Notch in the concrete foundation for the floor joist

Q:

I’m putting an addition on the back of my house and I need to have a couple steps down to it, but I’m worried that the new floor joist are going to be too close to the ground, what can I do?

A:

If you can’t lower the grade around the new addition then you’re going to have to put a “notch” in the foundation walls.

What is a notch you ask, basically you set the height of your foundation walls like you usually do and then you find out the height of your floor joists. Usually around 11 ¾’s (this varies depending on who is supplying you with your floor package). You then have the foundation sub contractors pour an extra 11 ¾ inch wall on top of the original wall (at the same time they pour the rest of the concrete wall), this extra wall is not the full width of the wall, so if the concrete wall is 8 inches thick then you would pour the extra wall so that it is half of that at about 4 inches. This 4 inch thick wall should be from the outside of the foundation wall, what this does is leave a 4 inch shelf or lip or “notch” on the inside of the foundation. This 4 inch’s will allow you to place the floor joist on top of it.

With the floor joist inside sitting on the foundation notch that allows you to raise the grade around the foundation without the ground getting to close to the wooden floor joist.

Doing this notch is important for many reasons;

• Allows you to raise the grade of the ground around the addition so that you can have drainage away from the structure.

• This allows you to raise the level of the waterproofing (as the grade is raised) around the addition giving it more protection.

• This keeps the wood floor joist away from the ground and the splashing of water which overtime with damage them and create mold and mildew problems.

• Leaving the 4 inches usually is enough to bear the weight for the floor joist which is what the engineering would call for.

This will cost a little more money for your concrete sub contractors to install as a notch takes more time to assemble and pour then a conventional concrete foundation wall, but it’s better to pay now and not have to repair rot and damage later.

Rob Abbott
Village Builders Inc.