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In the Middle Ages most buildings were made from timber. After large fires timber was restricted or outlawed altogether and brick and stone became the preferred building materials, e.g. in London after the Great Fire in 1666 (Source), or Lübeck after 1251 (Source).

Today fairly dense new neighbourhoods are designed using exclusively timber construction, e.g. Berlin Schumacher-Quartier (Source).

How is the fire risk reduced in these dense contemporary timber neighbourhoods?

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  • $\begingroup$ I know of a fire inspector that wanted to "fail" a wood chalet type construction. The owner set up a large balk of timber with a big gas torch and lit it then took the inspector into the kitchen so it could not be seen and made him have a cup of tea. And this inspector was getting really worried so he made him have a second. Then they finally went out to look a the balk of timber and it was hot but not alight... Inspector signed off on the building permit. $\endgroup$
    – Solar Mike
    Dec 2 '20 at 21:16
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    $\begingroup$ England basically ran out of decent softwood timber. Any they could get there hands on went for building naval vessels. They didn't really have to outlaw it, there wasn't any. It mostly came from the Baltic, and England ran a trade deficit with the Baltic for centuries due largely to lumber imports. $\endgroup$
    – Phil Sweet
    Dec 3 '20 at 0:43
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They protect the combustible framing by cladding it in fire-resistant materials like gypsum boards.

Fire code is an integrated and elaborate part of the building code.

It assigns fire protection requirements and demands fire ratings of structure and its contents depending on the usage and location of the building.

It sets standards for access and egress and fire escape routes.

It sets ratings like how long a gypsum board should resit a fire of a predesigned intensity. what class and rating windows and doors and roofs should be. it assigns higher protection to more critical components of the building. it requires a fire hose connection at hallways and landings. it requires sprinkler systems in certain buildings. It sets the size and location of fire-stop partitions and automatic lock-off fire gates in large areas.

It is a multi-faceted all-encompassing system of regulations involving from conceptual design to routine post-construction fire inspection to accessibility to water and fire hydrants and location of fire departments.

The fire department can and has issued non-conforming citations and ordered the buildings in violation of the code to be declared unsafe and to be vacated till fixed.

Here is a detail for an exterior two-hour fire rated wall.

firwall detail

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For England, the relevant techniques can be found in section 11 of requirement B4 of Approved Document B of the Building Regulations. The measures essentially consist of:

  • spacing buildings sufficiently far apart to limit radiative heat transfer from one to another;
  • using, for exterior construction, materials (whether timber or otherwise) that have demonstrated a particular level of resistance to the spread of fire in laboratory tests by methods prescribed in European standard EN 13501-1:2007+A1:2009;
  • the use of sprinkler systems to limit the intensity of fire within any one building.

To some extent, increased attention to any one of these three measures is allowed to compensate for reduced rigour in the others.

(There has recently been a fairly well-publicized case involving falsification of the EN 13501-1:2007+A1:2009 test results, but that concerned a metal/synthetic polymer sandwich material, not timber; I say "falsification", not "alleged falsification", because AIUI, the perpetrators confessed while giving evidence at a public inquiry in the UK.)

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