Building Pathology

Question 1

What are the two main forms of rot?

Answer 1

• Wet rot (Coniophora Puteana)
• Dry rot (Surpula Lacrymans)

Question 2

What are the characteristics of wet rot?

Answer 2

• Brown thread like strands
• Darkening of timber
• Knobbly fruiting body (rarely found internally)
• Cracks typically follow grain of timber (can also be cuboidal)
• Optimum moisture content of timber 50-60% (cannot survive below 43%)

Question 3

What are the characteristics of dry rot?

Answer 3

• Cuboidal cracking to timber (more severe than wet rot)
• Grey strands – become brittle when dried
• Fruiting body with white edge and rusty middle
• Ability to penetrate through materials
• Ideal timber moisture content between 20-35% (i.e. damp, not wet)

Question 4

What is the life cycle of wood rotting fungi?

Answer 4

1. Spores
2. Hyphae (strands)
3. Mycelium (mass of hyphae)
4. Fruiting body

Question 5

What are the remedial treatments associated with dry rot?

Answer 5

• Identify and eliminate source of moisture.
• Remove timbers up to 300-450mm beyond visibly rotten sections.
• Replace timbers where necessary and treat remaining timber with a fungicidal solution.
• Adjacent masonry should be irrigated (fungicidal solution pumped into masonry).

Question 6

What is the difference between settlement and subsidence?

Answer 6

• Settlement is movement caused by the weight of the building when a new building is built. Further settlement can occur if additional weight / storeys are added.
• Subsidence is the movement of a building in relation to the ground beneath the building, caused by shrinkage or loss of strength of the ground. For example, shrinkage of clay soils in close proximity to trees or washing away of soils from leaking drains in non-cohesive soils.

Question 7

Can you give me an example of where you would see differential cracking?

Answer 7

At the junction of two different structures or materials e.g. junction of original building and extension.

Question 8

How would you identify thermal movement / cracking to a wall?

Answer 8

• Straight, vertical cracking, with all displacement horizontal.
• Crack pattern can be influenced by openings and weak points in walls.
• Crack of equal width (slight increase at top of walls as loadings reduce).
• Do not extend below DPC.
• Cracks generally to south/west facing elevations.

Question 9

How would you identify cavity wall tie failure?

Answer 9

• Horizontal cracking through mortar joints, typically every 4-6 courses where the ties are set.
• Cracking is more likely to be seen in the first instance to the top of a wall where loadings are less on the bricks.
• Exacerbated by use of black ash mortar, orientation (south-west facing elevations)
• Properties are generally affected which were built between the 1920s (first cavity walls) and 1981.

Question 10

What other causes of horizontal cracking in brickwork are there?

Answer 10

• Sulphate attack to mortar joints.
• Results in crumbling and expansion of mortar joints.
• Thermal expansion of concrete deck.
• Sulphate attack to a concrete floor, pushing out the walls at DPC level.

Question 11

What causes deflection of brickwork over openings?

Answer 11

• Overloading or absence of lintels over openings, resulting in stepped, triangular cracking over openings.
• Rotting of timber lintels can result in deflection.

Question 12

What remedial options are available for deflected masonry?

Answer 12

• Retrospectively installing lintels will assist in carrying the imposed loads.
• Ensure minimum end bearing of typically 150mm either side of the opening.
• Prop brickwork above opening prior to removal of brickwork.

Question 13

Can you list any concrete defects?

Answer 13

• Sulphate attack
• Carbonation
• Chloride attack (chloride induced corrosion)
• Conversion of HAC concrete
• Alkali aggregate / silica reactions
• Structural failure of RAAC

Question 14

Can you describe the typical symptoms of sulphate attack to a concrete floor?

Answer 14

• Cracking and bulging of floors due to expansion of the concrete.
• Traces of white salts to floor surfaces.
• Pushing out of external walls at DPC level.
• Properties built between 1940s – early 1970s.

Question 15

How would you advise a client if sulphate attack was suspected to have occurred to a concrete floor?

Answer 15

• Depending on the extent of damage, undertake initial monitoring of the condition of the slab and any cracking.
• The process is gradual, however planned replacement of the concrete slab and incorporation of a DPM should be undertaking when funding is available.
• Replacement of the floor will require improving thermal performance (0.18 w/m2K).

Question 16

What is carbonation and what remedial works might you recommend if carbonation has occurred to concrete?

Answer 16

• Reduction in PH levels of concrete (around 9 PH) due to absorption of atmospheric carbon dioxide, resulting in the breakdown of the protective passive layer around steel reinforcement, leading to eventual corrosion and cracking/spalling of concrete.
• Remove delaminated concrete, clean down corroded reinforcement with a wire brush, apply an anti-corrosion treatment to the reinforcement and ant-carbonation paint to concrete before undertaking patch repairs to concrete.

Question 17

What is the minimum depth of concrete cover to reinforcements?

Answer 17

25mm.

Question 18

How is phenolphthalein solution used and how can it determine if concrete has suffered from carbonation?

Answer 18

• Phenolphthalein solution can be applied to concrete to determine whether carbonation has occurred.
• Pink – Carbonation not occurred.
• Colourless – Carbonation has occurred.

Question 19

What is Reinforced Autoclaved Aerated Concrete (RAAC) and when/where was it commonly used?

Answer 19

• A reinforced form of lightweight concrete used to form panels or planks. It has no aggregate unlike common concrete. These were mainly used in flat roofs but also in some floor and wall panel construction in the UK from the mid-1950s to the mid-1990s.
• RAAC was used in a range of building types, both public and private sector, but is believed to be more common in schools, hospitals and public buildings.

Question 20

What are some of the problems associated with RAAC?

Answer 20

• Short lifespan (circa 30 years).
• Considerably less strong or durable than traditional concrete.
• Poor original installation, cutting the reinforcement bars on-site, can dramatically reduce the end bearing capacity of the planks. It can fail suddenly, hence the recent action by the UK Government. Later RAAC planks are known to use galvanised steel or stainless steel reinforcing bars, and are of less concern provided the roof is kept watertight.

Question 21

How might you identify RAAC?

Answer 21

• Typically, panels are 600mm wide.
• There are distinctive, regularly spaced V-shaped grooves.
• Panels are white or light grey.
• When accessing records and drawings relating to the building, planks may be referred to by proprietary names such as Siporex, Durox, Celcon, Hebel, Xella or Ytong.
• The inside of the planks will also appear bubbly, like an Aero chocolate bar.
• There will be no visible stones – aggregate – in them.

Question 22

What are deleterious materials?

Answer 22

Materials that are harmful to human health or can result in damage to and failure of buildings.

Question 23

Examples of deleterious materials (human health)?

Answer 23

• Asbestos
• Silica dust
• Lead
• Lime plaster (Anthrax)
• Urea Formaldehyde foam (carcinogenic)

Question 24

Examples of deleterious materials (damaging to buildings)?

Answer 24

• RAAC (Reinforced Autoclaved Aerated Concrete)
• Calcium silicate brickwork (highly susceptible to shrinkage cracking)
• Calcium chloride concrete additive
• High Alumina Cement (HAC) – strength can decrease significantly under high temperatures and humidity (conversion process). Used between 1954-1974.
• Woodwool slabs – failure when becomes damp, loss of fire resistance and corrosion of reinforcement
• Spray foam insulation
• Combustible materials such as ACM cladding panels

Question 25

Can you explain how you would go about installing a lintel retrospectively?

Answer 25

• Prop brickwork above window (support first and second courses separately)
• Remove window and brickwork above window
• Install lintel and reinstate brickwork (include any weeps and cavity trays.
• Reinstate window and make good internally.

Question 26

What type of lintel would you install within a solid wall?

Answer 26

• Concrete, timber, steel, brick
• Corrugated, channel or box lintels

Question 27

What type of lintel would you install within a cavity wall?

Answer 27

Steel lintel with provision for insulation.

Question 28

At 77 Millfield Lane, you identified that defective lead flashings were contributing to internal dampness. Can you please explain why the flashings were defective and what your advice would be regarding repairs?

Answer 28

• The stepped flashings had become loose and were no longer chased into the external wall.
• Ensure flashings are inserted/chased into bed joints by a minimum of 25mm and secured with lead wedges.
• A minimum upstand of 75mm should be provided, with a minimum lap of 100mm.
• The flashings should not exceed 1.5m in length.
• Mortar joints should be sealed with a lead sealant rather than mortar to accommodate expansion/contraction of the lead and to avoid cracking.

Question 29

What code of lead would you specify for flashings at a roof/wall abutment?

Answer 29

• Code 3 soakers flashings.
• Code 4 step flashings.

Question 30

What should you consider when specifying leadwork?

Answer 30

• The use of the lead (chimney flashings, gutter lining, roofing etc).
• Exposure.
• Life expectancy of the building.

Question 31

What is Japanese Knotweed and what are the problems associated with it?

Answer 31

• Tall herbaceous perennial with bamboo like stems. Often grows into dense thickets. Characteristic leaves and stems, persistence of last year’s dead canes and distinctive rhizome (underground root-like stems) enables year round identification.
• Reputation to be damaging to lightweight or poorly built structures e.g. garden walls, hardstandings – unlikely to cause damage to actual buildings.

Question 32

How might you identify Japanese Knotweed?

Answer 32

• In spring, reddish-purple fleshy shoots emerge from crimson-pink buds at ground level. These grow rapidly, producing in summer, dense stands of tall bamboo-like canes which grow to 2.1m (7ft) tall (can grow around 10cm a day). These canes have characteristic purple flecks, and produce branches from nodes along its length.
• Leaves are heart or shovel-shaped and up to 14cm (5½in) in length and borne alternately (in a zig zag pattern) along the stems. The stems die back to ground level in winter, but the dry canes remain for several months or longer.
• The creamy-white flower tassels produced in late summer and early autumn reach up to 15cm (6in).

Question 33

What is spray foam insulation?

Answer 33

• Spray foam insulation is a polyurethane foam, typically applied to the underside of a roof, to increase the thermal performance of a roof.
• In some instances, spray foam is applied directly to the underside of roof coverings which are in a poor condition.

Question 34

At 131 Fossway you determined the movement to be related to clay shrinkage subsidence. How might you gone about preventing further movement?

Answer 34

• Implement a vegetation management plan, involving reducing the crown volume of the trees and regular pruning. If trees are required to be removed, planting of replacement species may be appropriate to prevent overhydration of the soil and subsequent heave.
• A management plan should be specified by a qualified arboriculturist.
• Checks should be made to ensure that trees do not have a TPO (can be fined in excess of £20k).
• Install a root barrier to reduce the likelihood of root damage to the foundations.

Question 35

What was the depth of the foundations at 131 Fossgate and what should they have been at to mitigate foundation related movement?

Answer 35

• The foundations extended to a depth of 500mm below ground level (the foundations were 200mm in depth).
• In accordance with NHBC standards, foundations should be at a minimum depth of 750mm (low plasticity), 900mm (medium plasticity) 1000mm (high plasticity).
• The depth would have likely been increased due to the presence of trees.
• The depth could be calculated using the NHBC foundation depth calculator tool.

Question 36

How might you determine the required depth of foundations?

Answer 36

• Advice from a Chartered Structural Engineer.
• Advice from a Chartered Building Control Surveyor
• NHBC foundation depth calculator.

Question 37

You mentioned reviewing a CCTV drainage report as part of your investigations. How might drainage defects influence foundation movement?

Answer 37

Depending on the soil type, leaking drains can either result in washing away of soil beneath a building’s foundations (non-cohesive soils) or softening of the soil which will reduce the bearing capacity of the soil (cohesive soils). Heave can also occur within cohesive soils.

Question 38

How might the pattern and direction of cracking assist in determining the cause of movement?

Answer 38

• The direction and cause of movement is generally perpendicular to the direction of cracking.
• Tapering of cracking is indicative of rotational movement which is commonly associated with subsidence or heave.
• Cracks extending below the DPC and/or ground level are indicative of foundation related movement.

Question 39

At HSBC, Burnley, what did you recommend to the client regarding replacement of the salt contaminated plaster?

Answer 39

• I recommended that the new base coat included a salt inhibitor.
• An NHL 2 was recommended for the top coat to allow the wall to breathe.

Question 40

What are the risks associated with moisture being trapped within masonry from the use of cementitious renders or mortars?

Answer 40

• Frost damage.
• when the moisture in the masonry freezes, it will expand and resulting in spalling of the face of the brick/stone.

Question 41

How might you mitigate the risk of decay to joists built into solid external walls?

Answer 41

• Provide a waterproof barrier between the timber and wall e.g. Visqueen sheeting (DPC).
• Cut the joists back to the internal face of the wall and suspend them on joist hangers.

Question 42

In what age residential buildings are you likely to find that lintels have not been provided over window and door openings?

Answer 42

Common to see in buildings built between 1945-1975 when original timber framed windows are replaced with UPVC.

Question 43

How might the orientation of a building influence defects associated with damp?

Answer 43

• North East facing elevations are less exposed to sunlight and are subsequently cooler and more prone to condensation and subsequent mould growth.
• South West facing elevations face prevailing wind and are more prone to penetrating dampness.