705.5 Firewall continuity/ termination

[steveray]

705.5 Horizontal continuity.

Fire walls shall be continuous from exterior wall to exterior wall and shall extend at least 18 inches (457 mm) beyond the exterior surface of exterior walls.

Exceptions:

1. Fire walls shall be permitted to terminate at the interior surface of combustible exterior sheathing or siding provided the exterior wall has a fire-resistance rating of at least 1 hour for a horizontal distance of at least 4 feet (1220 mm) on both sides of the fire wall. Openings within such exterior walls shall be protected by fire assemblies having a fire protection rating of not less than ¾ hour.

2. Fire walls shall be permitted to terminate at the interior surface of noncombustible exterior sheathing, exterior siding or other noncombustible exterior finishes provided the sheathing, siding, or other exterior noncombustible finish extends a horizontal distance of at least 4 feet (1220 mm) on both sides of the fire wall.

3. Fire walls *****shall be permitted to terminate at the interior surface of noncombustible exterior sheathing***** where the building on each side of the fire wall is protected by an automatic sprinkler system installed in accordance with Section 903.3.1.1 or 903.3.1.2.

Situation....3 story wood frame VB apartment building, 13R (mostly).....Common apartment facilities (exercise, offices, etc...) seperated out by firewall (NFPA13 on that side).....Building(s) sheathed with OSB except for maybe an 18" strip of Densglas at firewall termination..... Plan review was done before I got here.....Seems to meet exception 3 even though I do not beleve that is the intent....any input? Is having a strip of noncombustible sheathing just as wide as the firewall the intent of Exception 3?

 

[RLGA]

steveray: I agree with your assessment--that was not the intent. If they want noncombustible sheathing, then Exception 1 should be used.

 

[steveray]

Thanks Ron!....You always have very good input.....They have just gotten to that point with the framing and have buried me with lousy mechanical drawings, I did not do the "building" review so I have to go back and see if anything was spelled out on the plans .....

You did mean "combustible"...correct....?I do not believe the exterior walls are rated, but I will have to check. I would see no reason the use the densglas if they could achieve the rating with OSB.....

 

[RLGA]

steveray: Yes, I did mean "combustible" -- Thanks for the correction.

 

[steveray]

Apparently...that is the detail "we" approved.....They are certainly not going to like me......The exterior wall does not seem to be rated in that area....Thank You Ron!

 

[lpiburn]

Could you clarify something for me? Exception 1 is pretty clear. combustible = need fire rating. Exceptions 2 and 3 both deal with NONcombustible sheathing, but one uses the 4' rule and one uses sprinklers. If the distance of noncombustible sheathing is not mentioned in exception 3, then what IS it's intent?

It seems to me as though the 4' requirement in exception 2 is purposefully removed in exception 3 because of sprinklers. If that is the case, why wouldn't the detail shown here comply?

 

[steveray]

I kind of agree (the wording is not clear).....but why have the noncombustible sheathing at all if it were only required to be the thickness of the wall?

Could you clarify something for me? Exception 1 is pretty clear. combustible = need fire rating. Exceptions 2 and 3 both deal with NONcombustible sheathing, but one uses the 4' rule and one uses sprinklers. If the distance of noncombustible sheathing is not mentioned in exception 3, then what IS it's intent? It seems to me as though the 4' requirement in exception 2 is purposefully removed in exception 3 because of sprinklers. If that is the case, why wouldn't the detail shown here comply?

 

[lpiburn]

I don't have a commentary, but if I had to guess I would say that the noncombustible sheathing (sort of) accomplishes the intent of the firewall continuity requirement. For a basic firewall, you have to go out 18". Then the exception is for 4' of noncom. sheathing, then if you have sprinklers, you only need the sheathing the wall thickness itself.

Either way the purpose is to prevent the spread of fire around the exterior of the building. The 4' of sheathing intends to cut off a fire before it reaches the exterior finish (makes sense), and the reduction is due to sprinkler protection. Personally it doesn't make a ton of sense to me because then what exactly is that sheathing "cap" really doing? If there is going to be a sprinkler exception, let it be for combustible sheathing like in exception 1.

 

[lpiburn]

-edit-

I meant to also say that while the reasoning doesn't make a whole ton of sense, I still think the detail complies with the letter of the code.

 

[RLGA]

Exception 2 requires finishes applied over the sheathing to also be noncombustible, thus the limitation to 4 feet on each side of the fire wall. Exception 3 permits combustible finishes over the noncombustible sheathing, but requires a sprinkler system. The commentary states that this is the difference between the two, but does not mention the difference of a 4-foot extension of the noncombustible sheathing as a difference. I would probably accept as a compromise, due to the inconsistency between the two exceptions, the extension of the noncombustible sheathing to 4 feet on each side of the fire wall and permit a combustible finish over the sheathing.

 

[lpiburn]

Sorry, I'm still not seeing it. Given the way it is worded and the many commas, I interpret it this way:

Fire walls shall be permitted to terminate at the interior surface of: noncombustible exterior sheathing {Or at noncombustible} exterior siding {OR AT} other noncombustible exterior finishes provided the sheathing {OR THE} siding {OR THE} other exterior noncombustible finish extends a horizontal distance of at least 4 feet (1220 mm) on both sides of the fire wall.

If you read it in this way, it is not limiting you to noncombustible finishes. It is giving you the choice of terminating either at noncom. sheathing OR noncom. finishes, but if you were to terminate at the sheathing, you could use combustible siding over it.

You are probably right about the intent here, but the wording is certainly suspect.

 

[steveray]

Our State BO says the diference in Ex 3 is the sprinklers (ex 1&2 don't care)........and the reason the NC sheating is there is just to make sure there is no direct combustible path as part of the firewall termination....not the exact wording but he agrees with the design as shown being compliant......

 

[AegisFPE]

I find that the code is silent on the extent of the noncombustible sheathing.

The code tells us what we have to do, so since there is no prescribed minimum, the RDP should be able to address the provision accordingly. It appears that the n/c sheathing detailed is at least equivalent to the thickness of the fire wall. As the code does not prohibit this, it does not not comply, and so then should be acceptable.

Admittedly, where this has come up in projects that I have been involved in, I have generally recommended an extension of the n/c sheathing on each side of the wall.

 

[steveray]

Looks like I will be "recommending" more in the future....but I can't require it now....Now we just have to do something about the FRTW for roof sheathing that didn't make it to 4 feet....