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I'd like to know if there's an argumentation-/science-based recommendation regardless of the specific laws in different countries. Boundaries/parameters of my question:

  • fire pit for making a(n open) camp fire with wood for <= 15 persons (max. 10 kg of wood burning at the same time[1]) on the ground inside a stone circle
  • building made partly or completely of wood (I guess that matters, so should be distungished in the formular in the answer)
  • fire is watched permanently

The choice of the distance should avoid transmission of sparks to fire receptive parts of the house, e.g. exposed parts of the root framework. What would a fire fighting specialist/consultant, assurance specialist, etc. say?

[1] I've no idea how to measure fire energy emission, so I give this "dimension" for the beginning

Tester101
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Kalle Richter
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  • It completely depends on the type of "fireplace". Could be zero, to tens of feet. Do you have a specific design in mind? – Tester101 Sep 07 '14 at 13:51
  • Given that some fireplaces are inside wooden houses, and that various devices exist for having small social fires on wooden porches, I agree that as posted the answer is "it depends". – keshlam Sep 07 '14 at 14:23
  • @Tester101 Could you please explain, what is too unspecific about "for making a camp fire with wood for <= 15 persons on the ground inside a stone circle" (also added some further information in an edit)? – Kalle Richter Sep 07 '14 at 14:55
  • @KarlRichter Guess I didn't read carefully enough. Maybe I was confused by the use of the word "*fireplace*", to mean a hole in the ground surrounded by rocks (usually referred to as a "*fire pit*"). – Tester101 Sep 07 '14 at 18:50
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    I'd recommend talking with your local fire department. They will have an opinion based on experience with local conditions. That opinion will be tempered with the idea of avoiding coming out to your house to extinguish a problem. Keep in mind that wind, the availability of nearby fuel sources such as dry brush, and a host of other factors may increase the risk. –  Sep 07 '14 at 21:54

4 Answers4

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The model building codes provide separation distances based upon radiant heat, construction type, occupancy, fire resistance of the exterior walls and the ratio of openings to solid wall in the building to be protected.

Keeping in mind that the fire separation distances given are in feet from the property line and that adjacent buildings have similar requirements one could use the combination of IBC [2009] table 602 and table 705.9 to determine separation. As a rule of thumb, 60' would be the maximum required between two buildings each located 30' from the property line.

However, I recommend discussing the issue with your local fire official. There may be additional considerations that limit or prohibit the use. The fire official may also be aware of additional environmental, utility infrastructure, or fire service availability factors that increase or decrease the risk associated with a large fire pit.

If you set the forest ablaze, the distance between the pit and the structure will become irrelevant.

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I think the most important aspect is the possibility of a long lived spark being blown to the house where it might contact cedar shingles or the like.

When burning dry firewood like western white pine or doug-fir, the longest lived sparks which I have observed last 5–7 seconds after which they are effectively extinguished.

Using seven seconds then depends on the maximum windspeed. Most people aren't interested in hanging around a campfire when the wind is much over 15 mi/hr (24 km/h) so using 20 mph (32 km/h) means the distance to a valuable burnable object should be 205 feet (63 m) minimum.

However, I have assumed quite a bit. To apply to other situations, adjust:

  • Lifetime of spark
  • Wind speed
  • Safety factor
wallyk
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  • Great, thank you! Were the longest living sparks blown by the wind or just sailing in the air. If not, I'd split their time of living in half, make ~31 m the new worst case avoiding distance and the half of it, i.e. ~15m, the minimal distance - not if my neighbour is a refinery, though - for pure egoist reasons ;) – Kalle Richter Sep 07 '14 at 15:29
  • @KarlRichter: I am confused by your question. My observation is that the lifetime of a spark is not affected by its speed. Higher wind causes more sparks, but each lives the same range of lifetime. – wallyk Sep 07 '14 at 15:45
  • I thought, that if the spark moves in the air its oxigen supply should be higher than if it floats. If your experience is different, it can't be right... – Kalle Richter Sep 07 '14 at 15:59
  • @KarlRichter: Ah! But the spark is moving with the air, at least after the first 1/4 second (during which it was blasted from the fire). – wallyk Sep 07 '14 at 16:22
  • Maybe we'll have to move that to physics Q&A or forum... – Kalle Richter Sep 07 '14 at 16:30
  • This is an interesting way to approach about the problem, but I think the answer you've reached is a little silly... 200 feet?? That seems excessive. – Hank Sep 07 '14 at 17:19
  • @HenryJackson: While 200+ feet surprises me too (I expected 75 feet to be adequate), it is basically a worst case estimate better than 99 percentile (probably) of that event. It does assume no sparks can hit the structure, which for practical purposes isn't true since no one has kerosene soaked siding. It also assumes a pretty strong wind which is uncommon in this situation. I have lots of friends who wimp out at winds of 5 mph. 75 feet is probably overkill too since most backyard marshmallow roasting is done with almost zero wind. – wallyk Sep 07 '14 at 19:39
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I don't have a specific formula for you (I don't see how you could derive one), but I will say that in general engineering tolerances are a balance between two factors:

  • How likely is something to happen?
  • How bad would it be if it did?

This is why buildings in earthquake-prone areas are built stronger than elsewhere (because an earthquake is more likely to happen there), and why hospitals are built more strongly than single-family homes (because a hospital collapsing is worse than a single house collapsing).

So in this case you would need to weigh how likely you feel the fireplace is to start a fire (which depends on how big a fire it is, what kind of hearth you've built, wind speed and direction, etc.) vs the consequences of an uncontrolled fire (is the building just a shed or a multi-family house? Do you have a hose nearby in case a stray ember lights something?).

Hank
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NFPA considers 35' to be the default distance from areas where sparks could contact combustibles, with the Permit Issuing Authority retaining the right to increase that distance when warranted. This assumes an area where there is no potential for existence of flammable vapors/gases or materials. Unless there are special topography, flora, weather, circumstantial/situational issues you should feel OK with such a distance, especially with a full time firewatch (equipped with means to extinguish incipient fire).

Jimmy Fix-it
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