Hi,

I have a very uneven concrete floor and want to have it redone and lowered by 1.5" to get more headroom. The exiting floor is 1.5" thick, and it starts just above the wall footing. The wall footing is 4" deep. Under the wall footing and the existing basement floor, there is compacted/packed sand. There is little gravel under the basement floor, maybe 1". The basement is very dry, we never had any water coming from outside, either through the floor or the walls.

I started reading about structural and moisture/condensation issues. I put together a plan - basically have a new 4" floor on top of 4" gravel, and have the top of the new floor at the same level with the top of the wall footing to gain 1.5".
(I have a drawing, but I don't know how to add it to the post... I shared it here, you can create a free account to see it: basementPlan )

Before hiring a contractor to do the work, I was thinking that I should consult a structural engineer to confirm the plan and to ask how the plan should be implemented. I am afraid that digging so close to the footing may create problems.

I have trouble finding a structural engineer for a consultation. I think they don't typically work with the home owner, but with construction companies... Any ideas? Or do I need to talk to an architect for this type of project?

I have a few more engineering companies to call these days. Meanwhile, I would appreciate any advice on my plan. Some other questions are related to how to do the digging. For the floor + gravel, I need to go 4" below the wall footing. I read that a common principle is not to disturb the dirt 45 degrees from the bottom of the footing, or the basement walls may collapse. Is that enough? Or should I ask the contractor not to dig lower than the bottom of the footing for a 6-10" wide area around the perimeter? In that case there will be no gravel next to the footing, and the concrete floor will go directly on the sand. Is bench wall needed? Does the whole floor has to come out at once, or is it better to work in sections? How do I find a qualified contractor for this?

Thanks!
Ana-Maria

Figured out an easier way to share my drawing here

Your post is a classic example of why structural engineers do not want to deal with home remodeling/alteration. There is a great deal of time and expense to find out what is really there before they even begin to give a solution. - This increases the cost and liability.

I do not know how deep your basement is below grade or anything about what is behind the basement wall. What the wall is built of is also not included and is really not a major structural consideration since the walls have similar strengths.

You concern about digging too close to the foundation is well founded. If you look at most residential codes, they require 3.5" to 4" of concrete slab over the footing to prevent the wall from moving inward. Most home builders are very sloppy in this area. Your drawing shows a keyway between the wall and footing, but you would probably be hard pressed to know that for sure. - This is an important structural consideration since very foundations are built that way and there is no guarantee that there is reinforcing steel or dowels connecting the walls and foundation.

I really appreciate your reply, I could not find anyone to give me a quick yes/no answer to my idea. I am an electrical engineer myself, so I understand why a structural engineer would not want to take such a project -- and the contractors I've talked to (so far) have been generally hopeless when it comes to this kind of stuff. So I am doing as much of the homework as I can, coming up with ideas and trying to get somebody knowledgeable to validate (or invalidate) them.

So since I cannot know for sure about the keyway, my original plan is not an option.

The basement is 60" below grade.
I don't know what kind of soil is behind the basement wall. I will find out this weekend and update the post.
The wall is made of cinder blocks. The wall footing seems to be poured concrete, but I can't tell if it was poured in one piece since the original floor is still covering most of it.

Is it an option to redo the floor, but keep the current floor level of 1.5" above the footing? The new floor would then be 4" thick throughout (plus another 4" gravel), with the exception of the footing, where I keep the current 1.5" height. We would have to dig 2.5" down from the top of the footing. The gravel would be added gradually sloping away from the walls, not to disturb the soil around the wall footing.

Even if this is an option, the same concerns go for the digging process: What happens to the walls while the existing floor is no longer there while the digging is taking place? Is it safe to dig even for 2.5" next to the footing if there is 1.5" remaining till the bottom of it?

A few questions: how do you know the footing is only 4 inches thick? That would be highly unusual. Usually, a footing thickness is at least equal to the wall thickness. Why is an additonal 1.5 inches of headroom so critical? This seems like a lot of trouble to gain such a small amount.

But concretemasonry is correct...the floor is usually needed to support the bottom of the foundation wall against horizontal movement. What happens when the floor is removed? Who knows? Much depends on soil types, type of backfill, presence of ground water at that depth, structural integrity of the wall and the joint at the footing, the depth of soil supported by the wall, the wall thickness...a whole host of factors, none of which are known.

Quite a lot of work for 1.5 inch drop in height.
But you can remove the slab and re-pour it thinner. You must use a higher strength cement to compensate for the lack of thickness overall. Be sure to use a lot of rebar or wire in the floor to keep it strong. The primary concern is that the existing floor is acting as the key to keep the foundation wall from pushing into the room. Many older floors relied on the cement pour to hold back the bottom course of blocks from pushing in when lateral loads are created by water and soil pressure on the outside of the wall.
When re-pouring the wall simply create a step up on the height of the slab around the wall edge to assure that the wall is against the block wall. While leaving an open slot around the edge may never be a concern, if one existed before, I would pour the new floor to come into contact with the wall to be safe.

Remember that all your center columns if they exist will need to be checked for footings. Its not uncommon to find that the footings were not properly placed only to find that the columns were set onto the floor and missed the footing completely.

Before you go to all of this expense, get yourself a architect to draw up the plans on how this is to be done. Your township will require this before they can issue a permit.

But if its a simple re-pour it should not be that complicated.

I just had a trench dug through the floor to replace the sewer pipes which are running under the floor, parallel to the footing. That trench hasn't been closed yet, so I have some access under the floor to see the existing structure. The house is from 1950s, maybe that explains the footing...

The footing is wider than the wall (by some 4" on the inside, I don't know about the outside), but it goes only 4" deep. Under the footing, I think I see sand. It's quite likely that I have all sand under the entire basement floor. I don't know how deep the sand layer is or what is under it. The sewer pipe is roughly 9" under the floor level, and at that depth, it is still all sand.

The reason I am exploring the option of redoing and/or lowering the floor is beacause the existing floor is currently uneven. I need it to be leveled so I can install the subfloor.

I think that there are 2 options to level it: 1. use a self-leveling compounds to level it to the highest point of the existing floor, or 2. redo the floor at the lowest level that is structurally safe (and gain however much headroom I can). The price differential between the two options is not huge, but the loss of height between them is quite substantial. That's why I'm trying to figure out if it is safe to replace the existing floor to begin with, given all the unknowns that were pointed out.

So the 1.5" is not critical if it is not completely safe. Same for redoing the floor. I just need to have an engineer take a look and point out potential issues. I have had five! masonry contractors give me an estimate for redoing the floor. Even though I asked about structural issues, ALL of them said that it is completely safe to remove it, and suggested installing a 4" floor with 4" gravel.

Maybe that's why they are masons and not engineers. =) A four-inch deep footing isn't likely to have a 1 1/2-inch deep keyway in it, so what is keeping the bottom of your foundation wall from sliding inwards is the bond of the joint between wall and footing (questionable), and the floor slab. A floor 1 1/2-inch thick isn't a great support either, but it's all there is.

Now, if you remove the floor, your foundation wall probably isn't going to leap inwards right away, unless there is considerable water beyond it, and soils which get heavy when wet. The presumed sand may work in your favor, if it's all that's out there, so removing the floor slab probably won't be unsafe. However, not putting a slab back in so that it can continue to support the base of the wall could be unsafe over the long term.

Generally, one should not dig below a plane extending out and down from the bottom edge of the footing at a 45-degree angle. That's a "rule of thumb", though, and may not apply in every case. The sand that you presume is everywhere may require that the angle of that plane from the horizontal be reduced, and it could be almost zero degrees.

How about a compromise: pour the floor slab with its surface 2 inches above the top of the footing, and taper the gravel base from four inches out under the floor to 2 inches deep at the footing base, over about four feet. That way, the floor will continue to support the wall bottom, and better than it was before, you can still have a four-inch slab over the whole basement, and you won't be digging dangerously deep at the footing. The drawback is that you could get a crack in the floor near the inside edge of the footing, where the slab suddenly goes from 2 inches to 4 inches thick. You can mitigate against that by placing pieces of reinforcing steel perpendicular to the wall, maybe something like #3 bars 18 inches long, 6 or 8 inches on center, all the way around the basement, as a seat-of-the-pants guess. The bars hould extend about 3 inches over the footing, and the rest out into the new slab.

Then the only thing left to deal with is the issue of whatever post footings there might be. Are the posts supported only on the floor slab? Or is the slab thickened at the posts? Or do they have separate footings of their own under the floor? You won't know until you start demolishing the existing floor. They should have footings that are considerably deeper than four inches, but they may not. Maybe someone (like your engineer or architect) should calculate the footing requirements for the posts, and then there's the issue of temporarily supporting the structure above while a new footing is placed beneath the posts

Like we said, it's a lot of work to gain a tiny bit of headroom..

I did some more thinking, you're right, too much work and unknowns for a tiny bit of headroom. We'll go with the self-leveling compound.

Thank you very much for all your help. I really appreciate the time you took to answer my questions.