IRC Code Question: How to Determine Code Requirements Beyond Published Charts?
Greetings everyone, Chapter 4 of the IRC details the foundation size requirements for residential buildings with various wall types, soil types and roof loads. Table R403.1(3) appears to be the correct table for a build using ICF walls, and a SIP roof. Problem is it only goes up to 3 story, slab on grade. I’m planning a 4-story, slab on grade. Of course I’ll hire an engineer if everything seems doable, but I’m not there yet- I’m still in the “Is it feasible?” stage. The dealbreaker in question is that this build (an addition) would be 4 ft from an old house foundation.
So would using the “3 story with basement” requirement give me a good general idea, or is there some other place where the wonderful folks who write the IRC publish additional charts that cover a bigger range? Or is this simply something that I can’t estimate at this time, before an engineer gets paid? Thanks!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Measure_Twice,
"All buildings within the scope of the IRC are limited to three stories above grade plane. For example, a four-story single-family house would fall within the scope of the International Building Code®(IBC®), not the IRC."
What in particular are you trying to find out about the foundation at this stage?
Ah I see. Thank you. I'm trying to find out the estimated width, to see if there's enough room for the build size we have in mind, and still meet setback requirements. The geotechnical analysis will be crucial to learn the final foundation size. However that testing can't be done until the site is fully ready (an existing addition needs to be removed first, and there are tenants in the house for a while still).
Normally the footers are made as wide and thick as they need to be, then the foundation wall is 8" or so and sits on top of the footer. In my jurisdiction at least the setback is measured from the outside of the foundation wall. So the size of the footers doesn't affect the setback.
In fact, the setback is measured at grade, it's legal to have a basement go all the way to the property line so long as the above-ground part is within the setback, I'm seeing this in new construction.
You need to find out how it's measured in your jurisdiction.
Make sure you understand your local zoning rules. 4 stories would be prohibited in most residential zones or have large set backs from the property lines.
My guess is if you are going up 60 feet 2/3 or so can be above grade and 1/3 will be below so the foundation is 20 feet deep, or the base must be wider than the height 75' square that is 60' tall.
Walta
The 4 story plan was not a problem with the city planner I was working with, until 1 day recently her email kicked back automatically saying she got a different job. So I called the department and left a message to be contacted by whoever was now handling our area. Nobody has called back and that was 3 weeks ago. Uh oh! We do have a 40 ft height restriction, but the plans worked within that limit.
60 feet?
10 feet per floor on top of a foot exposed foundation with an 8 foot attic is still under 50....under 40 with a flat roof and no long [IE tall] floor joists..unless my math is terrible today
If the OP is really doing 3 floors and a walk out basement, the basement floor is not sitting on the foundation, so probably does not affect the foundation plans one would think