A Closer Look at Decay Fungi in Timber Framing

December 17, 2013

The leaky building crisis in New Zealand, and in other similar parts of the world, has been and still is of great concern to many homeowners. It brought to everyone’s attention that houses can be very vulnerable if they are built with natural products such as Radiata pine timber framing. This timber lacks the ability to adequately withstand dampness if exposed to it over a prolonged time, as it is not very durable. For example the life expectancy of Pinus radiata D. Don heartwood is approximately 5 years if in ground contact.  However timber framing is mostly  sapwood with the durability being much less, so full sapwood penetration with appropriate preservatives is essential.

Fungal spores (like pollen from trees), or fungal fragments (like a cotton string cut up into little pieces), are nearly always in the air as wind is one of the major ways for their propogation and search for new grounds for suitable organic nutrients like bread, fruit and damp framing timber. Other means of spore distribution can be by water, animals, insects and humans. If the time and conditions are right, they will take hold. In this case the spores germinate and produce very fine and thin hyphae (long strings) that start to colonise the timber (or fruit or bread).

To give a basic understanding why the colonisation of construction timber through wood decay fungi is possible, this is best explained by highlighting some of the important factors that have an impact on fungal establishment, development and survival:

1) Moisture content of wood

The moisture content (MC) of wood is given as a percentage based on the oven dry weight of the wood species. For timber in all its uses, the critical moisture content is around the fibre saturation point (FSP), which is at approximately 30%. Generally, below FSP, all water is tightly bound to the wood cell walls and is unavailable to most fungi. Only when the water content in wood rises above the FSP in the form of free water in the wood cell lumina and/or other cavities does timber in use get susceptible to decay. Therefore, it is commonly accepted that wood kept dry will in most cases be protected from fungal attack.   However, this might not always be the case especially if the timber was pre-infected by fungal hyphae or so-called resting spores that can withstand longer periods of drought and might start to colonise timber again at MC below FSP.  Today we know that many decay fungi found in buildings can colonise timber well below FSP, in the MC range of 18 to 28%. One example could be the true dry rot fungus Serpula lacrymans that thrives best on MC between 25-55%.

2) Humidity

Wood is a hygroscopic material and is therefore responsive when exposed to changes in relative air humidity (RH). As there is no constant atmosphere with ever changing temperature and RH, wood is constantly (as every other hygroscopic material) trying to achieve a stable equilibrium moisture content (EMC) condition resulting in either gaining or losing moisture.

The EMC of timber in constructions (wall framing) usually ranges from 6 to 14% but it can reach EMC’s of 17-20% under high humidity conditions. 20% EMC in Pinus radiata would compare to approximately 90% RH at 15°C. 100% RH at 15°C would reflect a MC of very close to FSP (28%). In perspective, the general EMC for P. radiata sapwood stored outside and under cover in the North Shore Auckland can average to 14.3% in summer and 20.8% during winter.

3) Temperature

For most wood decay fungi, growth has been recorded between 12 and 45°C and their optimum temperature range is given between 20 and 35°C. Beyond this temperature range, wood decay fungi go into a dormant state and survive colder or warmer temperatures by forming resting or dehydrated structures.  Some fungal species can live, survive and degrade wood at the lower temperature range < 0-10°C like fungi found in the Antarctica and others are still active at higher ranges up to 65°C as for example found in decomposing compost or wood chip piles.

4) Oxygen

Most fungi are aerobes and oxygen is a prerequisite for their hyphal extension and growth. If there is too much water within the timber, it restricts access of fungi to oxygen and they grow less. On the other hand, a higher demand for oxygen by fungi is needed when reproduction is initiated where fruiting bodies and spores are formed. This explains why most fungi form their fruiting bodies at or near the surface of their substrate where the necessary oxygen demand can be found.

5) Nutrients

Wood decay fungi not only need organic matter and water for growth and development but inorganic nutrients as well. Moisture and/or water contains dissolved inorganic nutrients; when the ground, stucco or stone materials are in contact with the framing timber, they provide the mineral nutrients and phosphates necessary for fungal growth and development. Further, fungi can play an active part by directly extracting required nutrients from the building environment like plaster, fibreglass and the ground and especially wood.

In summary, all these factors together allow decay fungi to colonise and degrade construction timbers when the time is right.

How this looks like in the early stages is illustrated with a few pictures below. In principal, pine wood is made up of a bunch of straws called tracheids, with each straw having three cell wall layers called S1, S2 and S3/warty layer and are surrounded by a middle lamella that holds the straws together (Pictures 1 and 2). The fungal hypha first grows into the hollow centre/lumen of the tracheids from where it sets out to colonise the timber and attacks/degrades the cell wall layers (Pictures 2, 3 and 4). Over time, this is first causing a loss in structural integrity of the timber framing (Pictures 2 and 3) and ending in complete deterioration (compost) of the organic food source.

Dr Dirk Stahlhut, PRENDOS New Zealand Ltd

Picture 1) Structure and features of a Pinus radiata wood cube seen through a ‘magnifying’ glass.

Radiata tracheid cube

Picture 1 Scanning electron micrograph of P. radiata:

Face A: Transverse: T= cut ends of tracheids; L= latewood; E= earlywood; R= resin canal; Face B: Radial/longitudinal: WR= wood ray; Face C: Tangential/longitudinal: FR= fusiform ray; Source: Harris 1991.

early degradation of cell wall layers

Picture 2 Scanning electron micrograph showing early degradation of cell wall layers.  Transverse section showing hypha in lumena (pink arrow), bore-holes (white arrows), an intact S3 layer (yellow arrow) and the start of S1 delamination at the middle lamella/S1 interface (red arrow). Scale bar 10µm.

fungal hyphae growing in tracheids.png

Picture 3 Microscope picture showing fungal hyphae growing in tracheids and stained blue for clarity.  Tangential section showing ordinary clamp connections (yellow arrows), medallion clamp (red arrow), bore-hole (pink arrow), cell wall fracturing (turquoise arrows) and bore-hole and hypha measurements. Scale bar 10µm.

Oligoporus placenta

Picture 4 Scanning electron micrograph showing resting spore anchored into cell wall ensuring fungal survival during changing conditions.  Oligoporus placenta (Brown rot fungus) resting spore anchored to the warty/S3 layer of the tracheid (white arrow) and showing warts on the lumen surface (black arrow). Scale bar 1µm.

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