Separate dwelling units on a lot with a shared deck.

[Otter]

There are two separate detached single-family 2 story homes on one lot/parcel. They are spaced far enough apart to not require exterior wall ratings, assuming an imaginary FSD line is established mid-way between the two. They do, however share a common, uncovered wood deck that connects the two upper stories, about 9 feet above grade. The deck also has stairs that serve as the means of egress for one of the homes. Since the shared deck extends to and over the FSD line, would it be correct to consider the deck a violation of Table R302.2.1(1) – a projection less than 2 feet to the line? 2021 IRC. Thanks in advance for looking.

 

[Glenn]

There are two separate detached single-family 2 story homes on one lot/parcel. They are spaced far enough apart to not require exterior wall ratings, assuming an imaginary FSD line is established mid-way between the two. They do, however share a common, uncovered wood deck that connects the two upper stories, about 9 feet above grade. The deck also has stairs that serve as the means of egress for one of the homes. Since the shared deck extends to and over the FSD line, would it be correct to consider the deck a violation of Table R302.2.1(1) – a projection less than 2 feet to the line? 2021 IRC. Thanks in advance for looking.

You are asking a very difficult question that does not have an answer in the model IRC. Decks and porch roofs have never been addressed in FSD provisions in the IRC, and thus you will get a wide variety of responses. A ground level deck vs. an upper level deck, have fundamentally different responses to fire. They don't fit the "projection" provisions of the IRC, which are specifically in regard to overhanging soffits and ventilation (it's gotten a little weird, see the footnotes). The FSD "elements" need to be updated considerably with science that makes applications for deck and porch roofs more consistently interpreted.

I look forward to hearing the responses to this question.
If you do choose to require fire protection, the IRC methods are not practical for decks. I recommend looking at provisions in the IWUIC. Heavy timber beams, and posts, FRT joists, and Class A decking material.

 

[Robert]

Good question. Since they are the same occupancy (R-3) on the same property, is a FSD even required between them?

 

[Otter]

Well, that's a good question too. I suppose the FSD line is required. Logic being that if two dwelling units are butted up to each other, you have a duplex and therefore a 1 hour separation wall is required. Now suppose we pull them apart and what does that separation look like? I go to Table R302.1(1) and figure if the buildings are far enough apart to not need any ratings on the exterior walls. Once the line is established, do I apply it to the deck as well? If the homes were 50 feet away would there be much of a concern? I think this is one of those things where it is technically correct to say it is violation of code, but it feels heavy-handed.