Expansive soil conditions and wood basement flooring advise needed

My son is looking to purchase a home in Castle Rock, Colorado.
A 1993 home with basement. Expansive soil conditions are present. Existing basement floor is a floating wood floor. What are the pros and cons of this vs a slab construction. Now interject radon over *4 pCi/L. *Accepting advise suggestions and comments?

It would seem that the floating floor would have to be removed to see cracks and where the radon entry potential is. If it can be mitigated then the floor could be replaced.


Larry, It is a wood floating floor over soil, think floor joists with plywood on top. Several inches to 8 or 9 above grade for the main portion of the basement. And has a crawl space for another portion fo the split level also with an exposed soil condition with poorly laid visqueen for barrier.

Never heard of that construction technique. Sounds like Mexico to me.

Oh, I was thinking concrete slab with floating floor on top.

Your description sounds like a mess to deal with even without the radon issue.

I don’t know how you would seal that floor from radon entry.


Problem #1: It appears Castle Rock, Colorado is within the normal range for termites so a wood floor on the ground is a bad idea.

Problem #2: Castle Rock, Colorado location is rated high on the state radon map, which as you stated compounds the problem.

Problem #3: Concrete basement slabs help resist the inward pressure exerted at the bottom of basement walls. Depending on the footing arrangement this could be an issue.

My first thought would be remove the wooden floor, place 4" of open graded aggregate covered with 6 mil plastic and a new concrete basement floor. The open graded aggregate will allow you to install a radon extraction system under the slab.

My second thought is if you don’t know whats under the wooden floor you may want to keep looking for another house.

This house has been in place for 19 years. If it is experiencing no structural problems and conditions are not changing on the lot/site, there seems to be no major problems except for the high radon.

Assuming the basement subfloor is panel style (OSB chipboard or plywood), the radon can be mitigated by sealing all openings in the panels and at their outer edges. The partial crawlspace will allow access to the partition wall at the change in grade for airsealing at this location.

If airsealing does not reduce radon levels enough, this open space wood joist system will allow for good, easy under subfloor depressurization to remove emitted radon.

The existing floor removal and new concrete slab is an expensive undertaking. As I mentioned in another post: if for 19 years there have been no other major problems and none are developing, take care of the radon (which you probably will have to do in most other houses located in this radon “hot spot” area).

This might help to give you some ideas.

An HI,

Concrete basement slabs do provide lateral support to the bottom of basement walls with an unbalanced fill, if they are poured up against the wall. IRC incorporates the lateral resistance in their basement wall tables when calculating the unbalanced soil height. The IRC also requires engineering design of any basement wall supporting an unbalanced fill height greater than 48 inches when the wall has no permanent lateral support at the top or bottom (R404.1.3). Without a basement slab the resistance to the inward forces at the bottom directly depends on how high the soil is on the footing inside the basement. My comment was qualified depending on the footing configuration. The typical factor of safety should be 2.5 which I would have to verify regardless of how old the house was. A basement without a slab is very uncommon in Missouri, no sure about Colorado, so that alone warrants a little extra caution.

I live in an area where frost depths are 3-4 feet and used to live in a colder area where footings had to be 5 feet or more below grade. With these conditions, there is a lot more potential for inward pressures that are much higher than houses built in warmer climates but no special requirements are required- just use good building practice and key the concrete wall properly into the footing.

I have seen larger slabs in full 8’ basements that were not cut for shrinkage purposes (looks) where due to shrinkage the slab was 3/16-1/4 from the basement wall- no inward pressure there.

Another house I inspected just up the road from me had 8’ walls on footings, no slab -just crushed stone and was unheated for years (upper floor was heated). No problems were seen.

Many PWF’s have their basement walls sitting/nailed into 2"X wood footing plates sitting in/level with the crushed stone required. Many of these basements have only wood joist floor systems with no problems .

Last summer, I was called to consult on a 28-29 year old, 5,600 sq ft house with a PWF. This house was built by an engineer on an estate sized lot on a lake in a high end neighbourhood but it’s price was dropped drastically due to the PWF and length of time on the market. I looked at it and found it to be in great condition. In the end my client, a doctor, decided not to buy it. I would’ve, had I the cash! The wooden floor in this basement was not a problem.

I think we overthink many residential situations and imagine* the worst. That influences judgement and reports get written to cover our azzes… and a house that probably should have sold didn’t. I have seen this many times in my career.

*Mike Holmes said in one of the better (for laughs) programs that “an HI should be able to imagine what’s in the walls”!

An HI,

As a professional engineer I am bound by law to give my clients the best engineering advice I can, which doesn’t come from Wikipedia.

Then you should go to Wiki and correct/ change the information you consider incorrect/improper as a large population is exposed to it.

Do you consider the statement absolutely wrong or does it have a “mostly true” ring to it?

Kevin and Randy,

Great info and advice, Thanks for all you input. Learning something new every day.

Thanks again
Take care



Thanks for the link and info…

thanks again

Look at the comments on the diagram about the “key joint”. Note there is a few poor English terms as this was translated from French.

Here’s one from Oak Ridge National Laboratories: (Have a look at diagram 2.2 -the keyway)

YET! Or that any proficient HI has observed and reported.:wink:

Are you “imagining” what is going to happen in the future? (Remember Mike Holmes removes asbestos without air testing…if you have it in the house, get it out even if it is not an air quality issue!)

When is this failure going to occur? Next alignment of the major planets and moon?

Who said there have been no major problems?
I have not looked into the future.
Please read the post.