Structural protection of logs

It is often said how “back in the day people knew how to build log houses”. Of course, people also built poor log houses in the past, but they have decayed over time. The only buildings left standing are those with structural solutions that protect the wood from rotting.


The location of the log building was carefully selected, and it was inhabited in the summer and the winter. Log buildings were often draughty. The heating period during the winter dried the structure – houses left empty over the winter were ruined.


An essential part of log building was to protect the wood from rainwater and thus from rot-inducing fungi. The timber was also carefully selected and dried slowly.


Significant moisture stress caused by the environment and the weather

Holiday houses are often located on the shore of the sea or a lake. Rainwater can be driven by the wind and penetrate the structure on the side of the waterway. The wet log and the insulation between logs dry very slowly, resulting in a very high risk of rotting in the shore-side wall and balustrade structures. Rotting risk is also increased by snow piling up next to the wall, as its melting water soaks the log.


Long eaves, storm water control and tall plinths protect the walls from storm water. Regardless of this, slanted rain often wets the entire wall. The wall facing the sea or a lake must have a roof that prevents slanted rain from wetting the wall structure.



In archipelagos and on the coast in particular, log buildings have traditionally had plank linings that are cheaper and easier to repair than rotted log structures. The lining protects the log structure from rainwater. Encasing the log corners with plank lining also protects the sensitive cut surfaces of the logs.

Structural errors


If a log building has structural errors, a surface treatment agent cannot correct the defects.


Rot damage is always caused by the high moisture content of the log structure. The moisture can come from the ground, storm water or an indoor water leak. Wood requires long-term exposure to moisture to rot, with the wood moisture remaining above 25% for a long time. Insects may also infest rotting wood and move on to eat healthy wood.


Rot damage is most common in the bottommost logs, with causes including:

  • A plinth that is too low or has sunk over time, or the rising of the outside ground level
  • Cracks in the logs through which rainwater infiltrates into the wood  
  • The plinth and steps are built in a way that guides rainwater into the logs. This can be caused by the concreting of the plinth or non-existent moisture proofing.
  • A soil bench structure which rots the logs from the inside (empty space below the floorboards, and insulation implemented by increasing the ground level inside the house, and often also outside, above the joint between the footing and the bottom log).
  • Poorly ventilated subfloor space





Rot damage may also appear in the logs at roof height, under the windows and around water mains connections. There may also be damage at the base of chimneys and behind the firewall. The moisture in concrete is sufficient for the growth of some rot fungi, so if there is no moisture proofing between the foundation and the log, this can also expose the lower part of the wall to rot.


Old log buildings often have rot damage in exterior wall logs at roof height. Their roofs may have leaked at some point, wetting the roof insulation. From there, the moisture seeps into the rest of the structure over a long period. Rot damage will then be difficult to detect, as it appears precisely where the insulation layer is.

Uninsulated flyway corners and cross corner joints are also very susceptible to rot. Log ends absorb water quickly along the grain. Water penetrates inside from the corner joints and travels inside the logs and the insulation between them.


In traditional log buildings, log corners were protected with plank siding or notched with dog neck mitre joints. The ends of the ceiling supporting members were protected with roofing felt or bark to prevent them from rotting.


corner plank sidings

An example of modern structural protection with corner plank sidings


Massive logs and cracking

A log wall that has not been given a surface treatment will go grey and crack, and the elements will wear its surface. The logs of a new building crack when they dry, but also later during use. As the wood dries and dampens, the resulting stresses cause cracks in the logs.


Wood begins to rot when its moisture remains above 25% for a long period. Cracks in the logs allow rainwater to infiltrate the log. Laminated logs crack significantly less than massive logs, as they have been dried and the stresses do not build up to the same levels.






Log balustrades

Massive log balustrades always have cracks, allowing rainwater to seep inside the wood. Log balustrades have a high risk of rotting, particularly if the balustrade structure is uncovered or the roof does not entirely protect it from rain. A dampened log dries very slowly, so for long periods the amount of water absorbed into the wood may remain larger than the amount that evaporates from the wood.


We recommend using balustrades made from boards, or protecting the structure with a drip board or sheet metal cladding, preventing slanted rain from penetrating the structures.  A surface treatment agent is unable to remedy structural weaknesses, nor is it its task.


The durability of the wood material in log balustrades exposed to direct weather stress is always significantly poorer than that of the wall surfaces. Additionally, continuous weather stress causes the surface treatment to wear out faster. For this reason, we recommend semi-transparent wood finishes (Valtti Color, Valtti Akvacolor, Valtti Arctic) for the surface treatment of unprotected log balustrades due to easier maintenance painting.


Log house manufacturers have moved from massive log balustrades to lighter balustrades made from boards.