[project_guy]

Hello,

I am planning to put in a new stoop at the front of my house. Dimensions will be approximately 4'x12'.

Frost line here is 42", and the existing house has full basement with CMU construction. Footings are approximately 7' below grade. Cores are filled every 4' with one #5 rebar and a bond beam around the entire top course with 2 #5s.

I plan to dig to the frost line, pour a 16"x8" footing with 2x #5 rebar, and build up to the stoop level with 8" block. I will then fill and compact the interior, and pour a slab over the top. My question is whether I should pin the new footing and block to the old foundation.

I have a couple concerns- if I can't line up with a filled core, I wouldn't want to pin into a hollow block as I worry about any movement in the stoop cracking the foundation.

Also, this creates another entry point for water, as the foundation (built 1950s) wasn't waterproofed well. Soil has a lot of clay in it here and we get a lot of freeze/thaw (I'm in Minneapolis).

The other option I can see is going all the way to the house footing, but this seems a bit excessive. However, I want to do this once and do it right, so I am not opposed to it if it is the best course of action.

So, any advice? Should I rigidly attach the two foundations? I've seen a common failure mode of the stoop tilting out away from the foundation, so could I pin just into the bond beam on top? Any other suggestions as to how others have done it?

I'd appreciate any advice. Thanks!

 

[jklingel]

I just looked at this situation w/ an engineer not long ago. Were it mine, I'd pin the new to old wherever I could. I'd dig below frost, compact gravel 8" or so deep under the new footings (then that same dimension wider than the footing), and use footings as wide as local code calls for. I'd then compact gravel under the slab and pour. The tilting out problem is settling, no? I would suspect settling over heaving, but I am no engineer. If it was not clear above, if you compact gravel 8" deep under a 16" wide footing, have the gravel 8+16+8 wide, etc. The footing's weight will spread at about a 45 degree angle through the gravel, so you effectively get a wider footing cheaper than pouring concrete.

 

[jomama45]

A few things:

- A 16" x 8" is the bare minimum for an 8" block wall. 20" is always better especially if it's not formed in the exact location, it gives you a little margin for error.

- The foundation depth really can't be determined until you start excavating. Ideally, the backfill aginst the house, after about 60 years, will be extremely firm when you get to 48" deep. If not, it's best to go all the way to the existing footing.

- The bond beam & pilasters would be a good place to dowel into where possible. Otherwise, just fill the jambs where they meet the existing house with grout and a rebar, let the rebar run above the block a foot or so and bend down into the stoop pour later to help hold it all together.