Forest Products Notes on...

Design of Stacks in the Timber Drying Yard

Much of the discussion below relating to the construction of “box” (or “flat”) stacked (or “piled”)  square-edged timber is also important when such timber is being dried in a kiln.

The correct CONSTRUCTION and ORIENTATION of stacks of timber is very important as it influences the air circulation, which is a significant determinator of the rate and even-ness of drying. Furthermore improper piling can cause distortion of boards, whilst careful, appropriate piling can reduce the degree of defects which might otherwise occur.

Stacks of sawn timber should not be built directly on the ground but should be raised off the ground on bearers (or foundations) – usually of brick or concrete (but it should be ensured that lower boards in a stack are not damaged, by indentation, due to the resilience of such hard materials) or treated hardwoods (to avoid the possibility of fungal or insect infection). These stack foundations must lie all in same plane, so the boards are not caused to distort.

In some countries it is useful to be able to look underneath stacks of timber in the yard to check for evidence of insect attack – dust might be seen suggesting that termites are at work within the stack!

Stickers

These are the pieces of wood which are used to separate layers of pieces of timber in the stack so that air can gain intimate contact with the surfaces of all the boards.

The cross-sectional size of stickers can be used to control the rate of drying of the stock (remembering that some species can tolerate rapid drying without developing defects – whilst other species CANNOT). Thus stickers of short height are useful when stock must be dried slowly (the air flow across the boards is reduced) whilst “thicker” stickers can be used for other species which can be more rapidly dried. It is important however that the stickers are not so narrow that the stack may be unstable (i.e. the stickers must not be likely to fall over) – but wide stickers must also be avoided as the areas “hidden” from the drying air will remain wet and so be prone to degrade (staining or fungi).

The stickers must be of a wood species that is softer than the stock being dried as otherwise “sticker marks” will be left on the dried pieces of timber where the stickers have embedded themselves into the board surfaces.

The closeness of stickering (i.e. how far apart they are – or how many stickers are used along the length of the stack) can be used to control any distortion that the boards might sustain (“bow etc – see later). Thus “close-stickering” is appropriate to restrain the distortion of certain species known to distort unacceptably during drying.

A well stickered stack (here ready to enter a kiln). Note the stickers are aligned vertically above each other.

The volume of timber comprising the numerous stickers that are required in a timber yard is considerable and it is sensible therefore to look after the stickers, protecting them from breakage etc.

"Close piling" – where no stickers are used (attempting to separate drying stock by using the boards themselves) is a poor technique as unacceptably large areas of the boards remain wet and prone to degrade.

Stacking square edged sawn timber

A number of methods are used to stack square edged sawn timber, although some are uncommon nowadays:

· Flat (or box) piling: This is the commonest method. A rectangular “box” of boards is built up, usually less than 6 feet (2 metres) wide and less than 3 times as high (to reduce compression damage in lower boards and to ensure stack stability). Sometimes weights are used on the top of the stacks to reduce the possibility of distortion occurring in some species as they dry (e.g. elm, beech). These stacks might be built “by hand” (when it is important to ensure the principles of good stack construction are followed – see above) or in large mills “stickering machines” can be used at the end of the sawing line.

· “Crib” piling: Less common than the above. Sometimes used to stack dimensioned stock. Not a good method. No stickers are used - the boards themselves are the "stickers", but high wood - wood contact areas can cause poor drying and sap - stain.

· Bristol piling: Used in the past to stack dry timber at a port as a ship is unloaded. The “self-roofing” structure ensures the bulk of the timber is kept dry. With the almost universal acceptance of containerisation this system is probably obsolete.

· End and Cross Racking: Useful to achieve the fast, superficial drying of board surfaces where necessary to protect them from stain (sycamore is prone to a damaging stain if the board surfaces are not rapidly dried).

Uncommon methods of stacking timber. Left to right: Bristol stacking, end racking and cross racking.

Logs

De-barking speeds up the drying of logs which might be destined for use as poles or fencing.

Valuable logs destined for use in furniture manufacture, especially if they have attractive “figure” (grain patterns) are frequently sawn 'through and through' (a number of cuts running the complete length of the log) and then the log is “re-assembled” (with stickers) and stored whilst it dries "en boule". This is so that when the furniture is constructed, wardrobe doors etc. can have matching grain, as the doors can be made from boards sourced from positions next to each other in a log:-

End view of a log sawn “through and through” and “reassembled” in the yard “en boule”: each board is separated from its neighbour by stickers.

A well stickered timber stack ready to enter a kiln.
Uncommon methods of stacking timber.
End view of a log sawn “through and through” and “reassembled” in the yard en boule.
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