When a sawmill enquiry arrives, it usually arrives as a drawing with a size on it. The size is the least useful number in the brief.
A sawmill is not one space. It is at least three, running in a line: the log yard, where raw material waits in the weather; the saw line, where the machine runs and the dust and heat come from; and the stacking area, where graded sawn timber sits for days or weeks with its value depending on staying dry.
In a mill that sells graded timber into an export market there is usually a fourth, and it is the one that never appears on the enquiry: the inspection bench, where pieces are turned, measured and marked before they are allowed into the stack. Put it in the wrong place and every later argument about a shipment starts there — once a piece is buried in a stack it is no longer easy to re-measure. It wants dry, good light, and a clear approach for a forklift; it does not want to share air with the saw line.
Put all of these under one roof and they start arguing with each other — the saw line wants air, the stacking area wants none of it.
This is the order I work through before a drawing exists. It is written from the sequence of a real enquiry, not from a catalogue.
1. Decide the zones before the span
The first question is not ‘how wide’. It is: which end has to be sealed, and which end can stay open? In most mills the answer is that the stacking area gets walls and closable openings, and the log yard end stays open or becomes a canopy. Once that line is drawn on paper, the frame stops being one box and becomes two or three bays with different side conditions — and the tonnage follows from that, not the other way round.
The second thing to draw is how many ways material leaves the building. A mill can be running three at once — whole logs out to a ship, sawn timber out in containers, and smaller parcels collected by local buyers — and each one wants its own gate, its own turning space and its own length of yard. A drawing with one door will work for whichever of the three was in the buyer's mind that day, and the other two get improvised on site, usually in the rain.
Get the zoning wrong and no amount of gutter detail saves the timber, because the wet zone and the dry zone end up sharing the same floor plate.
2. The four numbers that set the frame
Span, bay spacing, eave height, and what has to be lifted or moved inside. A sawmill usually wants a generous eave height at the log end for the machine and the handling equipment, and a lower, tighter volume over the stacking area — lower volume is easier to keep stable and cheaper to keep dry.
None of these have a universal answer, and anyone who quotes a ‘typical’ span for a sawmill without asking about the saw line is guessing. That guess gets paid for twice: once in steel, once in the first wet season.
3. The column and the crane are one decision
The question that follows the span is always the same: can the middle stay clear? It is usually asked as if it were a structural question. It is not.
An interior column is acceptable or not depending on the production route, and there are only three things to check. Does something have to travel the full width in one piece — a log entering the line, a bundle leaving on a forklift, a machine that may be replaced later? Does the column land where timber will be stacked, or where a vehicle has to turn? And will a lifting device ever be wanted over that bay — because a crane runway and a clear span are not the same building.
If all three answers are no, a column takes steel out of the frame and takes nothing out of the process. If any answer is yes, the column is permanent: it is not added later, and it cannot be moved once the slab is poured.
And the log decides more of this than the drawing does. A big log has to be lifted, and how it is lifted is chosen before the frame: a wheel loader working from the yard floor needs an opening wide and high enough to drive through, and the frame at that opening takes the hit. A gantry crane carries its own legs on ground rails, so the hall can stay clear-span — but those rails fix the yard layout permanently. An overhead crane sits on runway beams fixed to the building columns, which is the most direct way to move a log down a line and the only one of the three that puts the column decision and the lifting decision on the same drawing.
The weight that sizes the crane is not the number on a datasheet. Green timber straight off the truck is heavier than the same piece after drying, and on dense tropical hardwoods it can be heavier than the same volume of water — which is also why those logs cannot be kept floating in a pond and have to be stacked on a drained hardstand instead. Two rules follow. Size on green weight, not on finished density. And size on the heaviest species the mill will ever take, not on the average of the species list: a crane picked from an average works perfectly until a good log arrives.
That is also why the eave height at the log end is not a comfort item. It comes from the log, the machine and the hook — not from what looks generous.
The mistake is the reverse one: clear span for the look of it, then a crane is wanted and there is nothing to sit it on, or a loader arrives and cannot turn. That decision is made on paper, before anything is ordered.
4. Water: the chain, not the downpipe
Buyers ask for gutters and downpipes because that is the part they can see. But a downpipe only carries water from the gutter to the ground. What decides whether the timber stays dry is where the water goes after that.
The chain is: roof pitch, gutter capacity, outlet, discharge point, and the fall of the yard. Every link has to work. A generous gutter feeding a downpipe that empties against the wall plinth is worse than a modest gutter emptying into a lined channel that carries water away from the building — because water that lands at the wall comes back in under the cladding, quietly, at the base course.
The second water source is the underside of the roof sheet. Warm humid air meets a cooler sheet overnight and drips onto the stack. It looks exactly like a leak and is regularly diagnosed as one. The fix is a controlled interior face or a deliberately ventilated roof space — which of the two depends on how the mill runs, not on preference.
5. Air: the tension nobody writes down
Dust, heat and humidity all argue for open sides. Rain argues the other way, and the same opening that clears dust also admits driven rain at an angle. This is a planning decision, not a detail: seal the zone that stores value, ventilate the zone that generates dust.
Where the saw line sits under the same roof as the stacking area, the separation is usually a full-height wall with its own doors, not a half wall. Half measures let the humid air through and keep the rain out only on calm days.
6. Floor and plinth
A sawmill floor takes wheel loads, dropped logs and wet-season mud on the same slab. Two decisions matter more than the slab thickness: whether the finished floor sits above yard level, and whether the wall base has an upstand above that floor. Without the upstand, water that reaches the wall walks in at the bottom course, and the bottom course is where the stack starts.
Drainage inside the building is worth planning as seriously as drainage off the roof. Wash-down water and rain blown in at openings have to leave somewhere that is not under the stack — and debarking and washing are a separate flow from rainwater, with its own solids to settle out before it goes anywhere.
On a coastal site there is a second constraint that inland drawings ignore: the water table. A floor set above yard level achieves nothing if the yard itself cannot drain, and on flat coastal ground the fall has to be created rather than found. That decision is made once, with the earthworks, and is very expensive to revisit.
7. Programme in a two-season climate
Where rain comes in two seasons rather than one, the erection programme is built around the roof, not the frame. A closed roof is what turns an open steel frame into a place where work continues through bad weather. That means roof sheeting and flashings are ordered and shipped as a priority package, and the site sequence puts purlins and roof sheets immediately behind the frames rather than behind all the walls.
It also means the frame is detailed so that the roof can be closed before the walls are complete — which is a detailing choice, made months earlier, not a site improvisation.
8. What to send before drawings
Eight answers change this building more than its size does:
- Which zones sit under the roof, and which stay open or under canopy.
- What the saw line is, what has to be lifted over or moved around it, and whether anything must cross the full width without a column.
- The largest and heaviest log the mill will ever take — by green weight, not by finished density — and what lifts it: loader from the yard floor, gantry on its own rails, or overhead crane on the building columns.
- The sizes and grades the timber is sold in, and where it is measured and marked before it goes into the stack.
- How long sawn timber sits before it ships, and in what form it is stacked.
- Whether the slab is at yard level or raised, and where wash-down water goes.
- How the yard drains today, and where water is allowed to be discharged.
- Whether the building will be extended later along one bay — drainage and bracing drawn without that in mind get cut through.
None of these are steel questions. All of them change the steel drawing. That is the part of a sawmill brief that a quote cannot fix later.