Monsoon Sal Boards for School Desk Tops
In late June, a school workshop in Nalbari receives 40 sal planks, each 2.4 m long, 200 mm wide and 25 mm thick. The planks were converted three weeks earlier from freshly felled logs and have been stacked flat on the ground under a tarpaulin. A pin-type moisture meter reads 38–45% near the surface but 55% at the centre of a freshly cut check. The workshop has an open-sided shed, no kiln, and must supply 30 desk tops in six weeks. Within a week, two planks cup about 6 mm across the width and develop end checks 80 mm long. The instructor wants to plane all boards to final thickness immediately and screw the tops to welded steel frames. The budget can cover either kiln-dried stock at about 25% above the local air-dried price or a small solar dryer. The supplier can deliver kiln-dried sal in two weeks.
1. Why did the planks cup and end-check, and why is immediate planing to final thickness a poor decision?
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Cupping and end checking point to a steep moisture gradient, not simply to a bad batch of timber. The planks were converted from fresh logs and then kept flat on the ground under a tarpaulin: the exposed upper face dried faster than the lower face and the core, while the tarpaulin restricted air movement and trapped humid air. Wood shrinks more tangentially than radially, so the drier face contracts more and the board cups. End grain loses moisture much faster than side grain, so the ends shrink first and checks open. The meter readings confirm the gradient: 38–45% at the surface against 55% in the centre.
Planing now removes the drier surface and exposes a wet core, but it does not equalise the moisture gradient. The board will continue to shrink after planing; new stresses will be released and the top can warp, cup or split around fasteners. Screwing a wide top rigidly to a steel frame also restrains cross-grain movement. In Assam’s humid air a 200 mm wide sal top can move several millimetres between wet and dry seasons, so fixed screws can split the board or shear. The correct decision is to re-stack and season the boards, or buy kiln-dried stock for the tops, and to detail the fixings so the top can move.
2. Propose a seasoning and stacking plan that could meet the six-week deadline. State a target moisture content, sticker spacing, and how you would monitor progress.
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A sensible target is 10–12% moisture content for indoor desk tops in Assam, with no more than a 2–3% difference between shell and core. Sort out the worst checked boards. Seal the ends with wax or paint. Re-stack under the shed, not on the ground: bearers at least 150 mm high, stickers 25 mm thick and about 300–400 mm apart, aligned vertically over the bearers, with a weighted cover on top and open sides for airflow. Keep the stack out of direct rain and sun.
A six-week air-drying window is tight for 25 mm sal during the monsoon. Use the solar dryer for the boards intended for tops: load the stack, keep the collector vented so humid air leaves, and hold the temperature moderate-not above about 50°C at this stage-to reduce the risk of case hardening. Measure with a calibrated meter on a freshly cut sample from a sacrificial board every week, and weigh a control board to track drying. If the core is still above 12–14% at week four, buy kiln-dried sal for the 30 tops and use the air-dried stock for less critical parts. Do not plane to final thickness until the boards have equalised for at least a week after drying.
3. How should the desk tops be attached to the steel frames to accommodate seasonal movement?
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The top must be held down but not imprisoned. Use slotted or oversized holes with washers, figure-eight fasteners, or wooden buttons in a groove, rather than fully tightened screws through round holes. Fix the top at one central point and allow the rest to slide. For a 200 mm wide sal top, allow roughly 3 mm of movement at each side-about 6 mm total-between the wet and dry seasons, and provide a small gap at any wall or frame stop.
Screws should be long enough to hold but not so long that they risk breaking through the top. Do not over-tighten: a slightly snug screw with a washer is better than a crushed bearing surface. Finish all faces of the top, including the underside, to slow uneven moisture uptake. If the steel frame has fixed cleats, elongate the holes in the cleats rather than in the top, and check the assembly after one full humid season. This detail costs little but prevents the split tops and lifted corners that rigid fixing would cause.