Replacing basement floor below grade
I’ve read several of the blogs (Martin Holladay) and other research available, and posted a question about my current situation (high humidity, no vapor barrier, high salts in concrete floor) – recommendation was to waterproof from the inside.
I’m toying with the idea of ripping out the floor, installing french drains on the interior (I have exterior footing drains) and installing a proper vapor barrier. The cost doesn’t seem to be much more than alternative solutions that have no guarantee.
The articles mention the use of rebar or mesh in the floor. As this is a residential application, is this necessary? The one thing I don’t want is the floor to curl or crack too much. What I got out of the blog was to ensure that the company uses as little water as possible, is this enough?
This is obviously an existing house, so I have other things to consider, like moisture while the floor is curing/drying causing issues on the wood floors above.
I plan on finishing the space, should I use the ridged foam board under the poly? Will it make that much of a difference compared to additional cost?
What else should I be considering?
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Check with your local building department if they’ll require mesh reinforcement in the floor slab. Mesh is a common requirement commercially, but not so much for residential slabs. Mesh isn’t very expensive though and I’d put it in if it was my own floor slab.
If you’re going to be replacing the entire slab, it probably makes sense to go all out and put the foam under it too. See if you can get reclaimed EPS or XPS in your area that you can use beneath the slab. The cost for the foam won’t be very much, and the labor is minimal since you’ll generally be using mostly full sheets laid flat (it’s very easy to do). Most of your costs will be in the demo of the old slab and the pour of the new one.
Here's what I wrote on this topic for a JLC article:
"Most nonstructural residential concrete slabs don't need much reinforcement. Barring unusual soil conditions, if a residential slab is being placed on a well-compacted subbase, wire mesh — or, for that matter, synthetic fibers — can be safely omitted. Depending on whether you want to control either plastic shrinkage cracking or drying shrinkage cracking, either synthetic fibers or wire mesh may be used. And if a residential slab actually needs reinforcement — either because it is partially unsupported or because it is load-bearing — then what you need is rebar, not wire mesh or synthetic fibers."
Personally, though, I'd include the mesh. And I agree with Bill -- It's a good time to include rigid foam (although you didn't tell us your climate zone -- so the decision on rigid foam may depend on your location).
Ideally, these are your layers, from the bottom up:
1. A layer of well-drained crushed stone, connected to your footing drain system.
2. A continuous layer of horizontal rigid foam.
(It's also a good idea to include vertical rigid foam at the slab perimeter.)
Thanks Bill and Martin.
I'm in climate zone 5.
You didn't mention whether headroom is a problem or not. If not, you can place a layer of foam and poly above the existing slab and then pour your new slab on top of that. With exterior footing drains, installing interior ones is probably redundant.
I think with this solution, headroom would become an issue. I have an 8 foot basement, but with HVAC, it drops quite a bit. I'd have several low spots for sure.
I think I have a spring of some sort under the house, and it seems that the water is trapped in there, there are no weep holes in the footings.
If you have the elevation for exterior footing drain that can to daylight and the price affordable at all, I say go for it.
With all the interior systems you will always have to worry will the power go out during this storm and flood the basement? The question is not will the pump fail someday but what day will the pump fail and will I notice before the basement floods.
If you have a sump pump and find it runs frequently, then it’s worth getting a backup. There are both battery backup sump pumps and hydraulic ones that use city water pressure to run. If you’re on a well, only consider the battery backup kind.
Either type of backup pump is independent of the main sump pump, and set to come on a little after the high water level of the main pump. This way if the main pump fails, or can’t pump fast enough to keep up, the backup pump kicks in. These simple systems can save you from a flood and are very much worth having!
Hi Scott -
In this Hammer and Hand best practice (https://hammerandhand.com/best-practices/manual/7-basements/7-4-basement-retrofit/), note that they leave sections of the slab in place and I assume that this is for lateral stability of the basement foundation walls. I am not a structural engineer and it could be that how much you need to stabilize the foundation walls when you remove the entire slab depends on soil type/conditions, but I would definitely take this into consideration.
Will do Thanks Peter.
Looking into rigid foam, there is significant cost to adding this into the project - It appears that I can get some reclaimed, if it becomes available. The real cost is in the removal of the 2" of existing stone. Overall, to add 2" inches of insulation under the slab, it's an additional $4,000 - $5,000 (it's a big basement).
I'm looking at wire mesh vs double the fiber mesh (per Martin's article), making sure that the wire is on stands.
As for drainage - I already have an exterior footing drain that sees daylight, it drains 365 days/year. The plan is to install the interior footing drain, drill through the wall and connect to , or run parallel to, the footing drain pipe to daylight. I will install a sump just in case, but I dont' anticipate needing it. I have a whole-house standby generator and would probably get a water run backup (town water). There are no weep holes in the footings... something that may have prevented my entire issue in the first place.