Ask Me Help Desk - Foundation for addition

Quote:

Originally Posted by jessechenven

Hi,
I am planning an addition to the back of my house, aprox. 28' x 32'. My current house is built on wood joists and is about 14-16" above grade. I am trying to figure out the best foundation to build with it considering my constraints. FYI, I do plan to permit it and perhaps find an engineer to do the drawings, but I'm just in the early stages and would like so opinions.

I could just pour a slab, but that would make the addition drop down, which I don't want. A raised slab seems out of the question, because I have limited backyard access (no alley) so, bringing in all that fill would be a lot to do. And concrete is so expensive now, plus I like the ability to cange thins with wood floors.

So, to build on wood, seems like the solution. But, with only 14"-16" above grade it doesn't seem like I could build a stem wall 6" above grade (the requirement here) and still have room for joists (unless I used 2x8, but I'd rather use 12" or TJI). Plus I would need to excavate under the joists to give me 18", and putting that dirt around the back yard is going to raise my grade level.

A builder friend of mine suggested doing something like this, (I will try to attach a jpg.) Build a stem wall all the way up to floor height for your walls (in my case 14") and build an interior ledge for your 12" joists. Then, when you go to excavate, your stem walls are so far above grade it doesn't matter if you raise the ground level much.

Help! What seems like the best solution given these circumstances.

Thanks, Jesse

I have built an addition to a house with wood floors that was about 2' above the ground. One very important factor is to make sure you have a good foundation.
I would suggest you excavate, 'shovel' at least 16"-20" in the ground, stack 2 high concrete masonry units to build your footing wall and fill the blocks with concrete and rebar.
It will be helpful to pour some concrete or a mudsill no less than 8" thick at the bottom of your excavation, 'with rebar running continuous in the concrete.
I would make sure that there is solid or hard dirt at the bottom of your footing. you should try to remove all roots from trees and grass.
If you can put some course aggregate or rock, 'at least sand' at the bottom. Keep the footing dry prior to pouring the 8" mud sill. If you have the means compacting the ground with a sand tamp prior to pouring the mudsill is highly recommended though not required
The 8" mudsill is for your block to sit evenly on. you should run continuous rebar the length of your footings in the mudsill, 'use rocks or concrete bricks to hold the bar off the ground.
keeping in mind a concrete masonry unit or CMU is about 16" in length, 'with two holes' typical centering is 4" and 8" for the holes. Get some rebar that has a 90 degree bend in it. One leg 6" the other leg about 20" I would think a number 4 bar, '1/2" dia.' would be fine. Tie the 6" leg to the pair of continuous bars on a spacing of no more than 8", starting from the corners of course. you will need some tie wire for this and a pair of pliers. The 12" leg will be for the CMU's to slide down on for structurability. In footings more is better. After you have filled the CMUS with rebar and concrete then use the 2x12's to build the floor. The 2x12's should be spaced no more than 16" apart and I would not recommend using less than 3/4" T&G plywood with screws and construction glue. Use a good wood for your joist. A number 2 wood has less knots in it is typically more straight and not as expensive as a number one wood. When you are building to an existing elevation say 16" above ground you want to work down from there to determine your overall trench depth. I would use 2x12 floor joists with a rim joist for the end at that length and width I would excavate a trench in the middle of the area and install another footing wall. 2x12's get expensive over 20' long the quality goes down and deflection increases. You should consider using a rigid foam insulation board around the perimeter of the footing on the inside of the wall. Insulating the floor and soil poisoning is not a bad idea either. I can draw you a picture if you need more help.