In a timber-framed building the frame carries the roof and the infill between the posts does the sheltering, so the infill you choose sets how heavy, how fire-resistant and how defensible the walls are. Heavy infill such as earthbag or rammed earth needs its own footing and must not push on the posts, while light infill such as wattle and daub gives weather protection and privacy and little else.
Timber framing for defense is the use of a post-and-beam skeleton to carry a roof, with the spaces between filled by earth, stone or lighter material that supplies the wall.
Separate The Structure From The Wall
A frame lets you put the roof up first and fill the walls later, and lets you change the infill without touching the structure. That separation is what makes it useful for defence: the part that must stay up (the roof) is carried by a few strong members, and the part that takes damage (the infill) can be repaired in place. For the joinery and species choices, see Timber Framing and Post and Beam Construction.
Large timbers are slow to burn through compared with small framing members, which is one of the advantages the Timber Framing article lists. It does not make the frame fireproof: a frame with a thatched or board roof is a fire problem, and the roof is what you protect first; see Roof Reinforcement.
Choosing The Infill
| Infill | Mass | What it is good for |
|---|---|---|
| Wattle and daub | Light | Weather, privacy, cheap and quick; see Wattle and Daub |
| Straw bale, plastered | Light | Insulation; fire resistance depends on a continuous plaster skin; see Post-and-Beam Infill and Plaster as a Fire Barrier |
| Earthbag or rammed earth | Heavy | Mass; needs its own footing; see Earthbag Wall Construction |
| Stone or brick | Heavy | Durable and fireproof; needs mortar or careful dry laying |
On the defensive value of light infill. The settlement-planning ballistic table lists plastered straw bale and softwood log walls as unproven, because their mass per square metre is three to five times lighter than the earth and masonry rows. See Defensive Architecture. Do not build a framed house with light infill and call it protected.
Method: Frame First, Then Fill
Step 1: Set the footing under the posts and under the infill. Heavy infill needs a footing along its whole length; posts need separate footings. Do not assume the frame can carry the infill.
Step 2: Raise the frame and the roof. Brace diagonally until the infill is in; an open frame racks sideways.
Step 3: Fill the bays. Place the infill in courses between the posts. Leave a flexible joint (a gap filled with a compressible material) between heavy infill and the posts so the infill does not push on the frame when it settles or is wetted.
Step 4: Render and protect. Cover earth infill with plaster and protect the base from splash. Keep the plaster continuous over any straw.
Worked Example: What Heavy Infill Does To The Footing
Earthbag infill 0.4 m thick, 2.5 m high, at an assumed 1,600 kg/m³:
- Mass per metre of wall = 0.4 x 2.5 x 1,600 = 1,600 kg, which is 1,600 x 9.81 = 15.7 kN per metre.
- On a footing 0.5 m wide this is 15.7 / 0.5 = 31 kPa on the ground.
That is in the same range as the figure for earthbag walls in Rubble Trench and Grade Beam. Whether the ground carries it depends on the soil; see Soil Bearing Capacity.
Common Mistakes
- Letting the posts carry heavy infill. They are for the roof. Give infill its own footing.
- A rigid joint between infill and frame. Movement cracks one or the other.
- No bracing. An open frame sways and racks before the infill is in.
- Treating the infill as protection when it is light. Weight, not thickness, is the comparison.
A frame without bracing falls in
A raised but unfilled and unbraced frame can rack and collapse in wind or when a post is struck. Brace it fully before anyone works inside, and keep the roof off until the frame is tied. Do not remove temporary props until the infill and bracing are complete.
What’s Next
- Learn next: Roof Reinforcement — the part of the building the frame is for
- Combine with: Earth Construction — the heavy infill options