Types Of Wooden Roofing

The cleft log roof consists of trunks cleft in half and laid over each other (Figure 15.18).This type of roof is widespread in Finland and Sweden. Cleaving the timber gives a much better water-resistant surface than sawing (see 'Splitting', page 170).The cleft log roof therefore has a longer lifespan than others, as long as the slope is adequate.The bottom layer is often made of planks instead of half-rounded timber and is therefore easier to lay, but this reduces the durability.

15.18

Cleft log roof.

15.19

Plank roof.

Source: Norwegian Building Research Institute.

The plank roof consists of planks in two layers overlapping each other and running down the slope of the roof (Figure 15.19).The plank roof is often also used as a basefor other roof coverings.

High quality pine should be used in widths less than 15 cm to reduce the chance of cracks forming. There should be grooves on the edges of the upper (pointing downwards) and lower planks (pointing upwards) fordraining water.The planks are laid so that they press against each other when they swell in damp weather.The face with the inner grain of the tree must face upwards, especially in the case of the top planks.The part of the plank closest to the base of the tree has the best quality and should be on the lower part of the roof.

The plank roof can also be laid with horizontal boarding. This can only be used for steep roofs and is occasionally found on small towers or ecclesiastical buildings. It was often used on the oldest stave churches.The boarding is nailed with about 5 cm overlap, the innergrain facing upwards.

15.20

System for cleaving softwood shakes. Oak shakes are always cleft radially in the wood. Source: Vreim, 1941.

15.20

System for cleaving softwood shakes. Oak shakes are always cleft radially in the wood. Source: Vreim, 1941.

Shakes.Timber that is to be used as shakes has to come from mature trees and be well grown without any through knots. The trunk is first sawn into 30 to 65 cm long sections and then split into quarters (Figure 15.20).The pith is scraped away. The pieces are often boiled to reduce the chance of cracking when being cleft, but heating to over 70 °C also makes the resin melts out, and impregnating effect is lost.

Cleaving is then performed using aspecial knife which is 35 cm long and has a handle on each end. The sharp blade is usually placed radially on the end of the log and knocked in. The shakes should be from 1 to 3 cm thick. It is also possible to cleave shakes by machine.

The shakes are fixed on battens using the feather boarding principle with 2-3 mm between them to allow for shrinkage and expansion. A normal covering consists of two or three layers.They are nailed with wire staples so that the holes are covered by the next layer. Usually one staple per shake is enough.The staple should not be so long that it penetrates the roofing felt. Laying details are shown in Figure 15.21.The shakes can be shaped in many different ways, the most complex often being reserved for ecclesiastical buildings.

15.21

Traditional technique for laying shakes in three layers. Source: Eriksen.

Shinglesaresawn byacircularsaw.Theyare40 cm longand10to12 cm widewitha thickness of 1 cm at the lower end and 0.5 cm at the upper end. They are laid next to each other with a spacing of about 2 mm, usually in three layers, which means that the distance betweenthe battens is about13 cm. In the nineteenth century the majority of buildings in NewYork were roofed with shingles.

15.22

A less processed version of shakes laid in three layers with overlap sideways.

15.22

A less processed version of shakes laid in three layers with overlap sideways.

15.9.2 Timber cladding

Timber panelling has a long tradition as a cladding material, first as external wall panelling and later as internal wall and ceiling cladding. The different types of cladding have changed slightly through the twentieth century, particularly to suit mechanized production. Special panelling can include cladding with shingles and shakes. Cladding with twigs and branches also has a long tradition in certain countries. Juniper is widely used and gives functional, long-lasting weather protection.

Panelling for external walls should preferably be of high quality timber. The planks should be sorted on site and the best ones placed on the most exposed parts of the building. Nailing through two planks should be avoided; they may split through differential movement. External cladding should be nailed at an upward angle to avoid water seeping in and settling.

Timber panelling on an external wall is usually far more durable than the equivalent panelling on a roof. It is still important to choose the right system of panelling and use the correct form of chemical or 'constructive' timber treatment (see Chapter 19). A solution of water-glass (see page 90), can be used as a flame retardant.

Interior wooden cladding has a very resilient finish compared with many alternatives, and the surface has good moisture buffering properties if untreated, or treated with surface finishes such as soap or lye.

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