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various types of building materials. Cast reinforcement should be positioned in the DPC used as a slip plane
stone products in general experience two courses directly above the opening
long term shrinkage, conversely, clay and in the first and third course below
bricks suffer from irreversible moisture the opening. The reinforcement should
expansion and so these materials should project at least 450mm either side of the
be separated because their movements are opening. Whilst additional tensile and
different in both magnitude and direction. flexural strength may be obtained by
To reduce the potential for movement it is the use of bed joint reinforcement, the
also important that the units are correctly size of the bed joint may preclude it’s
bedded on a full bed of mortar, except one use or effectiveness. In terms of walling
piece cills and thresholds. DPC’s should materials such as ashlar, the risk of
always be sandwiched in the joint so that cracking increases where the length of
they are bedded on both sides. Under no a panel exceeds twice the height. Low
circumstances should cast stone units be horizontal walls such as spandrel panels
laid dry on top of DPC’s. are particularly vulnerable to damage.
Where two differing materials are used Extra consideration should be given to the
together in the same construction, then South and West facing elevations as these
consideration should be given to the use of a suffer more thermal gain. Movement joints
slip plane. Slip planes should be designed to should be positioned wherever there are
allow parts of the construction to slide, one changes in thickness or directions of walls.
in relation to the other, to reduce tensile and Lightly restrained details such as parapets
shear stresses in the adjacent elements. or boundary walls will need extra thought
Often these details double up as DPC cavity as these are more prone to movement.
trays. The slip plane may need to contain
two layers of smooth incompressible sheet The designer should also consider that
material or an applied coating to form brickwork can have movement joints
a separating membrane. In principle all positioned at up to 15m centres. Slip planes
details should be checked to ensure that will therefore be necessary to separate the
any potential movement is accommodated two materials. Always ensure that movement
without adversely affecting the stability and/ joints and slip planes do not impair the
or performance of the elements and the stability of the wall or its other functions. Use
structure as a whole. dowels or straps to provide lateral stability.
Please refer to BSI Standards Publication Movement joints are usually filled with
PD 6697: 2010. an easily compressible joint filler of
either polyethylene or polyurethane foam
4.2.2 Movement Joints however, in narrow joints for cast stone
they are usually formed butt jointed.
The risk of cracking in buildings can be Optimum performance in butt joints is
minimised by incorporating movement obtained when the depth to width ratio
joints according to British Standard of the sealant is in the range of 2:1 or
recommendations. With cast stone 1:1 for two part polysulphide sealants.
masonry, movement joints should be no
more than 10mm wide and positioned at Further advice on the design and
a maximum of 6m centres. In external construction of movement joints can
walls with openings, the movement joints be found in BSI Standards Publication
may have to be more frequent or bed PD 6697: 2010, BS 6093: 2006 and BS
joint reinforcement included to restrain 6213:2000.
the masonry. Around openings, bed joint
