Detail for Wood-Framed Floor over Rock(?)

[soiset]

I'm working on an innovative basement design for existing homes, with an emphasis on speed and economy of construction, in addition to long-term performance. The basement floor will bear on competent grey limestone, and will support interior columns, but will not support the perimeter walls. Support for the outer building walls will be outside of the basement and vapor barrier.

I'm trying to design a wood-framed floor over the competent rock, that will provide a strong, level floor, with solid anchoring for the columns and interior load-bearing walls. The system must also provide drainage and a vapor barrier.

The traditional approach would be to put down 3" of drain rock, a vapor barrier on top of that, then a 4" concrete slab over that. At the columns, concrete pedestals would be poured directly on the rock.

With rock below, the load-distributing effect of a rc slab is redundant, so I'm looking for a way to eliminate it entirely, with a system that is cheaper and faster, while still being highly reliable.

Ideas?

 

[msquared48]

So, other than the external bearing walls, how is this different than a normal wood framed crawl space?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[MiketheEngineer]

Agreed - just pour the footings/pedestal all at once - sized accordingly and set your beams/joists on that. Done all the time.

 

[soiset]

Fair question. I guess the main difference is in the overall depth of the system. A regular crawl space/pier and beam system has the depth of the piers above ground, plus the depth of the girders, plus the joists, which usually means an overall depth of almost 36". Also, drainage is less of a concern at or near grade than it is at 8' below grade.

I want a system with a total depth of maybe 5". I imagine maybe that if I wanted to build it with 2x4 joists, and no girders, that I could use many micro-piers, over which I'd drape the vapor barrier, and set the joists on top of that. Except at the columns, there wouldn't need to be any provision for lateral forces, as the floor is not connected to the perimeter.

I'd guess that the water would flow between the rock and the vapor barrier, especially if the barrier had many high points over whatever the micro-piers are.

I'd want to avoid the need for shimming at each pier. They could be cast to an exact height, or I could use an adjustable system, perhaps a threaded rod anchored in the rock.

 

[msquared48]

If you use wood though, you need a minimum of 18" clear to the soil/rock for the joists and 12" for the beams (which you are trying to eliminate).

I do not think 5" is reasonable here. Plus you will have venting issues. You will not be able to use forced air heating in the crawl, only other ways of heating will work.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[soiset]

I definitely do not want anything in the "crawl space," so hvac is not a concern. Supply and return air will be handled elsewhere. Those clearance issues should be addressed by, one, using treated wood appropriately, and two, the continuous vapor barrier. The IRC requires those distances only for "exposed" ground.

In the attached, I've shown a concrete "footing" that is 3.5" high, and 6" across. They are drawn at 24" oc, and the vb is draped over them. They could be continuous, or 6" squares. Along those I've shown a treated 2x4 plate, and then 1" plywood (untreated). Hardwood would go on the plywood.

This is just a concept, but it shows what I am aiming for. Obviously, all those tiny footings could take a long time to form.

 

[msquared48]

What about the issue of insulation under the floor?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[boo1]

will be hard to have a level floor system

 

[csd72]

msquared48,

I assume that it would be insulated like a raised floor or a stud wall.

soiset,

have you considered the additional cost of detailing the external walls to be self supporting. Also it sounds like this system could be more labour intensive.

 

[soiset]

msquared48, In Dallas, you wouldn't want to insulate the basement floor, because at that depth, you have a 70 degree earth temperature, which would aid in both summer cooling and (mild) winter warming.

csd72, for putting a basement below an existing pier and beam home, with the right design (which I can't reveal right now), it is a lot easier not to build it with the same design as a traditional basement, where a slab retains the bottom of the wall. Just trust me on that for now.

boo1, what I am looking for is a way to avoid a concrete slab that will, relatively easily, cheaply, and quickly, give me a level floor. I'm not sure that way exists, but I'm looking for it.

I appreciate the responses.

 

[csd72]

Have you considered prefabricated floor panels that can be lifted into place already assembled. It is common over here in the UK to do it this way.

 

[soiset]

csd72, do you have a link for that kind of product? I'm not familiar with it.

 

[MiketheEngineer]

Do a search on SIPs - structurally insulated panels. Couple of things to remember -

Something ALWAYS goes wrong under the floor - wiring, piping, etc. At only 5'' I would also worry about venting, moisture control, rats, snakes, etc.