Larch – technical assessment

Why we don’t consider spruce, fir, Siberian larch suitable for some outdoor structures.

Excerpt from an expert opinion of a specific larch horizontal structure.

The subject of the assessment are wooden ceiling beams above the 2nd floor, or their sections extending outside the interior of the building, where they basically form a pergola above the terrace of the 2nd floor, in the other direction the overlaps are rather an architectural element. The basis for the assessment was an on-site inspection and identification of a wood-decaying fungus that appeared on the beams and caused their destruction. A sample of damaged wood with mycelium was taken in a place without fruiting bodies of wood-destroying fungi and from another place with fruiting bodies of various sizes.

  • Evaluation of samples of damaged wood and fruiting bodies of wood-destroying fungi

On the basis of macroscopic assessment and microscopic evaluation of taken samples of wood, mycelium and fruiting bodies, the cellulosic wood-decaying fungus Rusty gilled polypore – Gloeophyllum sepiarium (Wulf. ex Fr.) P. Karst was clearly identified. The identified wood-decaying fungus Rusty gilled polypore was found in controlled samples at various stages of development, including fruiting bodies. The active state was proved. This wood-decaying fungus is active mainly on dead wood of spruces, pines and firs. It occurs everywhere where there is a long-term accumulation of moisture or in places with a repeated supply of water. It causes intense brown destructive rot, in the last phase of decay the wood loses its strength and thus its functionality. Rot occurs inside the milling profile and the infection often manifests itself only when the infected part of the structure is in a state of emergency. Not always do the fruiting bodies appear “in time” on the infected wooden structure or its parts, which signal the fungal attack. The fruiting bodies are annual, biennial or even perennial.

The fungus germ may already be in the wood and is waiting for suitable conditions to start its destructive activity. Rusty gilled polypore is, otherwise, mainly multiplied by producing spores that are present in the air, because this wood-decaying fungus is truly abundant. Dry cracks, loose contact of parts (joints) and moisture are then a breeding ground for the infection.

Rusty gilled polypore is dangerous not only because wood decay takes place inside the profile, practically unnoticed, but also due to its ability to survive in extreme climatic conditions. It withstands even stronger frosts, can withstand relatively high temperatures, and after infecting wood is not too demanding on moisture. When the humidity conditions are eliminated, this wood-decaying fungus suspends its activity; in the event of favorable conditions, the resumption of its destructive activity cannot be ruled out. It makes full use of all these properties on outdoor structures (fences, terraces, pergolas, balconies, playgrounds, garden furniture…). In this given case ceiling beams crossing from the interior to the exterior, where it forms an outdoor pergola which is affected by all climatic influences. Due to the fact that Rusty gilled polypore develops inside the wooden profile, usual surface finishing with a fungicidal coating or spraying is insufficient.