UPDATE: See new average grade plane calculator, linked below.
In the past few months, I've had several clients ask me questions regarding grade plane calculations - most often related to determining whether a particular story counts as a story above grade plane or as a basement. Here is quick rundown of how to do the calculation and the implications.
Average Grade Plane
Chapter 2 of the IBC defines Grade Plane (often referred to synonymously as Average Grade Plane) as:
A reference plane representing the average of finished ground level adjoining the building at exterior walls.
So if you have a completely flat site, the elevation where the finished ground level adjoins the building exterior walls is your grade plane. If your site is sloped, you need to take several elevations where the finished ground level adjoins the building and average them to determine the grade plane.
The most common question I receive on this: how many elevation points do you need to account for this in average?
First, the IBC does not give any specific requirements or instructions to answer this question. Your local jurisdiction could potentially have some guidance on how they want to see the calculation performed, but in my experience, most do not.
Typically, I advise clients to base the number of elevation points on the slope of the site. If you have a relatively flat site, a small number of elevation points will give you an accurate grade plane elevation. You may only need one elevation point per face of the building. On the other hand, if you have an irregular, highly-sloped site, you will need many elevation points. I once worked on a building that was built into the side of a steep hill - there we measured the elevation every 10 feet in some areas. If in doubt, the greater number of elevation points in your calculation will result in a more accurate measurement.
In the simple example above, the grade plane measurement is just the average of the elevations at the end of each wall segment (e.g. for the east wall, (280+270)/2 = 275'). However, with a more severe grade, particularly where the slope is irregular, you will need to take additional measurements, as shown in the example below.
Another common question: What if my site slopes perpendicular to the exterior walls?
The Implication - Determining a Story Above Grade Plane
Why does the grade plane calculation matter? On my projects, this always comes up when a client is trying to determine whether a particular story is actually a story above grade plane or a basement.
Chapter 2 of the IBC defines "Story Above Grade Plane" as:
Any story having its finished floor surface entirely above grade plane, or in which the finished surface of the floor next above is:
This definition has caused some confusion in the past, so here is a step by step breakdown:
There are numerous code requirements that differ between a story and a basement, but the most common one is complying with the allowable number of stories based on occupancy and construction type. I had a project this year where the floor surface of the level above was 5 feet, 10 inches above the grade plane elevation, just barely passing as a basement. 2+ inches more and the building would haven been one story taller, resulting in requirements for a more robust construction type, higher shaft ratings and substantial additional cost.
If your situation is that close, I suggest including a plan in the permit package that clearly shows the grade elevations around the building and shows your grade plane calculation. This makes it easy for plan reviewers to follow your logic and hopefully agree with your approach (remember, there are no specific IBC instructions or requirements for how to do the calculation).
What do you Think?
Have you had a project that required a detailed grade plane calculation? Have you every had a plan reviewer or code official disagree with your calculation? Let me know in the comment box!