Fire resistance comes from material that cannot burn (stone, earth, brick), from plaster that keeps oxygen and flame off compressed straw, or from timber thick enough to char slowly, and the roof matters most because settlement fires spread by roofs. A 150 mm timber post chars at roughly 0.65 mm a minute, so after 30 minutes in a fire only 55% of its cross-section remains and its resistance to buckling has fallen to about 30%.
Part of Fire Prevention in Settlements
Fire-resistant building materials are those that do not burn, or that burn so slowly or are so well protected that a wall, floor or roof keeps its job through a fire next door or inside.
Walls Resist In Three Different Ways, And Roofs Decide The Rest
The fire guide ranks wall materials: stone, earthbag, adobe and rammed earth excellent because earth does not burn (stone may crack in extreme heat); plastered straw bale very good, with a 2-hour fire rating when properly plastered; brick very good; heavy timber of 15 cm or more good, charring on the surface but keeping its integrity; light timber framing poor; and thatch very poor, an “ember magnet”. Three mechanisms sit behind that ranking: the material cannot burn, the material is protected from oxygen and flame, or the material burns slowly enough to last.
The straw bale rating needs a caution. The plaster article says it does not state the plaster type, coat thickness or test conditions behind the 2-hour figure, so a hand-plastered wall should not be assumed to reach it, and any exposed straw at a window reveal, roof junction or flue defeats it (Plaster as a Fire Barrier).
Roofing is critical, says the fire guide, since most settlement fires spread by the roof. Salvaged metal is the safest; sod or earth roofs are fireproof; tile is excellent; thatch is dangerous. The guide offers a lime wash on thatch as something that reduces flammability somewhat; no test of that is given in this corpus, so treat it as no substitute for a non-combustible roof, and keep thatched buildings at the doubled spacing.
Worked Example: A Timber Post In A Fire
Inputs: a post 150 mm square, the fire guide’s lower bound for heavy timber, burning on all four faces; a charring rate of about 0.65 mm per minute (a figure recalled from timber-design practice, not read from this corpus; check it); resistance to buckling of a square section proportional to the fourth power of its side (I = b⁴ ÷ 12).
- Char depth after 30 min: 0.65 × 30 = 19.5 mm on each face, so the sound core is 150 − 2 × 19.5 = 111 mm square.
- Remaining area: 111² ÷ 150² = 12,321 ÷ 22,500 = 55%.
- Remaining buckling resistance: (111 ÷ 150)⁴ = 0.74⁴ = 0.30, about 30%.
- After 60 min: char 39 mm per face, core 72 mm, area (72 ÷ 150)² = 23%, buckling resistance (72 ÷ 150)⁴ = 0.053, about 5%.
So “chars on the surface but keeps its integrity” is true for a time that depends on the load the post carries, not for ever. A post carrying a loaded roof needs spare capacity, or a fire-resistant cover, to last the half hour a settlement needs to get out and wet its neighbours.
What You Need
- Stone, brick or earth for walls near fire sources (Earthbag Building)
- Complete plaster on straw bale walls, with no exposed straw
- A non-combustible roof: metal, tile or sod
- Timber of at least 150 mm where timber must be used, and extra section for loaded members
- A survey of existing roofs and walls, listed by material
Method: Choose By Exposure
Step 1: Rank the buildings by exposure, with fire-using workshops, stores and the assembly hall first.
Step 2: Fix the roofs first. Replace thatch near forges, kilns and stoves with metal, tile or sod.
Step 3: Plaster the straw, closing every reveal and junction.
Step 4: Give loaded timber extra section. Add the char depth for the time wanted on every exposed face to the section needed for the load.
Step 5: Use the shorter spacing only for non-combustible walls and roofs (Building Spacing & Fire Breaks).
Common Mistakes
- Fireproof walls under a thatch roof. The roof burns and the wall is irrelevant.
- Trusting “heavy timber” without checking the load. The sound core is what carries the roof.
- Unplastered straw at a window. The wall becomes a chimney of smouldering straw.
- Assuming lime wash makes thatch safe.
Safety
Fire-resistant is not fireproof
A wall that does not burn still passes heat through, spalls when it is soaked while hot, and can crack and fall. Stay out of the collapse zone, which for a free-standing wall reaches at least as far as the wall is high, and never shelter in a building because its walls are stone without checking its roof and openings (Shelter-in-Place Preparation).
What’s Next
- Learn next: Ember Protection — closing the gaps that fire-resistant walls leave
- Combine with: Waterproofing and Fire Safety — protecting bale walls from both