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Students Zone

Case Study Eight

External walls

Formation of traditional external walls:

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In order to resist the passage of moisture from the ground it is necessary to install an impervious layer or damp-proof course into the walls at low level. Here we see the black DPC passing over the threshold of a door opening in both inner and outer leaves. Note the weep holes beneath.

Wall ties are built into the wall to link inner and outer leaves. Here the outer leaf has been built first with the ties set as required. At junctions with internal walls it is important to provide a means of tying the two together also.

Cavity ties are fitted with restraint clips to hold insulation against the inner leaf. If the insulation falls across the cavity a possible route for water is created.

Where openings are formed it is essential to provide appropriate details to retain the performance of the wall. Here we see the galvanised steel lintel above the opening supporting the inner and ouer leaf. The cavity is closed at the reveal by turning a block from the inner leaf to abut the brickwork of the outer. At this point a vertical damp-proof course is installed, clearly visible here.

Where openings are formed in the external walls for the provision of windows and doors it is necessary to close the cavity. This is to resist the passage of fire and to prevent ingress of moisture, vermin and contaminants. It is common to build the window and door frames into the wall structure as work proceeds. Alternatively, as seen here, cavity closers may be used as formers for the opening. Window frames are then connected to these later.

Where the cavity is closed there is the risk of moisture penetration and the possibility of ‘cold-bridging’ occurring. In order to resist these it is necessary to provide dpcs around all openings. Those used here are insulated with an effective insulation material bonded to the black/silver dpc material.

Here we see a selection of steel lintels stored on site awaiting installation. Wherever openings are formed using lintels the creation of horizontal surfaces upon which moisture can settle poses some problems. These are dealt with by the installation of DPCs and the provision of weepholes (circled) to allow the water to escape to the exterior.

All building materials suffer from a degree of dimensional change as a result of variation in temperature and moisture content. Masonry walls must be considered in this respect and it is necessary to include movement joints at varying distances, depending on the material used.