier Guarantee technical guidance please refer to your Risk Management Surveyor and the latest version of the Premier Guarantee Technical Manual.
Reference: TS 3174