Masonry Walls - What do they do?
The purpose of masonry walls is to:-
- Be of suitable construction for their exposure and resist the passage of moisture into the building.
- To provide adequate support to the roof structure and carry loads safely through to the foundations.
- Provide adequate insulation to the Building.
- Be Resistant to the passage of Fire.
Standard Cavity Wall Construction
Masonry walls are constructed in the main from stone, brick and/or concrete blocks. Our current standard wall construction is a 300mm cavity wall incorporating an external leaf of brickwork, 100mm cavity and inner leaf of blockwork. The two leafs are secured together through the use of wall ties. 50mm of Insulation is installed into the cavities against the inner leaf of blockwork. Weepholes are installed below the damp proof course and at the head of lintels and cavity trays to allow moisture to exit the cavity.
Stone work requires a higher level of skill on the part of the mason, especially when using natural stone. Whereas cast stone is shaped and easier to work with, natural stone is used mostly as it suggests in its natural form (rubble), therefore a straight forward bonding network cannot be used.
Solid Brick Walls
Prior to the 1920’s where solid masonry walls were constructed from brickwork this was traditionally built one brick thick. The two most popular types of bond used were English and Flemish bond. English bond consisting of alternate courses of stretchers and headers while Flemish bond incorporated alternate stretchers and headers in the same course and was more popular than the English bond during the 1800’s as it was considered to be more aesthetically pleasing.
Solid Masonry can be highly effective at preventing the penetration of water into the property assuming it is of sufficient thickness. Walls over 450mm thick act as good environmental barriers. Unfortunately due to economy in the use of bricks, many properties built with solid external walls, experienced some water ingress, particularly in exposed areas of the country. Different methods of reducing penetration of water are to install a weather tight render to the facing brickwork, improve the quality of mortar joints to prevent leaks through this medium and increased use of cavity walling.
Development of Cavity Walls
Cavity walling became common practice throughout the early 1900’s primarily to reduce damp penetration, and also to reduce the amount of heat lost. Early versions comprised of two half brick leafs with a cavity of 50-70mm held together at regular intervals by wall ties.
Dense concrete blocks have been in use since the 1930’s and have increased in popularity from then due to the cost advantage over bricks. Early blocks were often made from local aggregates; usually industrial waste products such as breeze and clinker. The density of the aggregate affects the thermal and sound insulation qualities of the blocks.
In their early years, dense concrete blocks were commonly used for internal walls and partitions. They did not take over in popularity from brick work for the inner leaf of a cavity wall until the late 1950’s and were the most common material used through until the 1970’s. They have changed very little in the last 60 years and are usually made from cement, fine aggregate and coarse aggregate in differing strengths. They have poor thermal insulation and readily absorb water.
Changes in Building Regulations in the 1960’s put greater emphasis upon the thermal insulation properties of materials used in construction. As a result, lightweight concrete blocks became the most commonly used block for internal cavity walls in the 1970’s as they had a lower U-Value and were lighter and therefore easier to lay. Dense concrete blocks are still used for the external leaf of a cavity wall with cladding or a rendered finish applied for aesthetic reasons and to prevent moisture penetration. More recently aerated blocks are used. These provide better thermal insulation than the lightweight blocks and have a closed pore system which limits water ingress.
Traditional mortar mixes consisted of lime putty and sand which gave a degree of flexibility but were very slow to set and readily absorbed rainwater. The patent for one of the first ever Portland cements was taken out in 1824 and the use of this material gradually increased but did not become the norm until the 1930’s. This was considered superior to the lime because it set in a matter of hours and was less absorbent. For a brief period during the wars black-ash mortar was popular in Industrialised areas of the country. They were very economic but were not as durable or resilient as the lime or Portland mortars. One further disadvantage was that they contained acids which attacked steel ties in cavity walling.
Installation of a DPC
Damp Proof Courses
Problems caused by rising damp led to the introduction of Damp Proof Courses (DPC’s) which were a barrier within the brickwork to prevent the water rising up through it. They were introduced at the end of the nineteenth century and consisted of three layers of slate, a course of engineering bricks or hessian soaked tar. These materials were quite brittle and over time became prone to cracking. Bitumen based products became more common in the mid twentieth century before giving way to the flexible black polythene material which is commonplace today.
The purpose of a lintel is to create an opening in a wall without compromising the overall structural stability of the wall. Historically openings have progressed from keystone archways to stone or timber lintels, evidenced from tudor times right through until the introduction of concrete lintels in the 1940’s.
As concrete developed, it proved to be cheaper while still retaining suitable strength in which the structural stability will not be compromised. Concrete lintels are still used on internal walls. The late 1960’s and early 70’s then experienced a new evolution of lintels when steel lintels were developed. There was a massive boom in building and a new high demand was needed to speed up construction with easier handling. Another advantage was that the lintel was not visible in the brickwork. Today, 95% of new build developments incorporate steel lintels.
Where lintels bridged the cavity wall, water was able to cross to the inner leaf causing problems with damp, leading to the introduction of cavity trays. The purpose of a cavity tray is to direct any water within the cavity towards the outside wall and out through a weephole. They did not become common practice until the 1980’s but early examples were made from the same materials as early DPC’s with weepholes created by leaving a gap in the mortar fill between bricks. Lead became popular due to its malleable nature but was very expensive before giving way to preformed trays, sometimes combined with the lintel and prefabricated weepholes.
Wall Ties are incorporated in cavity walling to hold both skins together and reduce lateral movement. Early wall ties were made from iron were found to be too brittle and prone to rust over a long period. This can lead to failure resulting in costly remedial works. The most common type used throughout the early 1900s was mild steel wall ties. These are usually galvanized with zinc or given a bitumen coating to minimize corrosion. A British standard for these ties was introduced in 1945 but due to discoveries that the life expectancy of the coatings was well under 60 years this was amended in 1981 to triple the thickness. Stainless steel wall ties were introduced in the 1960’s and standardised by Building Regulations.
Air bricks have been installed in walls for hundreds of years as a means of ventilating the room and floor space and after the introduction of cavity walls, to ventilate the cavity. Their use In this medium was identified as a fire risk in the mid twentieth century as they created a wind tunnel effect within the cavity which lead to the introduction of ducts between each leaf known as sleeves.
Click Here to Read About Alternate Foundation Designs
- Building Foundations
Alternate foundation designs which may be required when building a new extension and the factors which affect the decision.
What type of wall construction is found on your property?
© 2014 Rachael Tate