How to Define a Building Height?
How do we determine building height?
This question has a few different applications, especially since we're talking about allowable building height, or we could be talking about standpipe requirements. To answer this question, it really comes down to the definitions and the adopted building code.
Let's first start with the International Building Code 2021 Edition just as an example.
Chapter 2 of the International Building Code is dedicated to definitions. What we tend to love about the definitions and the ICC processes that they're all organized alphabetically, so they're relatively easy to find. Here I search for building height which directs us to see quote height, building, and then we look that up in the actual definition is the measurement vertically from great level to the average height of the highest roof surface.
What is grade plane?
That is also defined in Chapter 2 for definitions, and it's the average ground level adjoining the building at the exterior walls. It gets a little bit more complicated when the ground slopes though. When the ground slopes away from the exterior wall, then the point at which grade plane is considered is the lowest point between the building and the lot line, or if our lot line is more than 6 feet away from the building, then it's the lowest point between the building and a distance 6 feet away from the building. This all comes down to the definitions out of Chapter two.
Then later on in IBC, we get into chapter 5 which talks about allowable building heights. And there are a small handful of exceptions that apply to a level of building heights if we have towers or steeple's spires, you know, something that‘s common for churches, or as long as it's not used for habitation or storage, and then those are allowed to extend up to 20 feet above the allowable building height with combustible materials or unlimited when we're using noncombustible materials. These are found in IBC section 504.3 as it relates to allowable building height above grade.
Section 504 would direct us to 507 when we have an unlimited area building that can affect the building height.
And then, that same section 504.3 also will direct us to 1511, which addresses rooftop structures. This section carries some limitations for what types and sizes of structures that we can have on the roof deck itself.
We're seeing more and more with residential large apartments when we've got an occupiable roof that they like to do canopies or pergolas up on top. And this rooftop structure section would address those.
Now, when we're talking specifically about standpipe systems, the code section for standpipe requirements actually defines its own not based on building height, but on the highest occupied floor level compared to the lowest level of fire department access.
So, that distance is actually measured differently. It's not based on building height at the highest occupied floor level versus the lowest level of fire department access. The definition there is a little bit different than the overall building height that we get out of Chapter 25 when we're talking about allowable building height.
Now, if the applicable building code is say like NFPA 5000, we can check Chapter 3 out of that code and see that the building height is defined in a similar way.
The grade plane though is the average ground level of a joining grade along the exterior walls, which is a little bit different than the IBC definition.
NFPA 5000 goes on to specify height limitations in Chapter 7, or it also goes on to allow for increases for things like sprinkler systems and mass timber construction.
If we're looking only through a sprinkler lens, why does building height matter to us?
Some standards carry limitations for the height of the building. Like when we get into NFPA 13R, let's just say the 2022 edition limits NFPA 13R buildings, two buildings that are 60 feet in height or less. So, if we've got a 62-foot-tall building by the definition, we can't use NFPA 13R. And there's other limitations like the amount of floor levels, and there's other things that apply there, but when we're strictly talking about building height, we can only use 13R with buildings that are 60 feet or less in height. So that's where this measurement really comes into play.
So how do we determine building height? Well, first we check our applicable building code and we're probably gonna end up in the definitions out of the model building codes like IBC and NFPA 5000. The building height is defined as the vertical distance between grade level and the average height of the roof.
I'm Joe Meyer, this is MeyerFire University.
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