13. Building Pathology

Question 1

What is building pathology?

Answer 1

Building pathology is defined as the study of defects and their causes, processes, development and consequences. There are four main stages to consider:
1) Inspection
2) Diagnosis
3) Prognosis
4) Remedy

Question 2

What types of flat roof material are there?

Answer 2

1) Asphalt roofing
2) Liquid Waterproofing
3) Single plywood
4) Bitumen felt

Question 3

What is damp?

Answer 3

Damp is defined as the presence of moisture in the air or on a surface.

Question 4

Name 4 different types of damp?

Answer 4

1) Penetrating Damp
2) Rising Damp
3) Condensation
4) Water leaks

Question 5

What is penetrating damp? How is it caused? What are the remedial methods?

Answer 5

This is the development of moisture through the walls of the building. More common in traditional buildings with solid wall construction.

It can be caused by defective pointing, porous brickwork, defective rainwater goods or cracks it render.

Visible through damp patches, puddles or drips contained to a single (or sometimes multiple) areas.

Undertake an assessment, assess the cause and make remedial methods. For example, if it is defective pointing, scrape out and repoint.

Question 6

What is rising damp? How is it caused? What are the remedial methods?

Answer 6

This occurs when moisture from the ground travels upwards through the pores in the brickwork. This is known as capillary action.

Ground water often contains salts. When this travels up the wall it often forms white marks (usually up to 1m).

It can be caused by a breach in the DPC or build up of debris or dirt in cavity wall.

Other signs include tide marks and staining, damp wall paper or damp skirting boards.

Question 7

What are deleterious materials?

Answer 7

These are materials that are prohibited from site. At one stage, these materials would have been considered satisfactory to use during the build.

Question 8

Name 3 types of deleterious materials that may affect the structure

Answer 8

Structure
- Stramit board: These are compressed strawboards made from wheat. They lose their structural integrity when wet.

-RAAC: Lightweight cementitious material with no coarse of aggregate. It contains steel reinforcement bars. Cheaper and quicker to install than normal concrete. Used between 1950s to 1990s. Life expectancy of 30yrs. Susceptible when exposed to moisture.

Wood Wall slabs: Manufactured predominately from softwood shavings that have then been coated and bonded with Portland cement and compressed to make medium density boards. Issues when used as permanent shuttering as it can lead to insufficient covering to the reinforcement of the floor. Known as voiding.

Question 9

Name 3 types of other deleterious materials

Answer 9

Glass
- Nickle sulphides: This is a spontaneous fracture of toughened glass panels. Cracks in a butterfly shape. This is not an issue in untreated glass but when heat is added to make it toughened, it produces nickel sulphides. Can be reduced through heat soaking.

Insulation
Asbestos: Use of asbestos as insulation.

Metals
Lead: Can be used for external gloss paints and domestic pipework. Lead poisoning is known to cause health problems.

Other
Silica Dust: This is the release of dust particles from cutting, drilling and machining. This can affect the respiratory system when inhaled. Can come from activities such as bricklaying, concrete works, grit blasting and demolition.

Question 10

What is condensation?

Answer 10

Condensation typically occurs when warm, moist air comes into contact with cooler surfaces that are at or below the dew point (i.e. windows). Water condenses on those surfaces.

It can be either:
1) Surface condensation
2) Interstitial condensation

Question 11

What is interstitial condensation?

Answer 11

This occurs between the layers of the building fabric - typically as a result of air diffusing from the warm interior of a building to the cool exterior and reaching its dew point within the construction of the building itself.

Question 12

Where does interstitial condensation occur?

Answer 12

This tends to occur on the warm side of vapour resistant layers.
- VCL can be positioned on the warm side of insulation, however this must be carefully sealed and penetrations avoided.
- Ventilated cavities can be provided near the cooler side of the construction.
- Cold bridges can be eliminated
- Moisture in the building itself can be reduces

Question 13

What is the impact of condensation in a building?

Answer 13

Condensation can affect the performance of a building. It can cause problems such as:

1) Mould growth - Can cause respiratory allergies
2) Staining
3) Slip hazards
4) Damage to equipment
5) Poor performance of insulation

Question 14

How can condensation be controlled?

Answer 14

Condensation can be controlled through several factors.

• Limiting the source of moisture.
• Increasing air temperatures.
• Dehumidification
• Natural or mechanical ventilation
• Increasing surface temperatures
• Avoiding cold bridges - This is where there is a direct connection between the inside and outside through one or more elements that are more thermally conductive than the rest of the building. More common in traditional buildings. In modern buildings, thermal bridging can occur because of poor design or workmanship.

Question 15

What is cracking?

Answer 15

Cracking can be caused by a variety of different reasons. These may not always be severe and may not cause any further problems. It is important to monitor cracks to see if these have stopped or continue to move.

Question 16

What examples are there of cracking?

Answer 16

1) Settlement of made up ground
2) Thermal movement - Due to expanding or contracting can cause cracks
3) Lintel failure - Due to poor design or materials at end of life
4) Roof spread - Overloading pitched roofs
5) Cavity wall tie failure - Old wall ties can corrode and fail causing bulges in middle of walls.

Question 17

Are you aware of any guidance for cracking?

Answer 17

BRE Digest 251 ‘Assessing cracks in houses’

This document splits the cracks into 6 main categories. In general, categories 0, 1 and 2 with crack widths up to 5 mm can be regarded as ‘aesthetic’ issues that require only redecoration. Categories 3 and 4 can generally be regarded as ‘serviceability’ issues, that is, they affect the weathertightness of the building and the operation of doors and windows. Category 5 presents ‘stability’ issues and is likely to require structural intervention.

Question 18

What is subsidence?

Answer 18

Subsidence is when the soil beneath a building is unstable and sinks downwards.

As a general rule of thumb, if the cracks are larger at the top and taper from the head downwards then this is consistent with subsidence.

Cracks tend to be diagonal.

Question 19

What are the causes of subsidence?

Answer 19

1) Caused by ground movement
2) Changes in ground water levels
3) Leaking drains
4) Past or present excavation works

Question 20

What are the remedies for subsidence?

Answer 20

This very much depends on the cause. However, generally:

1) Remove the cause
(i.e. remove source of water if leaking drains)
(i.e. reinstate excavations)

2) Structural measures
Installing new members, brickwork stitching etc.

3) Soil stabilisation

4) Underpinning

Question 21

What is heave?

Answer 21

Heave is the upward movement of the ground usually associated with the expansion of clay soils which swell when wet. These can also do the opposite and shrink.

It is heavily linked to trees and cohesive soils.

As a rule of thumb, cracks occur which are larger at the bottom.

Cracks tend to be vertical.

Question 22

How does heave occur?

Answer 22

When trees are removed from clay soils. The cohesive soil then fills with water and swells. The opposite can happen when a tree is added.

Heave can also occur as a result of leaking drains or water supplies.