Opening limitations

[Sifu]

2018 IRC t302.1(1)

Got a question asked of me I am not sure about. Consider a house with an exterior wall that is >5' away from the property line. It has a bump out that is only 4' from the line. The bump out has a window in it. The window is limited to 25% of the wall area. The question is the wall area. Is it 25% of the rated wall area that is the bump out? Or is it 25% of the entire wall, including the non-rated portion? I have searched a lot of places and can't find clarification.

From the commentary: “the total area of the openings in a wall with a fire separation distance that is equal to or greater than 3 feet, but less than 5 feet from the lot line, be less than 25% of the maximum wall area”.

On one hand it says wall with a fire separation distance equal or greater than, so you measure the openings in that wall, and I would think it is measured against the wall area of that same wall. On the other hand, it says maximum wall area, so I wonder if it means you measure the area of openings against the total of the entire wall.

Anyone bump up against this before?

 

[steveray]

I am going to say % of wall that is in the "setback" zone......

 

[Glenn]

Definitely what Steveray said. Definitely. What if that bump out was 10 feet deep, and the other wall was 14 feet away from the property line. That wall has no impact on the amount of opening of the wall in question.

 

[Teeshot]

Totally agree with steveray and Glenn. An owner of a property has no control over what occurs with an adjoining property. The opening limitation helps to limit exposure and transmittance (typically from a fire) from one building to another. Only the wall area within the 3 to 5 zone would be applicable, e.g. bump out, garden window, etc.

 

[Sifu]

Those are my thoughts but the code isn't super clear. If using the remaining wall, it could easily be big enough to allow the wall within the exposure zone to be 100% openings, I don't think that would be the intent of the code. I searched several other AHJ's, several code commentaries, as well as several code books and websites and can't find anything that directly addresses this. The closest is building code essentials.

 

[Robert]

Those are my thoughts but the code isn't super clear. If using the remaining wall, it could easily be big enough to allow the wall within the exposure zone to be 100% openings, I don't think that would be the intent of the code. I searched several other AHJ's, several code commentaries, as well as several code books and websites and can't find anything that directly addresses this. The closest is building code essentials.

I had a similar situation where we needed the opening (in this case a door) so we actually added additional wall in the setback zone to achieve max 25% opening. And it varies depending commercial or residential, one is "per floor" and the other is not (entire building).

 

[Sifu]

Yes, adding a little bit of wall would be a solution. Probably more palatable than reducing the window.

 

[Mr. Inspector]

I don't understand how making the wall longer makes it safer? The window will still be the same size.

 

[tmurray]

I don't understand how making the wall longer makes it safer? The window will still be the same size.

It's really not. It's just a work around. The only saving grace is that this section of the wall is rated where before it would not have needed one.

 

[Sifu]

A 300^ft wall with a 25^ft window has the same opening as a 50^ft wall with a 25^ft window but the code is written to prevent the 300^ft wall with 200^ft of window. I guess the intent is the affect of an opening on a given wall, and they came up with a percentage of acceptable effect. I imagine they could put qualifiers on it like a minimum, or a logarithm like an if/then. But how many more pages of code do we get if we start doing that? This code, like others is painting with a broad stroke, and is actually pretty easy to comply with...as long as you do it before your build the wall and install the window. So safer? probably not. Compliant to the letter of the code? Yes. Given the opportunity to do it to the letter of the code from the beginning, I will do it every time. After the fact-each of us will look at it differently, and each of us may do it differently at different times depending on the circumstances. Would I require an additional 4" of wall to meet the letter with no actual benefit? Depends on the circumstance, (Repeat offender? Master plan? History of previous enforcement? Honest mistake or intentional belligerence?) but I would do it knowing the action or inaction I take today WILL affect me tomorrow.

 

[Mr. Inspector]

To me it seems to be less safe for the code to force them to build more wall withing 5' from the property line to get the size window they want.