Building Pathology Flashcards
Describe brown rots.
- Cuboidal cracking.
- Loses strength very quickly.
- Timber becomes darker in colour and dry.
- Friable.
Describe white rots.
- The timber develops a fibrous consistency.
- Cracking is along the grain.
- The wood will become lighter in colour.
- Generally, a higher moisture content is preferred.
Explain the life-cycle of wood rotting fungi, (5)
- Spores are dispersed from the fruiting body.
- Spores germinate (in the correct conditions).
- Spores develop Hyphae (root like filaments that penetrate the wood).
- Mass Hyphae become Mycelium which become visible.
- Mycelium develops and produces a fruiting body.
What are some conditions required for rot?
- Food - Sapwood contains nutrients.
- Moisture - The moisture content of the timber is key.
- Temperature - Fungal growth can be killed off in low or high temperatures.
- Air - Fungi require oxygen.
What is dry rot? Describe the conditions required.
Dry rot is a type of brown rot, which becomes dry and crumbly. The moisture content that makes timber susceptible to dry rot is 17-22%. Dry rot prefers damp still air where timber is in contact with brickwork and ventilation is poor, i.e. sub-floor voids and cellars. In these atmospheres the rot develops a fluffy/cotton wool appearance. Dry rot is considered the most serious of the rots due to its impact on the structure of the timber.
How do you identify dry rot?
- Timber becomes dry and crumbly.
- Timber becomes lightweight and a dull brown colour.
- Cuboidal cracking.
- The strands are grey and white.
- Mycelium may look leathery or like cotton wool balls.
- Spores cannot be seen but on mass they appear like red rust dust.
- The fruiting body is often pancake shaped with a white margin and red spores on the surface.
What are the causes of dry rot?
These conditions are usually found where water is coming into contact with the timber directly. The most common defects allowing water ingress are:
- Failure of roof covering or rainwater goods.
- Defective rendering.
- Inadequate ventilation.
- Defective DPC.
- Plumbing leaks.
What are the implications of dry rot if not remedied?
Spores can survive with interruptions to its living conditions and its mycelium can spread across masonry and attack other areas.
What are some remedial measures for dry rot? (7)
- A detailed survey to identify the areas of timber and masonry affected - intrusive techniques likely required.
- Strip off plaster to 1m beyond the last point of infection.
- If the timber has been structurally weakened replacement of the weakened timbers
with preservative pre-treated timber, concrete or steel members is required. However, as much decayed timber as possible should be removed. It is standard to cut out the infected areas + 500mm of adjacent timber to ensure the infected timber is removed. - The source of dampness should be eliminated and the timber allowed to dry.
- Timber can be treated with a fungicide to prevent decay, but to be effective treatment has to penetrate deep into the timber.
- Irrigation of surrounding walling.
- Dry rot masonry fluid can be applied to the surface of damp and infected walls.
Why does cut edge corrosion occur and what can happen if it is not treated?
When the cut edges of coated metal cladding are exposed to the elements, for prolonged periods of time, delamination may occur in the form of peel back of the factory coating. The unprotected metal substrate then becomes exposed to rusting and corrosion.
How can cut edge corrosion be treated?
A silicone paint application (e.g. Seamsil) can provide edge protection to the sheet metal. If the corroded section is badly affected then a replaced section of sheet metal may be required.
What coatings are usually found on sheet metal?
Plastisol is a PVC plastic coating often used on sheet metal. It is usually identified by a leather effect. An alternative to plastisol is pvf2 which is a thinner material and has a smooth finish.
How can coatings on a sheet metal break down?
These coatings can be affected by UV radiation, humidity and oxygen leading to a loss in colour, chalking and delamination.
What age of a roof is cut edge corrosion most likely to occur?
10-15 years old.
What are some common symptoms of deterioration of GRP rooflights?
- Exposed fibres
- Cracks
- Poor light admittance
- Moss and lichen growth
What are some common causes of deterioration to GRP rooflights?
- UV degradation breaks down the top coat.
- Temperature changes leads to brittle sheets
- Exposed fibres encourage moss and lichen growth
