Building Pathology Flashcards
What is relative humidity?
The amount of water vapour present in the air expressed as a percentage of the amount needed for saturation at the same temperature. Mould growth can occur if the RH remains above 70% but, for active growth, prolonged spells of over 80% RH are generally necessary or continued access to direct moisture supply within the material upon which they will form.
What is condensation and how is it caused?
Condensation - change of water vapour naturally present in air into liquid water. Too much moisture in the air produced by people and activities such as cooking and washing.
How would you identify condensation within a building?
- Wall has a ‘misty’ surface
- Stains or streaks of water running down a wall (particularly in bathrooms, kitchens and below windows)
- Damp patches with no definitive edges
- Dampness behind wall cupboards or inside wardrobes against external walls (areas where air circulation is restricted)
- Localised dampness at potential ‘cold bridges’
- Patches of mould growth
- Humidity (measured using a hygrometer), insulation and ventilation levels, as well as heating and living patterns, must also be taken into account
What steps would you recommend to eliminate condensation?
Reduce moisture generation (lids on pans, dry clothes outside, vent tumble dryers externally, do not use paraffin or bottled gas heaters, put cold water in bath before hot)
Increase ventilation to remove moisture-laden air (open trickle vents, open windows, mechanical ventilation)
Increase air temperature by heating - warmer air can hold more water vapour without condensing
Increase surface temperature by thermal insulation (external or internal)
What is interstitial condensation?
Interstitial condensation is condensation that occurs within the structure of an element, as opposed to on its surface. The dew point is within the structure.
What is the dew point temperature?
The temperature below which condensation will begin to occur. If the dew-point temperature is close to the dry air temperature - the relative humidity is high. If the dew point is well below the dry air temperature - the relative humidity is low. Any surfaces below the dew point of the air immediately adjacent to them will suffer surface condensation.
What are some examples of common condensation-related mould species?
Aspergillus fumigatus, Cladosporium cladosporioides, Penicillium chrysogenum. For example, Stachybotrys chartarum (sometimes called S. atra or ‘black mould’) is a toxigenic mould that can trigger significant respiratory problems for those exposed to its spores – particularly infants and the elderly.
What are dry bulb and wet bulb temperatures?
Dry bulb temperature is the air temperature.
Wet bulb - The temperature to which air can be cooled by water evaporating into it (or the adiabatic saturation temperature) . It’s measured by wrapping a wet cloth around a thermometer bulb and exposing it to the air. Difference between the two indicates the humidity of the air. Greater the difference the lower the humidity.
What is cold bridging?
Cold bridging occurs in localised spots where the nature of the construction allows heat to escape through the structure at a higher rate than normal. This can lead to isolated patches of condensation.
What are some examples of deleterious materials
The term ‘deleterious materials’ is a broad one, encompassing not only materials that are dangerous to health or which are the causes of failures in buildings, but increasingly, materials which are environmentally damaging. Asbestos, Lead, Clay hollow pot floor, Chlorofluorocarbons (refrigerants), chlorides.
What is dry rot?
An aggressive form of fungal growth also known as Serpula Lacrymans. It is a wood destroying fungi that feeds off the cellulose in timber in order to grow and spread. This process leaves timber in a dry and brittle state and can weaken structural timber in buildings as a result.
What causes dry rot?
Dry rot is caused when a source of moisture and a lack of ventilation combine creating the perfect conditions for an infestation to start. There needs to be a source of food for the fungal spore - typically susceptible wood - and when the spore lands on the timber with moisture content over 20%, the germination process can begin before producing hyphae. The spores themselves are very small, about 0.01mm, and almost invisible to the naked eye
However large numbers of these spores will frequently collect around a fruiting body and form a reddish ‘dust’, which is a key sign to identify a significant attack of dry rot is present.
How does dry rot spread?
Dry rot has a few ways to spread throughout its fungal life cycle. For example, during its mycelium stage, dry rot can grow over and through various materials, including brickwork and masonry, in its search for more timber to consume. If light activates the mycelium, a mushroom-style fruiting body called a sporophore develops. This releases spores that look like red dust. These spores drift on air currents and germinate on suitable surfaces. So the life cycle turns full circle. All of this means dry rot can spread throughout a property fairly rapidly if the conditions are right.
How do you identify dry rot?
- Timber shrinking and becoming darker in colour
- Cracks in a cuboidal manner forming
- A silky greyish coloured skin frequently tinged with patches of lilac / yellow colouration will form/
- White fluffy cotton wool-like mycelium develops with strands that become brittle when dry.
- Finally, the most recognisable sign of dry rot is fruiting bodies, which have a soft fleshy pancake consistency the surface of which is orange or ochre-coloured.
- Rust red coloured spore ‘dust’ can frequently be seen around fruiting bodies covering localised surfaces.
What are the four stages (or forms) of dry rot?
- Spores
- Hyphae
- Mycelium
- Fruiting bodies.
What is rot spore dust?
Rust red coloured sores in the air. Spores are very common and usually harmless. However, if fungal spores start to appear in concentrated patches of rust coloured dust, this is a sure sign of an active problem. Spores begin to produce hyphae when they come into contact with timber in damp and humid conditions.
What is hyphae?
Spores begin to produce hyphae when they come into contact with timber in damp and humid conditions. Hyphae are white/grey strands that look similar to spider silk. Hyphae act as the root of the rot, stringing fine strands to grow through the wood and timber. The hyphae will then feed on the sugars within the timber known as cellulose, hemi¬cellulose and lignin. The dry rot fungi / fungus produces enzymes to split the sugars, reversing the formation of the wood. These enzymes, however, are unable to break down lignin. The subtraction of these sugars results in cross grain cuboidal cracking, reducing the timber to an unsound structural state. Hyphae then multiply and colonise together, generating mycelium growth, a fluffy cotton-wool like substance.
What is mycelium?
Mycelium is a grey and white cotton-wool-like mass that dry rot produces when it spreads from timber it can no longer feed on. It can travel great distances to find new sources of food, and it is this ability to spread through various building materials (it can even go through bricks and mortar!) that allows a dry rot outbreak to progressively feed on timbers throughout an entire property. Following the structure of a property drying out, it can lay dormant for anything up to ten years, and has the ability to come back to life should the environment be right.
What are dry rot fruiting bodies?
The last stage in the lifecycle is the most visually striking - the fruiting body. These fleshy masses of dry rot fungus look like large rust-coloured mushrooms. The fungus grows when it needs to pump fresh spores into the air to find more timber and start the cycle all over again. The dry rot produces a self–reproduction organ known as a sporophore. This allows the spore-bearing surface of the sporophore to shed orange coloured spores into the atmosphere in the hope that that the spores can land once again in the right environment to carry on germinating and extending the growth period of the dry rot.
How can wet rot be treated?
Typical -
Cut out all affected timber and apply a fungicide to the area.
Use preservative-treated timber for replacement.
Environmental -
Promote drying out of the affected area. (Eliminate source of moisture where possible). .
Do not re-plaster of cover up affected timbers until dried out.
Isolate wet timber. Increase ventilation where possible.
Extra air bricks in floor voids etc. Regular schedule of inspection and maintenance.
What is wet rot?
Wet rot describes a group of wood-destroying fungi (Basidiomycetes) that attack the cellulose or lignin in timber to leave it brittle and weak. Both brown rot and white rot are types of wet rot with the exception of one brown rot - Serpula Lacrymans - also known as dry rot.
Wet rots generally thrive on a higher timber moisture content than dry rot, but do not spread through masonry, and fungus growth stops when the moisture is removed.
The most common type of wet rot is Coniophora puteana (cellar fungus). Other species include Fibroporia vaillantii (mine fungus) and Phellinus spp.
How does it grow? (Stages of growth)
Wet rot fungus grows in stages. Early in its life cycle it will develop in strands called hypha that look a bit like spider silk forming into fern-shaped patterns. Depending on the specific genus of wet rot growing the colour will vary from brown rot to white rot.
These strands will later develop a white skin or coating and eventually a series of small fruiting bodies that look like tiny “off-white” mushrooms. This is known as Mycelium and is often accompanied by a musty smell.
This rot fungus cannot spread across other surfaces, but it can continue to grow and emit spores into the atmosphere.
What are the differences between wet rot and dry rot?
The most important difference between the two types of wood rot is that dry rot is far more aggressive and a much greater threat to your property than wet rot.
Unlike all species of wet rot which will remain localised to the source of moisture that allowed to grow in the first place, dry rot is only caused by the Serpula Lacrymans fungus which can spread far beyond the initial source and will actually spread across masonry and brick surfaces to attack fresh sources of timber. This means that wet rot does not cause dry rot and wet rot spores do not turn into dry rot fungus.
Other differences are that wet rot requires a high level of moisture content to begin to sprout, while dry rot spores will begin germinating at just 20 to 30% water saturation.
It is important to know that both types of wood rot share many similarities, and both require a damp environment to form.
Where does wet rot grow?
Wet rot fungus occurs when there is excess moisture in the property. This rot needs a continuous source of moisture to germinate and thrive, as a result, wet rot will only affect internal timber if there is a significant damp problem. Common property maintenance issues like structural defects, broken plumbing or guttering and leaking pipes can leading to rotten wood, timber decay and wet rot fungal growth.
Wet rot will grow on wood with a moisture content of between 30-60%. Wet Rot thrives in timber with a high moisture content of around 50% or higher, whereas dry rot only requires 20% moisture presence before it can attack. This is usually caused by defects in plumbing, external guttering or pipework that create leaks. Wet rot thrives when these damp conditions combine with a lack of adequate ventilation. A challenge associated with identifying wet rot is that it often develops in hard-to-spot areas of a property due to unseen water ingress. As a result, wet rot can be commonly found in damp basements, under floorboards, behind skirting boards and underneath leaking fixtures and fittings such as baths, toilets and washing machines.
What is mine fungus?
Group of fungi called Poria Fungi. These fungi generally attack softwood in buildings. They require a higher moisture content in the wood than that required by dry rot, but they are tolerant of occasional drying and are therefore normally associated with roof leaks.
This group includes Amyloporia xantha, Fibroporia vaillantii and Poria placenta. They are commonly called white pore fungi or mine fungi. F. vaillantii can cause extensive damage in buildings. The mycelium of this group of fungi forms white or cream sheets or fem-like growths, which may discolour brown on contact with iron. The rhizomorphs may be up
to 3 mm in diameter, seldom thicker than twine, white to cream in colour, remaining flexible when dry, and they do not extend from their foci of infection. The sporophore is rare in buildings; it is a white, irregular lumpy sheet 1.5- 12 mm thick, covered with distinct pores, sometimes with strands emerging from its margins. Spore-bearing surfaces are white to pale yellow, occasionally with pink patches (P. placenta only). The decay damage to wood is similar to that caused by S. lacrymans, but the cubing is somewhat smaller, less deep and lighter in colour. When decayed, the wood crumbles between the fingers. It is not as powdery as that attacked by S. lacrymans, but slightly more fibrous and gritty.
Types of foundation movement?
Settlement or differential movement.
What affect can trees have on ground conditions?
If a tree is removed, the moisture returns to the soil slowly causing it to expand (heave) by as much as 150 mm. Because the effect of the tree is localised, the movements generated in nearby foundations are differential and can be much more damaging than uniform movements.
Tree roots can extract large quantities of water from soil: a fully grown poplar uses over 50,000 litres in a year. When the soil is of clay, this will lead to a drying shrinkage, the magnitude of which will depend upon the inherent properties of the clay and, of course, on the nature of the tree and its moisture requirements. If tree roots take up moisture from under, or near to, foundations, the latter will subside and such subsidence will almost inevitably be uneven
The greatest problems have occurred when shrinkable soils have dried excessively through the removal of moisture by nearby growing vegetation. Such drying is likely to be greatest at the corners of foundations. As the ground falls away, the weight of the building pushes the then suspended parts of the foundations down and the walls in that vicinity crack. Cracking is predominantly diagonal and follows the vertical and horizontal mortar joints in brickwork, unless the mortar is abnormally strong for the bricks used, when cracking may occur through the latter. The cracks are widest at the top corners of the building and decrease as they approach ground level.
What are methods of detecting high moisture content
Electronic damp meter (moisture meter)
Speedy carbide meter
What are the limitations of an electronic damp meter?
Meters are only calibrated for timber,
Hygroscopic salts can exaggerate reading (it is important to test for salts),
Different species of timber have different resistances,
The following can provide exaggerate readings;
- Copper, Chrome or Arsenic treated timber
- Foil lined wallpaper
- lead paint
What is a carbide speedy meter?
A calcium carbide meter is a sealed vessel which is used to mix measured samples of masonry with calcium carbide. The calcium carbide will react with moisture present within the material and produce acetylene gas. The proportion of gas released is directly proportionate to the amount of moisture present in the material, therefore by measuring the gas, we can derive the total moisture content of the sample tested. The flask incorporates a calibrated gauge which is used to measure the pressure excreted in the cylinder by the gas which is read as a moisture content percentage.
What is an electronic damp meter?
Damp meters usually work by measuring the electrical resistance between two pins. Often displaying the moisture content as a percentage.
What are the limitations of a carbide meter?
Destructive technique
Generally not as quick as other methods.
What are some crack monitoring techniques?
Crack width gauge,
Plastic tell tale,
Glass tell tale,
Brass screws and callipers,
Target and total station
What is crack monitoring?
Observing crack width changes is one technique used to monitor structural damage due to movement.
What is a Crack width gauge used for?
Steel ruler is simple instrument used to monitor crack width variation. The crack can be measured to the nearest 0.5mm. Typically used at the start of a crack investigation.
What are Plastic tell tales used for?
The plate with scales marked in millimetre units of measurement is fixed on one side of the crack and the other plate marked with cursor is fixed on opposite side of the crack. Pros include; They can measure cracks along two axis, can get corner crack monitors. Cons include; fixings can come loose especially if stuck on with adhesive or if knocked. Typically 1.0mm degree of accuracy.
What are Glass tell tales used for?
This technique used to measure crack width variation in the past, but it is not popular any more. It basically consists of strip of glass cemented on to the cracked structural element. This method is no longer used because there is no way of measuring the extent of the movement or direction.
