There are a number of causes for fissures and cracks in timber. The two principal reasons are:

Often timber components for structural use are also those timbers acting as architectural features. They may be installed with relatively high moisture content compared with what will be the eventual service class. The outline characteristics for each of the service classes are indicated below:

Service Class 1

Characterised by a moisture content in the materials corresponding to a temperature of 20ºC and the relative humidity of the surrounding air only exceeding 65% for a few weeks per year. In such conditions most timber will attain an average moisture content not exceeding 12%.

Service Class 2

Characterised by a moisture content in the materials corresponding to a temperature of 20ºC and the relative humidity of the surrounding air only exceeding 85% for a few weeks per year. In such moisture conditions most timber will attain an average moisture content not exceeding 20%.

Service Class 3

Characterised by climatic conditions leading to moisture contents higher than Class 2. Timber should be installed at a moisture content close to that which it will attain in service. Strength graded timber to be used in Service Class 1 and 2 should be assessed for fissures and distortion at a moisture content of 20% or lower, with no reading to exceed 24%. These recommendations do not apply to timber with a target thickness of 100mm or more. Timber supplied in a dry condition should be adequately protected. Wood-based panels are manufactured with a low consistent moisture content; if variation in moisture condition during service is likely then this situation should be considered and addressed before installation.

Router

Jigsaw

Often softwood featured exposed columns and beams, ie for structural or pseudo-timber framing, exhibit cracks and fissures on a continuing basis as the timber dries. Quite frequently, the timber is in an external position, when the fissures will be affected by intermittent wet and dry and often open and close seasonally. In order to minimise this affect, the following action has proved successful over a number of years:

1. Establish the date when the timber was installed and, identifying its moisture content, take a number of regular readings of a minimum of a 2-month period or through a seasonal variation - all in order to check both variability of crack width and depth and moisture content.

1.1 Measurement of the fissures is most appropriately carried out by using the Avonguard tell-tale system. These movable tell-tale units can be fixed across the fissure in the timber and regular recordings of the movements made on the standard sheet. The resulting information will enable a decision to be made regarding the most appropriate method of repairing the fissures.

2. Ensure that the timber is below 22% moisture.

2.1 Where necessary, enlarge the existing fissures using a jig saw or a router. The aim is to ensure that the fissures which are to be in-filled are regularised in such a way that the vertical sides are parallel as apposed to natural scalene triangle effect which is apparent in normal timber shrinkage.

3. Remove all dust and other foreign material.

4. Prepare over-size timber slips of the same species and moisture content as the timber component to be treated. (Variation in moisture content not to exceed 1%)

5. If the length of the fissure may be irregular, these slips will be required to be made in a number of lengths. The final visual appearance will be dependent upon the quality of the carpentry coupled with the type of final finish.

6. It is possible in some cases that the regularising and removal of the timber does not remove the full depth of the fissures, i.e. there will be continuing fissures on the bottom surface of the freshly exposed timber. The procedure then will be to treat these small fissures with the injection technique described below. Once this work is completed, the insert timber can be installed as follows:

- Apply the CB10T SS lubricant adhesive to the sides and base of the regularised fissure.
- Apply sufficient CB10T SS to 3 faces of the timber fillet to ensure complete wetting.
- Place the pre-coated fillet(s) into the appropriate slots and firmly fix with a soft mallet.

Vertical fissure prepared by router technique

Vertical column fissure routed to 100mm wide 60mm deep

7. In the case of relatively small section timbers 150 by 150 mm that are heart wood centred, usually purlins, and where fissures exist on two faces especially at right angles, care must be taken not to apply undue pressure with the mallet, otherwise splitting may occur. When working on alternate faces slips should be inserted alternately from face to face rather than one series of slips down the whole length of an individual face.

8. Allow a minimum of 2 to 3 days at a temerature at least 18°C for cure of the adhesive.

8.1 Sand or plane down the surfaces as required.

8.2 Stain or apply suitable protective coating as required.

Completed and re-stained beams and columns.

References
1. Timber Engineering Fact Sheets 1-6 - Rotafix document FCST0093/1
2. BS 5268 Part II, 1996 - Sections 1 and 2
3. CB10T SLOW SET data sheet - Product Code 3552

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