Raised granary
Keep a year of grain dry, cool and away from rodents — the single highest-return building on the site.
Home: Storage and farm buildings, Level 1 · Foothold, level 1 of 3 in its track. Serves 3 steps across 2 subjects.
Every build on this site has hazards that injure or kill — quicklime, carbon monoxide, molten metal, collapsing ground. Read the safety notice before starting, and read this build's own failure modes below.
What it is and why this shape
Growing the grain is the hard part; losing it afterwards is the common part. Post-harvest losses in unprotected traditional storage routinely run 10–30%, and in a bad year the loss is total. A granary is not a shed — every dimension on it is doing a specific job.
Four enemies, and the building answers each one geometrically rather than chemically, because you will not have chemicals:
- Rodents — defeated by height, by a smooth barrier they cannot climb past, and by clear air around the building. Not by cats and not by hope.
- Damp — defeated by raising the floor clear of ground moisture, by ventilating beneath it, and by a roof with a generous overhang.
- Insects — defeated by getting the grain dry and cold before it goes in, and by admixture or sealing.
- Fire — defeated by siting, which costs nothing and is usually neglected.
The design below is a raised timber granary sized for a small settlement. It uses no metal fasteners except the rat guards, which are the one component genuinely worth spending scarce sheet metal on.
Cross-section
Fig 1
Earth / subsoilMasonry / brickTimberLoose fill / grainVoid / air
Floor framing plan
Fig 2
Timber
Rat guard — detail at post head
Fig 3
Earth / subsoilMasonry / brickTimber
Keyed parts
| 1 | Roof, gable, 30–40° pitch | Thatch, shingle or tile. Pitch must shed water fast; the overhang is what keeps the walls dry. |
| 2 | Eaves vent gap, ~100 mm continuous | The building must breathe or it will sweat. Screen with woven mesh, fine cloth or split cane against birds and insects. |
| 3 | Wall, boarded or wattle-and-daub | Carries the lateral thrust of the grain, which is substantial. Boards run horizontally so the studs take the load in bending. |
| 4 | Maximum fill line, 0.3 m below wall top | Freeboard. Filling to the top blocks the vent gap and puts thrust on the wall plate rather than the studs. |
| 5 | Floor deck, 40–50 mm boards | Tight-laid. A gap that passes a grain of wheat will pass most of your grain over a winter. |
| 6 | Joists at 0.6 m centres | Span the short direction. Grain is heavy — roughly 780 kg/m³ for wheat — and the floor carries all of it. |
| 7 | Post, 150 mm square, 6 off | Set on stone pads, not buried. A buried post rots at the ground line and is the usual reason these buildings fail. |
| 8 | Rat guard, 0.6 m smooth disc or cone | The one part that must be metal or fired ceramic. Smooth, unclimbable, with no gap at the post. |
| 9 | Discharge hatch and chute | Low, so the bin empties by gravity and nobody has to climb inside. See the failure notes on entering a bin. |
| 10 | Loading hatch | In the roof slope or gable, above the fill line, with a positive catch against wind and birds. |
Specifications
| Capacity | 4.3 m³ | ≈ 3.3 t wheat at 780 kg/m³; ≈ 3.1 t maize; ≈ 2.5 t paddy rice |
| External plan | 2.40 × 1.80 m | Internal 2.25 × 1.65 m |
| Wall height | 1.50 m | Fill to 1.15 m — the top 0.3 m is freeboard, not storage |
| Floor height above ground | 0.75 m min | 0.90 m is better where rats are established |
| Rat guard diameter | 0.60 m min | Smooth metal or glazed ceramic. This is not the place to economise |
| Roof overhang | 0.40 m min | All four sides. Keeps rain off the walls and off the vent gap |
| Roof pitch | 30–40° | Steeper for thatch, shallower for tile |
| Eaves vent gap | ~100 mm continuous | Screened. Ventilation is not optional in a grain store |
| Floor loading | ~9 kPa | 1.15 m of wheat. Size joists for this, not for people walking |
| Grain moisture at filling | ≤ 13% wet basis | ≤ 12% for oilseeds. The single most important number on this page |
| Clear zone around building | 1.0 m | Nothing stored, stacked or growing within it |
Bill of materials
| Material | Quantity | Notes and substitutes |
|---|---|---|
| Posts, 150 × 150 mm | 6 × 1.1 m | Durable heartwood — oak, chestnut, cedar, locust. Charring the buried-height portion helps even though it is not buried. |
| Stone pads | 6 | Flat, bedded level. Replaces the buried post that would otherwise rot out. |
| Bearers, 100 × 150 mm | 2 × 2.6 m | Across the posts, the long way. |
| Joists, 75 × 150 mm | 5 × 1.9 m | At 0.60 m centres, spanning the short direction. |
| Floor boards, 40–50 mm | ≈ 4.5 m² | Tight-laid, tongued or rebated if you can. Gaps lose grain continuously and invisibly. |
| Wall studs, 75 × 75 mm | ≈ 14 × 1.6 m | At 0.60 m centres. These carry the grain's sideways push. |
| Wall boards, 25 mm | ≈ 14 m² | Run horizontally. Wattle-and-daub also works if lined smooth inside. |
| Rafters, 75 × 100 mm | 10 × 1.6 m | Pitched 30–40°. |
| Roof covering | ≈ 8 m² | Thatch, shingles or tile. Thatch is free and burns; site accordingly. |
| Rat guards | 6 × 0.6 m dia | Sheet metal, beaten flat and smooth. Substitute: glazed pottery discs, or an inverted fired-clay bowl. Rough or dented metal is climbable. |
| Vent screen | ≈ 9 m run | Woven mesh, fine cloth or split cane against birds and large insects. |
| Pegs / trenails | ≈ 60 | Dry hardwood, driven into slightly undersized holes. |
Tools: Axe and adze · Saw · Auger or brace and bit · Chisel and mallet · Steel square or a 3-4-5 triangle · Plumb line · Water level
Construction
Site it before you cut anything
On the highest ground you can reasonably use, with natural drainage away on all sides. Well clear of the dwelling, the workshop and any fire, and upwind of them if the roof is thatch. Away from overhanging trees — branches are rat bridges and a fire risk. If you can, orient the long axis across the prevailing wind so the gable ends take the weather.
✓ Check: Stand on the site after heavy rain. If water lies anywhere within 2 m, move it.
Set the pads and level them
Six stone pads, bedded on compacted ground, at the post positions in the plan. Level them to one another with a water level — a length of tube filled with water gives you a true datum over any distance for nothing, and there is no substitute for it here.
✓ Check: All six pad tops within ±5 mm of one datum. Out-of-level here twists every joint above.
Stand the posts and fit the rat guards
Posts stand on the pads, never in the ground. Brace them plumb temporarily. Fit each rat guard before the bearers go on, because afterwards you cannot slide it down the post. The guard must sit tight to the post with no gap — a rat needs very little.
✓ Check: Every post plumb in both directions; every guard tight to the post and smooth on its upper face.
Bearers, then joists
Bearers across the post heads, housed into the tops so they cannot slide. Joists over the bearers at 0.60 m centres, spanning the 1.80 m direction. Check the frame for square by measuring both diagonals — they must be equal.
✓ Check: Diagonals equal within 5 mm. A frame out of square will not take a rectangular floor or a rectangular roof.
Lay the floor tight
Tight-laid boards, cramped up hard. Any gap that will pass a grain will pass a great deal of grain over a season, and you will not notice until the store is short. If your boards are green they will shrink — either season them first or lay them and re-cramp after the first dry season.
✓ Check: Sweep a handful of grain across the deck. Nothing should disappear between boards.
Frame and board the walls
Studs at 0.60 m centres, boards horizontal. Grain pushes sideways with real force — a metre of wheat exerts roughly 3 kPa against the wall, and the wall is a retaining structure, not a weather screen. Line the inside smooth so grain flows down rather than lodging in ledges where insects breed.
✓ Check: Push hard against the middle of a wall panel. Any visible flex means studs too far apart or boards too thin.
Leave the vent gap, then roof it
Stop the wall boards ~100 mm below the wall plate to leave a continuous vent gap, and screen it. Then rafters, then covering, with at least 0.40 m of overhang on every side. The overhang is what keeps rain out of the vent gap you just made.
✓ Check: Look up from inside on a bright day: you should see a continuous line of light at the eaves, and no daylight at all through the roof.
Fit hatches and a discharge chute
A loading hatch high in the gable or roof slope, above the fill line, with a positive catch. A discharge hatch low on one wall with a chute, so the bin empties by gravity into a sack. Design it so nobody ever has to get inside — see the failure notes.
✓ Check: Fill with chaff and discharge the lot without a person entering.
Dry the grain before it goes in
This is the step that decides whether everything above was worth building. Grain must be at or below 13% moisture before it is binned. Without a meter, use the field tests: a grain should shatter rather than squash when struck; a handful should not clump; bitten, it should crack cleanly rather than dent. Dry on racks or mats in sun and moving air, and bring it in before evening damp.
✓ Check: Seal a sample in a glass jar in the sun for an hour. Condensation on the inside means it is too wet — dry it further.
Fill, seal and start a log
Fill to the marked line and no further. Record the date, the weight or volume in, the variety, and the moisture assessment. Then check monthly: push a hand or a probe deep into the bulk and feel for warmth, which is the first sign of trouble and arrives well before smell or visible mould.
✓ Check: One month after filling, the grain at depth is the same temperature as the grain at the surface.
Tolerances that matter
| Dimension | Target | Why it matters |
|---|---|---|
| Pad levels | ±5 mm | Everything above inherits the error, and a twisted frame will not take a square roof. |
| Frame diagonals | equal within 5 mm | Out of square means the floor, walls and roof all need individual fitting. |
| Floor board gaps | 0 — tight | A 2 mm gap over 4 m of joint will lose a surprising quantity of grain silently. |
| Rat guard to post | no visible gap | The guard is a single-purpose part. A gap defeats it completely and the building looks finished either way. |
| Grain moisture | ≤ 13% w.b. | Above ~14% moulds establish; above ~15% the bulk heats and can be lost entirely. |
| Vent gap | continuous, ≥ 80 mm | Blocked ventilation causes condensation on the underside of the roof, which drips back into the grain. |
How this fails
Posts set into the ground. They rot at the ground line — the wettest, most aerated point — and the building sags into itself in five to fifteen years. Stand them on stone.
Filling with damp grain. The commonest total loss. Moulds establish above ~14% moisture, the bulk heats, and a heating bin can be beyond saving within weeks. Nothing about the building can fix grain that went in wet.
Rat guards fitted after the bearers. They cannot be slid down past a fitted bearer, so they get left off or cut and patched — and a patched guard has a gap, and a gap is the same as no guard.
Anything stacked against or under the building. Firewood, a ladder, a barrel, a growing shrub. All of them are ramps that make the height and the guards irrelevant.
Entering a bin of grain. Grain behaves as a fluid: a person sinks, and above roughly waist depth cannot self-extract, and above chest depth the pressure prevents breathing. This kills people every year in modern agriculture with rescue services available. Design for gravity discharge and make it a standing rule that nobody enters a bin with grain in it.
No monitoring. A bin is not a set-and-forget object. Warmth at depth is the early warning, and it precedes smell and visible mould by weeks.
How you know it worked
VerificationA full bin has come through a complete wet season with no warm spots, no mould, no rodent droppings on the floor and no measurable loss between the recorded fill weight and the recorded discharge weight.
Scaling and variants
Smaller: a single-post version at roughly 1.2 × 1.2 m holds about 1 m³ (≈ 780 kg) and needs one guard rather than six — sensible per-household, and it distributes risk, which matters more than efficiency.
Larger: do not simply scale the walls up. Lateral pressure rises with depth, so a deep bin needs horizontal ties or hoops, and above about 2 m of fill depth the wall becomes a serious structural problem rather than a carpentry one. Build more bins instead of bigger ones — separate bins also mean one spoiled bin is not the whole harvest.
Where rats are severe: raise the floor to 0.9 m, use 0.75 m guards, and set the posts in the middle of an open, swept, hard-surfaced area with nothing within 1.5 m.
Hermetic variant: where insects rather than rodents dominate, a sealed container — a lined pit, a rendered clay bin, or sealed drums — kills insects by oxygen depletion within a few weeks and needs no insecticide at all. It requires genuinely dry grain and a genuinely airtight seal, and it cannot be opened and resealed repeatedly.