Design of Stacks

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Design of Stacks

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Design of Stacks

Stacks should be located on good foundations to prevent timber from coming into contact with damp soil and to ensure adequate ventilation under the stacks. The design of foundations recommended for air drying stacks in the USA are as follows; 'The bottom row of boards should be 46 cm off the ground. The foundations should consist of masonry or preservative treated timber blocks, 15cm x 15 cm. These blocks should be spaced at intervals of 1.2-1.6 m in rows 1.8-2.4 m apart to correspond with the width of the stack. Two square beams 16 x 20 cm in cross section are laid over the two rows of blocks and the whole stack rests on cross bars, 10 x 10 cm in cross section, placed over the two main beams.

The method of stacking is important in preventing warping during drying. Each row of stickers should be held up by a cross piece lying on the foundations and all the stickers should be in perfect vertical alignment. Mechanical guides are often used to ensure that stickers are aligned perfectly.

Stacks of air drying timber

Stacks of air drying timber. (WIVJ)

As the vertical flow of air within the stack is important gaps are left between the boards in order to form vertical shafts with as few breaks in them as possible. This is easier if boards are classified according to width before being piled. The gaps between boards varies from 2.5-15 cm with the average being 20% of the width of the boards.

In most cases stacks are box piled, ie., the stack maintains its rectangular shape despite differences in board length. This is achieved by placing the longest board in each layer in the outside and sliding smaller boards alternately backwards and forwards so that they are flush with the side of the stack. Short lengths should be supported at their ends (see figure below). If possible different species and boards of different thickness should be stacked separately. If boards do overhang the end of the stack they should be supported at their ends. Box piling ensures even circulation within the stack.

Overhanging boards must be supported by additional sticks. These must be inserted beneath the overhang, otherwise lower boards will distort. (UCNW 1977)

Stacks should be roofed to shield the uppermost boards from the weather which would rapidly crack and warp the uppermost layer of boards (see diagram below). The roof is usually sloped and may consist of timber planking, plywood or plastic sheets. The roof is usually anchored to the stack by wire and a gap of approximately 15 cm is left between the roof and the top pile to ensure adequate ventilation.

Recommended stack construction for air drying of timber.(Pratt 1974)

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