Rocket Mass Heater Foundation

We will be moving to Kentucky and our 'new' little old farmhouse at the end of this week, and heating the place is on my mind.  I've got the plans for both Walker kitchen stoves and plan to build one of them in the kitchen of the 'new' house, with a heated bench extending into the living room.  I think I will have to cut a hole in the floor and build a foundation for the stove and bench on the ground, shoring up the floor joists around the hole, and supporting them somehow.  The house is on a crawlspace; there's probably only a couple of feet from the ground to the underside of the floor joists.  Has anyone else done this?  Any tips or advice on how best to do it?

Kathleen

I expect you will need a separate foundation for the stove, but you may not need one for the bench. If the floor joists run perpendicular to the bench, and are not so flimsy that you are concerned about the existing floor being bouncy, and the bench location is near an outside wall or main support beam, they should be able to support the added weight of a cob bench. At most, you might need to add a beam under the joists in line with the bench and support it with a few piers.

If you need to cut out joists for the stove foundation, I would double up the remaining joists on each side of the new opening, and add a double header joist to carry the load from the cut-off ones to the doubled ones. Cut the joists back enough to allow you to add the headers without choking the opening.

The amount of new foundation you need to add depends on the character of the soil under it. What do you have? Sandy, loamy, clay/hardpan, rocky...? If clay, what kind? There are expansive clays that swell a lot when wet, and those are tricky. There are other clays that are almost as good as bedrock for support.

Does the crawlspace freeze in winter? How deep is the standard frostline in your new area? What is your microclimate? (Warmer or cooler than average, drier or wetter?)

Thanks, Glenn, that sounds like good advice, and about what I was thinking as far as doubling joists and so on.  The house is located in south-central Kentucky, the soil is good silt-loam but probably has a little clay in it.  The location of the stove will be central in the house, next to the existing brick chimney, which I plan to use after it's been checked out and, if necessary, repaired.  (The house was built in the 1920's, I think; the chimney hasn't been used in a long time).  The ground under the interior part of the house stays quite dry for the most part, and in any case I doubt that foundations there need to be more than eighteen inches or two feet into the ground.  I've got to run or we'll be late for an appointment, but will check back here later.

Kathleen

If you have a chimney, it's got to be on its own foundation already, right?

In that event, I would be very careful to make sure the existing chimney foundation is properly underpinned before you dig anywhere around it. Were you to cause it to fall, well I don't know how large a chimney it is, but it's not inconceivable that it could do a fair bit of damage were it to collapse on your house.

For an idea as to what you need, I would look at the chimney foundation as you excavate. I might be wrong, but I would guess that you want your new foundation to be built at least as heavy as the first one.

I also don't think you want to attach the new foundation to the old. I think you'd want to allow for it to settle separately, on it's own, to hopefully minimise the effect on the position or orientation of the old one.

As an aside, the last bench I built used some of those reclaimed parking lot bumpers with the rebar as structural material to hold the interior fill together. It occurs to me that if these formed the base of a bench, instead of a full foundation, footings could be poured to support the ends of the bumpers.

Let us know how it goes, and good luck.

-CK