Victoria Cottage radiant floor using 2x4 sleepers and a suspended slab

[NavyDave]

I am beginning my Victoria cottage using a pier foundation on 8" Quickrete tubes which are anchored into bedrock at a slight angle with 1/2" rebar and portland cement. My question concerns the radiant floor that I plan on incorporating into my build. I plan on laying 2X4 sleepers onto my subfloor at 16" OC, fastening 1/2 PEX tubing into the sleeper "bays" and pouring 1 1/2" of concrete over the PEX tubing until it is flush with the 2X4's then following up with another layer of advantech subfloor which the wall framing would rest upon.

There is one main questions with a few sub-questions here:

1. Is the pier design/beam spacing something that would accomodate the additional concrete weight? According to the concrete calculator I used the following values of concrete would be required:

-The main 16X28 living section - 75 bags of 80lb concrete (6000 LBS).
-The 12X16 bedroom addition (increased length) - 33 bags(2600 LBS).
-The kitchen bump out - 14 bags (1120 LBS).
-The laundry/utility addition - 7 bags (560 LBS).

According to what I'm calculating this will be an additional 14 PSF load spread out on the entire footprint of the structure. (please keep me honest on this calculation)

Subquestions:
1. If the foundation/joist span would not accomodate this additional weight would their be a sizeable difference by adding a middle beam in the main and bedroom sections which would reduce the joist span by 1/2 to less than 8' and 6' respectively?

2. If the additional middle beam would not be required are the 8" quickrete tubes sufficient or should I upsize to 10"?

For reference, the link to what i'm planning with the 2X4 sleepers is on page 21 entitled "SUSPENDED SLAB" of the following link. http://www.radiantcompany.com/manual/Manual_web-2009.pdf

Thank you for any info and discussion provided.

[MountainDon]

I guess I don't understand the choice of building what more or less amounts to half a concrete slab on piers when the frost depth in TN is usually about 18 inches or less, likely only 12 inches in a great portion of the state. A slab on grade would result in a foundation with little worry about what the load on the piers would be. And it would meet any code and have much more resale value.  Probably be easier to insulate under the heated on grade slab as well as being better insulated to prevent heat loss.   Just my way of looking at it. 

[NavyDave]

My property slopes enough so the high corner is approximately 3 feet higher than the low corner. Seems like a lot of concrete would be required for a slab on grade with that much difference? I suppose I could have earth moving equipment come in to level it. Anyone else have this experience with sloping property and a foundation?

[Don_P]

We'll often step a footing, build block walls, backfill to level with #57 washed rock, foam, and then a slab. Or as you said it can be a cut. A friend just did an 18x24 that way with 44" of fall across the 24' dimension

[MountainDon]

We cut and filled our cabin location to avoid having one corner barely above grade and the diagonally opposite corner 4 feet or so in the air.

[John Raabe]

I concur - unless there is a compelling reason to do a redesigned pier foundation with the added weight, it should be much more straightforward to do an insulated slab on grade. You could buy a lot of additional blueboard insulation for the cost of all those piers and beams.

[NavyDave]

now i'm second guessing myself, especially with all of the combined knowledge/experience telling me the same thing.

I've built the beams and poured the footers. I suppose I could work the return of the materials for the remaining part of the floor - quickrete tubes, concrete, joists, insulation, 2 layers of advantech etc. and speak with a local excavator/concrete company.

OR I could forego the radiant floor altogether and install my "back-up" wood stove earlier than planned and continue with the piers. Not pouring the floating slab and returning the extra concrete/advantech would offset most of the cost of the woodstove. I wouldn't have the benefit of the cozy warm floor but the build would progress faster and I believe be a bit cheaper/less complicated. Until I consider that I'll have to harvest firewood for more than just a back-up.

OR I could consider the fact that my beams are built and footers poured a "compelling reason" as John stated to continue with the floating slab. With the discussion i've read I'm tending to lean away from this option at this point though.

This is interesting, I have to ponder this. Thoughts and personal experiences are welcomed.

[John Raabe]

Could the beams be useful as header material?

It's hard to have to backtrack on a planned method of construction, but sometimes it is best for the bigger picture. I think your idea of the center bearing beam may work to deal with most of the extra weight issue, but I would ask you to review the pier foundation locally for your new changes. The pier and beam isn't the best choice for many sites and the Victoria cottage pier foundation is more complex than most. If there are some grade changes that push part of the foundation higher, that further complicates the situation.

As Don_P mentions you can build up concrete walls and backfill with crushed rock for a bearing slab that while it involves machine work is actually a simpler and more stable platform - especially for a heated radiant slab floor.