Choosing High Quality Material

In old trees of most species, a large part of the trunk consists of heartwood, which has a strong resistance to fungi and insects. Not even the house longhorn beetle can penetrate the heartwood of pine. Heartwood was traditionally used in log construction and external panelling and, until the nineteenth century, in windows and doors.

Initially, pine was thought to be more durable than spruce, but this conclusion has been modified. The core of pine has almost no moisture absorption capacity, whereas the sapwood has a moisture absorption capability (lengthwise in the cells) 10 times greater than that of spruce. Pine cladding from the young core is therefore less protected than spruce. Birch cladding is even weaker; its permeability is about 1000 times greater than that of spruce. Generally speaking, the absorption of moisture increases in relation to the breadth of the growth rings.

Table 19.2 Minimum slope of roof to prevent water seeping in

Type of roof covering

Normal situation (°)

Exposed location p)

Corrugated metal sheeting

10

14

Concrete roof tile

15

22

Corrugated cementitious sheeting

14

18

Slate, single layer

22

30

Slate, double layer

20

25

Fired clay tile, interlocking

20

30

Bituminous shingle

18

22

Bituminous roofing felt, two layers, welded

3

3

Plastic membranes, welded

2

2

Timber, shingle and plank roof

22

27

The manner of sawing plays an important role. There is least warping and cracking in planks with standing annual rings (see Figure 10.4). This greatly reduces the likelihood of water penetration and fungal attack.

Timber should be felled in winter, because wood felled in summer has a much higher sugar content, making it more attractive to insects and micro-organisms. By removing the bark from the felled trees, attacks by bark-eating bugs are avoided. Sawn timber should be dried to 20% moisture content before spring, and logs that are not going to be sawn should be stored in water. In northern climates, pine for log construction should be felled in September and profiled on both sides during the spring. It should be dried during the summer and used as building material in the autumn. See more about preventative sawing and drying routines on page 169-170.

Material from a building that has recently been attacked by the house longhorn beetle or the common furniture beetle should not be re-used.

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