Drain tile (or lack thereof) and slab foundation additions

I came across an addition on a slab foundation that had very bad grading, and appeared to have the base of its framing below grade. The poor grading led me to question whether or not the slab foundation on the addition had proper footings or a drain tile system, which would be very desirable with drainage essentially down a small hill toward the addition.

Is it customary to tie into the drain tile system when building an addition on a slab, or is a proper footing considered adequate?

what weve seen is some do, most havent used any tile at all

Frank, a lot depends on what region you are located in. Many errors are made with slab foundations, particularly when the slab is poured over the top of the foundation walls without thickening and reinforcing. Unless it’s a structural slab, it should lie within the foundation walls, not on them. You can check for the presence of a footing by driving a steel rod down just outside the foundation wall to see if it hits a footing below. It’s possible, again depending on region, that the slab and stem walls were poured monolitically, and again, the slab should be thickened at the edges and reinforced. I don’t know that there’s a non-destructive way to inspect for that.

A slab on grade thickened at the edges will be plain concrete looking, and a stem wall will show form panel joints at two feet on center and form ties at 6" from the top.

Just a clue that sometimes might work.

Marcel :slight_smile: :smiley:

Two different issues/questions in the last line. Is it a true “slab-on-grade” or a slab supported on a 4’ or so partial foundation and compacted infill? The first probably may not need drainage as it should be elevated above the local grades to begin with. If it’s on a partial foundation with infill and located in a low spot that collects water in wet periods, it should be drained.

We have a local “horror story” house that was built “slab on a partial wall” and located in a low, wet area. I mentioned it in another post yesterday. It’s been on a national investigative TV show in Canada and on a another prominent TV “building disasters” show’s message board in the past 2 years.

Thanks, everyone.

Yes, Brian, I was referring to the wood faming apparently being below grade in the rear. The ground was snow-covered and frozen, but this was apparent from lifting a bit of the siding on the side of the addition where the elevation was a bit lower. At least the framing appeared to be treated lumber.

Yes, I do recommend digging a swale in a situation like this. Lots of digging to be done in this particular case. I’m also recommending they check city records for permits, inspections, etc. with the caveat that passing a city inspection does not guarantee proper construction.

Being in Michigan during a January (very) cold spell, the ground was frozen so solid it would not have been possible to probe it with any sort of rod. There wasn’t any way to tell how the slab was constructed other than to check city records if proper permits were obtained.

My main concern about footings here has to do with the effects of freezing and expansion of the ground with the clay soils that are present in most of SE Michigan. These conditions wreak havoc on a lot of foundations, particularly the flimsy hollow concrete block basement and crawl space foundations that were common when much of the area was built up in the early 1950s.


you prolly know,but we`ve run into slabs poured on…slabs.:roll:

other times itll be on 1-3 courses of block,others on block 4’ deep.

and certainly atop poured footings.

depending on size, have seen a few of these footings taken to depth of
basement-wall footing, only where footing for slab/porch meets house,
the rest is 4’ or less…most no drain tile.


  1. block footing under old small porch/slab at back door,goes all way down
  2. poured footing for newer/larger porch on back. they used the
    block footing for 1 end and poured for other end.Can`t remember
    exactly but poured footing was between 36-48"


and sometimes, like this house in Eastpointe, will have rebar.
they`ll sometimes drill holes through hollow block,bricks or near
top of wall.

here, they had 3-4 holes per each patio slab on back of house.
not good!

created leaks in basement and every slab did wind up settling back
towards the house.

have seen rebar that was tied into house for slabs too.
this added weight can sometimes cause crack(s) in basement wall

I hope it was the grade approved for wood foundations and not just “deck” grade. There is more preservative retention (in lbs/cubic foot) required for foundation grade wood. That’s why you’ll see deck posts meant to buried in the ground “incised” with little knife type cuts to allow more and deeper treatment.