Harden an existing building from the openings inward: doors first, then windows, then one interior room, then the roof. Decide what you are defending against, because forced entry, thrown objects and fire need different measures, and keep every occupied room with a way out that opens from the inside, since barricades that stop an intruder will also trap occupants in a fire. Check the floor will carry any sandbags or earth you add.
Reinforcing an existing structure is the adding of strength, mass or obstruction to a building that was not built for defence, usually at its doors, windows, interior and roof.
Start With The Threat, Not The Material
The parent article says an existing building can be significantly hardened, and that windows are its weakest point. What “hardened” means depends on the threat:
| Threat | What helps | Where |
|---|---|---|
| Forced entry by people | Strong frames, hinges, bars; delay and noise | Window and Door Hardening |
| Thrown objects | Mesh, shutters, a screen | Windows |
| Fire | Non-combustible roof and cladding, no fuel against walls | Roof Reinforcement |
| Refuge for those who cannot fight | One reinforced interior room | Interior Barriers |
| Projectiles | Mass; untested | See the warning below |
On projectiles: the ballistic table in Defensive Architecture marks its figures as untested and its two light rows (softwood log, plastered straw bale) as unproven. Adding boards to a window or plaster to a straw wall does not give protection from a named cartridge; this corpus has no test data. Mass is what counts, and mass has a cost, set out below.
Order The Work By Weak Point
Step 1: Walk the building as an intruder would. List every door, window, cellar hatch, loft opening and roof access. Count the ones that can be reached from the ground, a flat roof or a neighbour’s wall.
Step 2: Harden the doors. A door and its frame are usually weaker than the wall around them. See Window and Door Hardening.
Step 3: Deal with the windows. Shutters, mesh, narrowed openings; keep at least one openable from inside.
Step 4: Choose one interior room. Pick it for the fewest exterior walls and a solid door; see Interior Barriers and the safe-room requirements in Defensive Architecture.
Step 5: Look at the roof. Fire arrives by the roof. See Roof Reinforcement.
Mass Added To A Building Is A Load The Building May Not Carry
A floor or sill that was built for people and furniture can carry far less than a stack of sandbags. Sandbags 0.4 m thick and 1.2 m high at an assumed 1,600 kg/m³ weigh 0.4 x 1.2 x 1,600 = 768 kg per metre of run, which is 768 x 9.81 = 7.5 kN, sitting on a strip 0.4 m wide: 7.5 / 0.4 = 18.8 kN/m² on that strip. The worked comparison with ordinary residential floors is in Window and Door Hardening. On an upper floor, spread any heavy stack over several joists on planks, or put the weight on a ground floor.
Rules That Keep Reinforcement From Killing The Occupants
- Every occupied room keeps a way out that opens from the inside without tools. A barred door or sandbagged window is a delay for an intruder and a trap in a fire. See Escape Route Planning; the parent article itself advises concealed exits for a safe room.
- Keep fuel and similar stores out of the hardened space. If you hold gasoline, the hazard is vapour that is heavier than air; Fire Prevention covers petroleum vapour specifically and is the right page for a fuel store, not for building fire safety in general.
- Do not seal the building tight if you burn anything inside. A stove or lamp needs fresh air and a flue; sealed rooms collect carbon monoxide.
- Do not make the building look like a fort. Defensive Architecture warns that sandbagged positions and military-looking additions advertise that you have something worth attacking and escalate conflict.
Fire, collapse and carbon monoxide
A hardened building with no inside-openable exit becomes a trap when it burns. Added mass can collapse a floor onto the people sheltering under it. A stove, lamp or fire in a closed room can poison its occupants with carbon monoxide without warning. Check all three before closing up.
What’s Next
- Learn next: Window and Door Hardening — the first jobs
- Combine with: Sandbag Construction — fast mass where the floor can carry it