1. Introduction to Building

1.1 General

1.2 Dampness in Walls

1.3 Exposure

1.4 Drawings

1. INTRODUCTION TO BUILDING

1.1 General

In this chapter we are trying to cover three general aspects. Firstly, mainly in the following drawings, we show the use of the technical terms that you are likely to come across in discussions, in writing quotations for work, and from time to time, in reading reports on particular buildings.

It is important to understand the terms used by others – if an architect starts talking about ‘voussoirs’ or ‘rusticated quoins’ or ‘the parapet’, these terms are shorthand descriptions that you should try to become familiar with. Similarly from our point of view when preparing a quotation it will enable us to define precisely what we are offering to for the client.

Apart from the technical terms, the drawings also show different ways in which walls are constructed as load-bearing and panel walls, solid or cavity walls. Cavity walls are almost universally used in modern masonry construction since the same degree of resistance to weather and level of heat insulation can be achieved with less material than is required in solid wall construction.

1.2 Dampness in Walls

We think that there are likely to be occasions where repointing is being considered for an existing building where the external wall shows signs of rain penetration.

There are a number of possible causes for dampness in external walls and it is not always easy to determine which particular cause applies, on occasion a number of factors may be operating at the same time.

The indications for rain penetration are dampness (discolouration of plastering and decoration) generalised vertically up a wall. In contrast rising dampness is generally worse in the lower parts of the wall and may not rise higher than perhaps three quarters of the way up the lower storey. Another (perhaps the commonest) likely cause of dampness is condensation, this being at its worst during winter months. In all three cases wall construction is likely to be inadequate by modern code of practice requirements. Much housing until the between wars period was built with one brick thick external walls to the main part of the house, the present code of practice recommends a minimum of one and a half bricks for sheltered sites (for “sheltered”, “moderate” and “severe” exposure refer to next section). Very many of these “inadequate” walls do not show any signs of rain penetration, particularly in sheltered urban situations. However, for more exposed sites rain penetration is likely during the winter months when the wall can become saturated with water for long periods of time.

The solid wall obtains its resistance to rain penetration by providing an adequate “reservoir” within its micropore structure to absorb rain falling on the surface. The absorbed water eventually evaporates out when drier weather occurs. A wall of inadequate thickness will have insufficient ‘reservoir’ capacity to cope with rainfall in winter when natural evaporation rates are low. The precise mechanism of rain penetration is complex depending upon the absorbency of the brick and mortar and upon the presence, or not, of hairline cracks between the brick and the mortar. This discussion has been in relation to external walls of one brick thickness: in much Victorian housing of the cheaper type back extensions to the main part of the house were often only half a brick in thickness and even in sheltered sites are nearly always damp from rain penetration.

These walls of one brick (or less) thickness also have high rates of heat flow through them (poor thermal insulation). Apart from making the cost of providing heating high, the other major effect is that the surface temperature of the wall internally will be very low to the point that condensation on the surface is likely. Mould growth (of yeasts etc) associated with condensation can be very severe in areas where natural ventilation is poor (eg upper corners of rooms between ceiling and walls, built in cupboards (internally), etc).

Again this type of construction, particularly in the earlier part of the nineteenth century, often has no damp proof course thus allowing damp to rise from the ground in contact with the foundation.

Cavity wall construction overcomes the rain penetration problem by providing two separate ‘leave’ or ‘skins’ to the wall (see drawings). The half brick outer leaf is assumed to be saturated in winter conditions probably to the point where water can run down its inner face. However, the dampness cannot pass across the 50mm (normally) air gap between the two leaves. For reasons of structural stability the two leaves are tied together with metal ties (most commonly galvanized steel) : the tie is designed to prevent water wherever there is a “bridging” of the cavity (commonly at window openings), the separation between the leaves should be maintained by suitable dpc’s or walls). Theoretically, and generally in practice, the cavity wall offers better weather resistance than a solid wall. Most defects that occur are due to inadequate design or construction in maintaining the separation between the two leaves of the wall.

Other possible causes of dampness in walls that may need to be investigated are leaking rainwater pipes and gutters, leaking service pipes (water) or soil and waste pipes, displaced flashings (leadwork) around chimneys or at abutments, cracked cills, copings etc, hygroscopic salts used in brickwork.

1.3 Exposure

This is used as a technical term to describe the severity of weather conditions to which a building or part of a building is likely to be exposed. Fairly obviously severe weather conditions are going to demand a higher standard of construction in order to exclude weather from the building and to ensure that materials used are going to last a reasonable length of time.

The exposure grading for a particular site is based upon two Met. Office measurements – annual rainfall and average wind speed : these are combined to give a “driven rain index” from which a threefold classification is made – “sheltered”, “moderate” or “severe” exposure. Tall buildings in areas normally classified as sheltered will, in their upper parts at least, be graded as of severe exposure.

Similarly in any particular building some parts are subject to more severe conditions of exposure than the building in general. In this latter group are placed parapets, copings, cills, chimneys, retaining walls, free-standing walls (eg boundary walls,) and walls below dpc but above ground level.

Careful attention is needed to the above “elements” of a building since they are the parts most likely to be saturated for long periods and to be subject to freezing whilst saturated.

In your assessment of potential work particular attention should be paid to these elements since they are normally the areas where pointing may first break down and defects in materials show up. Later in discussing selection of mixes for repointing we shall see that mix proportions should be varied to suit degree of exposure.

(click images below to view full-size in a new tab/window)

          
Figure 1             Figure 1.1            Figure 1.2            Figure 1.3           Figure 1.4          Figure 1.5

       
Figure 1.6            Figure 1.7           Figure 1.8          Figure 1.9          Figure 1.10         Figure 1.11

 

The above information should not be taken as recommendations for any individual contract/project and are guidelines only. Consult your local licencee for advice on the projects in your area.

 

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