An exercise in installation rules per NFPA 13, deflector distance to combustible soffits

[fpst]

I'm trying to make sense of the logic of NFPA 13's obstruction rules so that I can follow them,

In NFPA 13 2013 edition it says,

8.6.4.1.1.1 Under unobstructed construction, the distance between the sprinkler deflector and the ceiling shall be a minimum of 1 in. and a maximum of 12 in. throughout the area of coverage of the sprinkler.

and later it says,

8.6.4.1.1.3 The requirements of 8.6.4.1.1.1 shall not apply for light and ordinary hazard occupancies with ceilings of noncombustible or limited-combustible construction.
...
B) Where the distance between the upper ceiling and the sprinkler deflector is less than or equal to 36 in., the sprinklers shall be permitted to be spaced as though the ceiling was flat, provided the obstruction rules are observed as shown in Figure 8.6.4.1.1.3(B).

So far, I take this to mean that, if the ceiling type is combustible, such as some type involving non-treated wood, then any soffits or irregular ceiling areas having a surface outside of 1 to 12 inches of the deflector nearby it require a sprinkler under it - even if, for example, it's a combustible-finish soffit beneath the sprinkler and the sprinkler is throwing water on it and under it, and the soffit is narrow such as 20 inches wide.

However, further down the standard says (for standard upright/pendent sprinklers),

8.6.5.1.2* Sprinklers shall be arranged to comply with one of the following arrangements:
...
(3) Obstructions located against the wall and that are not over 30 in. (762 mm) in width shall be permitted to be protected in accordance with Figure 8.6.5.1.2(b).

The figure then shows soffits along the wall under 24 in. wide and extending at least 18 in down from the deflector not requiring sprinklers.

So my question is, which rule takes precedence? They seem to contradict. It was implied videos by the authors of the documents that later sections generally over-rule earlier sections, as earlier sections are more generalized rules, and later sections go into more detailed cases. However, it was also said in several NFSA technical newsletters that we should use the most stringent requirement where requirements conflict.

 

[cdafd]

I will give you my shot, not an engineer or designer


To your first question to me 13 is talking about the cieling as a whole. If the feature is 36 or less, than the ceiling is treated as unobstructed. As far as placement of the head down from the ceiling.


Second question to me is an actual obstruction encountered than those rules apply.

 

[sfallows]

The 1-12 inches is about where the head is installed not where it is required. It says ever time you install a sprinkler it has to be 1-12 inches below the ceiling to work. It is not saying anything about the placement in terms of different ceilings.

The less than 36 in is allowing you to treat it as flat surface as long as obstruction to the head on the higher elevation is not to close to the change in elevation. Think about walking through an office where the ceiling goes to 10' to 9' and the sprinkler heads are 6' away in both directions (one on the low side one on the high side) if it wasn't for this rule you would have to go by 1/2 the sprinkler spacing. This says that doesn't matter as long as the head isn't too close.

The obstruction against the wall does not fit either of the other situations so go by those rules.

Each one is separate they are not related sections.