Walls Flashcards
What are weep holes?
Weep holes are small openings left in the outer wall of masonry construction as an outlet for water inside the cavity to move outside where it will run off and evaporate.
Typically what is the spacing between weep holes?
1m centres.
Where are weep holes typically found?
Where there are penetrations in the wall which breach the cavity and therefore require a cavity barrier.
Therefore, at the abutment of flat/lean too roofs with a cavity wall, above door or window openings, above the edge of a concrete slab or ring beam, above airbricks, meter boxes, ducts pipes or cavity liners.
What are the types of cavity wall ties?
Fishtail
Wire butterfly
Modern stainless steel.
Historically what are the types of cavity wall ties and why and when did they change?
Traditionally fishtail ties were made of galvanised steel or occasionally on high quality work bronze.
In the mid-twentieth century wire ties were widely used, again made from galvanised steel wire.
As time has passed many galvanised steel ties have deteriorated due to moisture in the outer leaf of brickwork. The corrosion may force apart the cement joints and even result in the collapse of walls if no remedial action is taken. Any cracks appearing in cavity walls dating from the twentieth century need to be investigated before irremediable damage ensues. Horizontal cracking is especially suspect. Failed ties have to be isolated and substitute specialist ties installed by drilling through inner and outer leaves from outside the building. The replacement ties may be fixed mechanically or with special adhesives.
Galvanised steel ties are no longer in use for this reason.
For a brief period, plastic ties were used but were not satisfactory.
Modern practice is to use stainless steel ties.
Typically what is the width of a cavity?
Traditionally, cavity walls were spaced 50mm apart.
Currently, due to the need for thicker insulation in exterior walls these days, a range of longer ties are now available so that cavities of up to 150mm can be constructed.
Typically what is the spacing between wall ties and formation?
450mm vertically and 750mm horizontally in a diamond pattern.
How would you identify cavity wall tie failure?
Bowing walls, horizontal cracking every 4-5 courses, staining if tie is close to the front of the mortar joint.
Why does cavity wall tie failure occur?
Corrosion of the cavity wall ties. The corrosion increases the size of the metal by up to 7 times its original size leading to expansion of the joint and the cracking.
Describe the remedies to cavity wall tie failure?
Use a metal detector to locate the position of the ties and a borescope to inspect their condition.
If it is wall tie failure then they need to be removed.
To remove again use the metal detector to locate their position then remove localised bricks and the defective tie and install replacement stainless steel tie either mechanically fixed or using a epoxy resin.
What is a movement joint and why are they used?
An expansion joint or movement joint is an assembly designed to safely absorb the heat-induced expansion and contraction of various construction materials, to absorb vibration, to hold certain parts together, or to allow movement due to ground settlement or earthquakes.
Typically what is the minimum spacing between movement joints?
BS 5628 : Part 3 ‘Code of practice for use of masonry’ refers to movement joints at 12m spacing for unrestrained or lightly restrained walls e.g. parapets and non-loaded spandrel panels built off membrane type DPCs and 15m maximum spacing for normal storey height walls.
The orientation of a wall in relation to the sun will influence the degree of thermal movement.
Window and door openings in the brickwork can induce local stress points leading to cracking around the openings.
The brick manufacturer’s recommendations should be taken into account.
What are the different structural forms that a building can take?
Loadbearing masonry in residential and commercial use.
Steel or concrete framed in commercial use.
Steel portal framed in industrial use.
How critical is the type of mortar used on the construction of a building?
The type of mortar can be important depending on what type of wall or building the works are on.
An older building will typically include lime mortar. Replacement for cement mortar can affect the brickwork, if moisture cannot escape it will lead to spalling.
Also, it depends on the use of the wall has, it may need high compression strength if carrying heavy loads.
What’s the general principle for choosing a mortar type?
The mortar should be weaker than the brick.
The mortar is meant to be sacrificial it should allow moisture to pass into and out of the wall rather than the moisture going through the masonry.
This then prevents damage to the masonry such as spalling due to frost damage.
