3.8.2020
Why wood? | Timber construction

Design of log houses

Modern log construction is making a comeback. The last time log construction was modernised was in the 1960s when Kaija and Heikki Siren were designing modern, log-built holiday homes. Nowadays, log construction is acceptable even for public buildings and offices, so we have prepared this article about the key design principles of log buildings.

Until the 1920s, logs were the principal building material in almost all forms of con­struction. Wooden frame construction replaced log construction, even for small houses, after the Second World War. From then onwards, logs were for a long time used mainly for building holiday homes. Nowadays, log construction is once again appropriate for use in buildings of different sizes and for different purposes. Use of logs is on the increase, es­pecially for public buildings and for blocks of flats.

Logs are used mainly in loadbearing wall construction, but non-loadbearing parti­tions can also be constructed from logs.

Traditionally, because of the length of the available timber, the maximum length of a wall constructed of logs was about seven me­tres. Today, with industrial manufacturing, laminated logs and finger jointing, it is in principle possible to manufacture extreme­ly long logs. If this is done, however, spe­cial attention must be paid to making sure the wall has sufficient transverse stiffening. Log walls are stiffened with crosswalls and/ or dowels. Dowels prevent the logs from be­coming warped out of position, especially in long walls and adjacent to openings. The spacing between dowels should be a maxi­mum of 2000 mm.

One special characteristic of the design of a log building is the control of sink or settle­ment and the movement of the wood, such as splitting. Apart from this, the design prin­ciples of log building are more or less as for other forms of timber construction. Log construction can be used to construct build­ings which are excellent in terms of energy efficiency, weather-tightness, fire safety and sound insulation.

One special advantage of log buildings is their safety as far as moisture content is con­cerned, something which is reflected in the quality of their indoor air. As the humidi­ty of the indoor air rises, the wood surfaces absorb moisture from the air and then re­lease it when the indoor air is dry. Research shows that, because of the untreated wood surfaces, the humidity of indoor air remains well within the recommended limits of 30–60% relative humidity. The phenomenon is weakened if the wood surfaces are treated with a finish which prevents the transfer of moisture.

Sink or settlement of logs

Typically, sink or settlement in log walls are caused by shrinkage resulting from the wood drying out and by the wood shrinking and the joints contracting due to the mass of the building. Depending on the quality of the logs, a log wall will sink or settle 10–15 mm per metre of height. A round log will sink most and a laminated log least. Because of the lower moisture content, internal parti­tions will sink a little less than external walls. The sink of a ‘non-sink’ log is comparable to other forms of timber construction.

For example, a hand-formed round log will need a sink allowance (painumavaraa) of 100mm over a two metre opening. The wall will actually sink about 60–70 mm, so there will still be room for the insulation above the sink allowance.

Sink or settlement must also be taken into account in all places where log construction which is subject to sink or settlement adjoins non-sink construction. This includes win­dows and doors, furniture, non-loadbearing wood-frame partitions, stairs, brick construc­tion etc. Sink must be allowed for in joints with non-loadbearing construction, but in load­bearing construction, screw-jacks are used.

Flues must be joined to the structure in such a way that the joint allows the structure to sink so that safe distances from the structure are maintained even after sink or settlement has taken place. High-level kitchen cupboards can only be fixed to one course of logs and the amount the high-level cupboard drops must be allowed for in any splash-back tiling.

Special attention must be paid to sink when there are changes of level in the building re­sulting from different numbers of courses of logs in different parts of the building. Sink is greater in those parts of the building where there are more courses of logs.

With roof structures supported on log construction, it must be noted that the sink or settlement of the roof at the ridge is great­er than that at the eaves. This is caused by the outward thrust of the rafters at the eaves. The rafters must be fixed to the top of the wall with fixings that allow movement, so that when they sink and settle they do not push the wall line outwards.

Shakes and splits

Shakes and splits are particularly characteris­tic of solid, round logs. They are a natural re­sult of the tensions which come into play as the wood dries out. As it dries, wood shrinks about twice as much in a radial direction as in a tangential direction. In addition, the in­ternal tensions in wood emphasise the fact that drying out starts from the surface. In log building, the logs can be rotated so that exces­sive splits come within the area that is to be worked to form the groove (varaus).

The magnitude of any shakes and splits is affected by the moisture content of the log and its size. In a heated indoor space, the moisture content of a log settles down at around 8% and in an external wall at around 14% of the dry weight. However, because of solar radiation and structural protection, there may be major variations in the mois­ture content of external walls. In winter, splits tend to open up, and in summer, they tend to close.

The importance of splits and shakes is mainly aesthetic. In indoor spaces, splits and shakes have a favourable effect on the ability of the logs to even out variation in the humidity of the indoor air, because splits increase the surface area of the interface be­tween the hygroscopic wood and the indoor air, through which diffusion occurs. The sur­face area is directly correlated with the abili­ty of the building to sequester moisture from indoor air and release it.

Weather-tightness

The weather-tightness of a log building is en­sured by the shape of the groove or ‘varaus’ between the logs, and by the way they are sealed. The most critical areas from the point of view of weather-tightness are joints be­tween different components in the external envelope, or holes in it for pipes or other components to pass through. Jointing prin­ciples are examined in the adjoining draw­ing. Where holes occur, it is recommended that special seals are used. Measurements show that a carefully constructed log build­ing can be extremely weather-tight.

Energy efficiency of log walls

The E-factor required for a small house built of log construction depends on the size of the building. With a maximum heated area of 120 m2 the E-factor may be 229 kwh/m2 per year and for a building with a maximum heated area of over 600 m2 the E-factor must not be more than 155 kwh/m2 per year. For heated areas between 120 m2 and 600 m2 the permitted E-factor is calculated according to surface area using a special formula.

The energy efficiency regulations do not apply to buildings which have a net heated area of less than 50 m2 or to holiday homes which are not designed with heating systems for all-year-round use.

In comparative heat-loss calculations for buildings, the heat transfer coefficient for a log wall is taken to be 0.40 W/(m2k) and the thickness of the logs is assumed to be 180 mm. In a holiday home with a heating system designed for all-year-round use, the heat transfer coefficient is taken to be 0.80 W/(m2k) and the minimum thickness of the logs is assumed to be 130 mm. This excep­tion does not apply to holiday homes with heating systems designed for all-year-round use, which are intended for the business of providing accommodation.

The average thickness of a solid log wall must be at least 180 mm to satisfy the 0.60 W/(m2K) U-value requirement. Insulated log construction easily reaches a U-value of 0.17 W/(m2K). U-value calculations for log walls can be easily carried out using the Puuinfo U-value calculator ‘Puuraken­teen U-arvon määrittämien’ (Specifying U-values for timber construction).

Values for round-log walls must be calculated using the effective thickness of the wall, i.e. with the wall thickness changed to a notional thickness which is the same throughout. This can be found in the Puuinfo technical information sheet ‘Hirsiseinän tehollinen paksuus’ (Effective thickness of log walls).

Long-term durability and the protection of log surfaces

The durability of logs is primarily affect­ed by the moisture content of the wood. Saprophytes and moulds do not begin to grow until the moisture content of the wood exceeds 20% and the temperature is above +5 °C. For the moisture content of the wood to exceed 20%, the relative humidity of the air must be above 85% for a lengthy period. The durability of logs is also affected by light. The ultravi­olet light in sunlight penetrates the wood to a depth of about 0.1 mm and destroys the lignin in the wood. If the conditions are favourable to wood, timber construc­tion can be extremely long-lasting.

The same principals apply to the protection of log construction as to other forms of timber construction. Logs must be kept sufficiently distant from the ground. The passage of mois­ture from the foundations by capillary action must be prevented. Wall con­struction must be protected from rain, splashes and surface water. Roof drain­age should be controlled using gutters and downpipes. Attention must be paid to construction especially the ventila­tion and drying of the joints between the logs which are particularly suscep­tible to the weather.

Log surfaces can be protected me­chanically, chemically or by surface treatment. Mechanical protection is usu­ally provided by cladding boards. These give the logs a ‘wearing surface’ that can easily be repaired or renewed.

The function of chemical protection or surface treatment is to prevent fun­gal spores from becoming attached to the wood surfaces, to prevent the wood from absorbing moisture, to eliminate the ef­fects of UV radiation and to form a wa­ter-repellent membrane on the surface of the wood. Such coatings can be either transpar­ent or opaque.

Dimensioning log buildings

Log buildings are dimensioned according to the Eurocodes. The strength class for planed logs is taken to be C22, for laminated logs C24 and for round-logs C30. The dimen­sioning recommendations given in RT 82-11168 for the loadbearing capacity of log construction are based on the VTT research report (RTE3818/00).

Fire resistance of log construction

Log construction is extremely safe in the event of fire. Logs char at the rate of 1.0 mmper minute so that the behaviour of log con­struction in the event of fire is predictable. As the wood surface begins to char, the charring begins to protect the wood from burning.

Logs are defined as being in Class D-s2.d0. REI 30 fire resistance is achieved with 92 mm-thick planed logs and 150 mm-thick round-logs. REI 60 fire resistance is achieved with 148 mm-thick planed logs and 236 mm-thick round-logs, and REI 90 fire resist­ance is achieved with 199 mm-thick planed logs. Insulating the structure improves its fire resistance. Dowel spacing must not ex­ceed 1600 mm.

The use of non-clad log surfaces becomes more difficult in circumstances where the fire regulations call for a surface which is in a higher Class than D-s2.d0. In such cases it may be possible to switch to a lower Class by using automatic extinguishing equipment.

Log construction can also be used for blocks of flats. In small blocks of flats which do not exceed two storeys, log surfaces can be left visible. In log buildings of over two storeys, leaving the log surfaces visible calls for performance-based fire design

Sound insulation in log buildings

The sound insulation of a log wall depends on the mass of the wall, the weather-tight­ness of the joints and the stiffness of the wall. These all improve as the thickness of the logs increases. Non-insulated log-wall construc­tion is calculated to have a sound insulation factor in air (Rw) which varies between 30 and 40 dB as the thickness of the logs varies between 95 and 270 mm.

Sound insulation of external walls against e.g. traffic noise can be improved by addi­tional insulation and boarding on either the inside or the outside of the wall. This way, the calculated sound insulation in air (Rw) of the log wall can be improved to between 43 dB and 54 dB, depending on the thick­ness of the logs and the insulation, and the boarding used.

Logs can also be used as wall construction for walls between apartments. The walls are built as double log wall construction with in­sulation between. Some manufacturers offer their own log type for this purpose, which has particular attention paid to the weath­er-tightness of the joints.

For more information on log construction in Finnish see e.g.: