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The Building Code Blog

Determining Grade Plane for a Building

2/12/2021

2 Comments

 
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.
Simple Grade Plane Image
A simple grade plane calculation where the site slopes uniformly.
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.
Advanced Grade Plane Image
A complicated grade plane calculation with an irregular slope.

​Another common question: What if my site slopes perpendicular to the exterior walls?
Fortunately, the definition of Grade Plane specifically addresses this:

Where the finished ground level slopes away from the exterior walls, the reference plane shall be established by the lowest points within the area between the building and the lot line or, where the lot line is more than 6 feet from the building, between the building and a point 6 feet from the building. (2015 IBC)

So you're looking for the lowest elevation point within 6 feet of the exterior wall (or less if the lot line is closer).
Picture

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:
  1. More than 6 feet (1829 mm) above grade plane; or
  2. More than 12 feet (3658 mm) above the finished ground level at any point.

This definition has caused some confusion in the past, so here is a step by step breakdown:
  1. Is the floor surface of the story in question located entirely above the grade plane elevation?
  2. Is the floor surface of the floor above the story in question located more than 6 feet above the grade plane elevation?
  3. Is the floor surface of the floor above the story in question located more than 12 feet above any of the grade measurements at any point along the building exterior walls?
If you answered yes to any of the three questions above, the floor in question is considered a story above grade plane.

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).

    New Average Grade Plane Calculator

    If you need to perform a grade plane calculation or are wondering if a particular floor is considered a story above grade plane, check out this new calculator at the link below!
Go to the Calculator

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!
2 Comments
Bernadeta Frohnen
11/4/2022 11:49:35 am

Really good article. Thanks for the straightforward explanation!

Reply
John TeSelle
1/24/2023 03:23:07 pm

Thank you for the article. One thought about how you are choosing the points to measure from -- it seems that perhaps they should be chosen based on perimeter and not using corners or other arbitrary building features such as mid-point of wall. For instance if you have a building with lots of corners on one end but only two corners on the other end, then the average would end up skewed toward the grades on the end with lots of corners.

A more "fair" way to choose points might be this -- consider a polyline (to use Autocad terminology) drawn around the perimeter of the building and then divide that polyline into a number of sub-polylines of equal length (how ever many you want). Then the end points of those sub-polylines will be the points where you measure.

This method still is not totally fair in that there are a number of ways to create the sub-polylines and one start point might end up with a different average than another. But the more points you have the more fair it becomes.

Reply



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  • Home
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  • Tools
    • Allowable Height & Area Calculator - Separated Mixed Occupancy
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    • Average Grade Plane Calaculator
    • Calculated Fire Resistance for Wood Walls
    • Fire and Smoke Damper Tool
    • Fire Wall/Exterior Wall Intersection Tool
    • Frontage Calculator
    • IBC Occupant Load Calculator
    • Plumbing Fixture Calculator
    • Stair Pressurization Estimator
  • Startup