Floor support, Step 2... I will have a RMH up and running before winter 2023.

So being the freak-out expert that I am, I have come to the conclusion after calculating the massive amounts of natural resources we wasted in our new home this last winter that a RMH is the only logical option.

I have the perfect room in the house with a great location and am ready to begin my RMH Journey.

I will be purchasing most any educational resources pertaining to RMH through Permies.com over the next few weeks and beginning my research.

I know about the Better Wood Heat DVD's, are there other specific books/plans/info I should be researching before beginning?

and

Being that our home has a 3' crawl-space underneath it, I am currently planning on getting something like these floor jacks (https://www.amazon.com/gp/product/B07M7XR11J/ref=ox_sc_saved_title_5?smid=A1UMBRA5ZTBCX8&psc=1) to support under the floor where the RMH will be built, is this a good plan or is there a better rout for supporting the floors that I should be considering?

Also, if I do install said jacks, would 4 of them likely spread-load the weight well?

and, for the part that presses against the floor from underneath, how should I reinforce that area so there are no weak-spots? Something intense like 4"x12" cedar? Can I hand plain my own cedar from my property and trust it?

I realize these are a load of questions and it's a bit ambiguous so early on into a project I plan to take 7-8 months to complete, I just need to keep learning before I start to get comfortable with the task I am about to undertake. If anybody is near expert-level and would be willing to be flown or have their travels covered to come direct the show (of course for time/labor paid for be us) in South Carolina come September timeframe please let me know.

Cheers Friends

The amount of added support depends on what your existing framing is like, how it is laid out relative to your proposed RMH configuration, and what style of mass you plan.

If you do a piped mass or bell bench with a long mass running crosswise to floor joists, you might not need any extra support. If you plan a tall bell, you will definitely need extra support for the concentrated load. A tall bell is best handled by cutting out floor framing (and reframing as required around the hole) so that you can run masonry piers up from a footer in the crawl space. This would be required by building code if you are subject to that.

Ernie and Erica Wisner's _Rocket Mass Heater Builder's Guide_ has the most comprehensive information including a variety of detailed plans for standard J-tube RMHs.

Glenn Herbert wrote:The amount of added support depends on what your existing framing is like, how it is laid out relative to your proposed RMH configuration, and what style of mass you plan.

If you do a piped mass or bell bench with a long mass running crosswise to floor joists, you might not need any extra support. If you plan a tall bell, you will definitely need extra support for the concentrated load. A tall bell is best handled by cutting out floor framing (and reframing as required around the hole) so that you can run masonry piers up from a footer in the crawl space. This would be required by building code if you are subject to that.

Ernie and Erica Wisner's _Rocket Mass Heater Builder's Guide_ has the most comprehensive information including a variety of detailed plans for standard J-tube RMHs.

Awesome, thank you. It’s in an addition built-on to an existing manufactured home. So imagine one or two rung(s)-up from the worse-case scenario… I’ll take pictures or make a short video showing all the “before” shots for posterity.

I want to do a huge cob style bench, extra long, so that it will fit three kiddo’s and a wife on it comfortably reading on a cold day in December.

I think you’re right about cutting out the floor & will start contemplating and researching the idea.

I’ll go ahead and pick-up the RMH Builders Guide, thank you for the resource.


Here is a picture of the closest I imagine ours will turn out:

The standard approach for adding support to a joist floor would be to create one or 2 beams at 90 degrees to the floor joists to split their total span or to support a point load. If the point load run in the direction of the floor joist sometimes the floor joist are doubled up.  A cement footing would be poured at either end of the beam or every 10 ft and your teleposts go over each footing. Single story support footings are usually 24x24" by 10 inches thick minimum. Single story beams are usually 3 layers of 2x8 lumber as a minimum with 3 nails every 16 inches from both sides...
Cheers,  David

For a long bench style mass, I would suggest a bell cavity in the bench, with little mass below the cavity. This will give you the best heat transfer and storage with minimal mass, and make supporting it easier. If the bench runs crosswise to the floor joists and along one edge of the span, you can probably support it fine by beefing up the beam or adding another parallel to the existing one under the "room facing" edge of the bench, as David describes.

The only thing not already covered in the support of joists/beams/footings department is that those jacks aren't meant for permanent use. At least it says so in the product description. There's likely 50% installed that never get replaced by something permanent...
Ideally, footings and piers of concrete blocks would be used (or the steel/concrete columns). As stated before, a lot depends on local codes, or what your inspector wants to see.

For a 3' high crawl space, I expect it would be not just better and more fitting code, but actually cheaper, to put a couple of concrete block piers under a new beam rather than jack posts. You might want one short jack post to raise the beam a hair to shim under it for each pier, but that is all. If you had a wood beam, I would just use a pair of opposed wedges on each pier to tighten up the support.

Glenn Herbert wrote:For a long bench style mass, I would suggest a bell cavity in the bench, with little mass below the cavity. This will give you the best heat transfer and storage with minimal mass, and make supporting it easier. If the bench runs crosswise to the floor joists and along one edge of the span, you can probably support it fine by beefing up the beam or adding another parallel to the existing one under the "room facing" edge of the bench, as David describes.

OOOOOOOOooooooohhhhh.... would it be crazy to cut out the section of the floor and build it from all 3 ft under the floor and extend it to create a heat transfer that runs the length of the floor?... I know it sounds like a lot but I'm pretty ambitious and really love being in love with outcomes...

You will probably be better off  to do cinder block posts as mentioned above. The link above looks a little lacking I'm more used to ones like these for permanent installation:   https://www.homedepot.com/p/Tiger-Brand-Super-S-Series-36-in-Jack-Post-J-S-36/100041510

Hi Chris,

Your dream picture showed one of the Wisners J tube builds but there is also batch boxes to consider as well.
A great resource to learn about them while you wait for your book is here: batchrocket.eu

Matt Walker has a lot of great info on his youtube channel as well: broaudio
and plans on his website: walkerstoves.com

and finally, our local Permies rocket scientist Thomas Rubino has a store selling RMH parts at: dragontech.com

If you had a basement or someplace that could benefit from being warmed, an underfloor/floor level system might make sense. In your case, you would be giving much of the heat to the crawl space, and I don't think that would work out well for you.

Hi Chris!

All things related to RMH can be found on the WIKI here: Rocket Mass Heater Resources

Lots of good resources there. I would definitely suggest Rocket Mass Heater Builder's Guide. by Ernie and Erica Wisner, if you haven't already got it.

Nancy Reading wrote:Hi Chris!

All things related to RMH can be found on the WIKI here: Rocket Mass Heater Resources

Lots of good resources there. I would definitely suggest Rocket Mass Heater Builder's Guide. by Ernie and Erica Wisner, if you haven't already got it.

Sorry to everyone I haven't directly responded to, all responses have been very helpful in the beginning of this journey.

Super helpful information, I really appreciate it... we went ahead and ordered the 8 disc RMH/RStove dvd's (for now)... I figure once I watch all of them 2 or 3 times I'll have a better idea as to if we should get the bundle from Ernie and Erica-- I will say they must be having a wild ride, I can't imagine my life's work being so interesting! Someday... maybe when I grow up

P.S. -- whats really fun about getting different Permie DVD's is my 2 yr/old daughters reactions and questions; she'll likely be more knowledgable about Permaculture by the time she's 12 than I'll ever be.

I do so so much appreciate all the input. luckily, a neighbor (3 doors down / 1/2 a mile down) is doing a work-trade with me & will be helping me install proper sleepers under my house to support the RMH. I’ll track the whole project when I start in 2 weeks.

Anyone have experience with hybrid RMH’s?… if I put mostly cob in the front for heat insulation and pebble on the backside to keep it slightly lighter will I be happy?… 😁

Chris Vee wrote:I do so so much appreciate all the input. luckily, a neighbor (3 doors down / 1/2 a mile down) is doing a work-trade with me & will be helping me install proper sleepers under my house to support the RMH. I’ll track the whole project when I start in 2 weeks.

Anyone have experience with hybrid RMH’s?… if I put mostly cob in the front for heat insulation and pebble on the backside to keep it slightly lighter will I be happy?… 😁

Can you do air paths underneath so the air can flow underneath and then up the back through the pebbles?

Jeff Bosch wrote:

Chris Vee wrote:I do so so much appreciate all the input. luckily, a neighbor (3 doors down / 1/2 a mile down) is doing a work-trade with me & will be helping me install proper sleepers under my house to support the RMH. I’ll track the whole project when I start in 2 weeks.

Anyone have experience with hybrid RMH’s?… if I put mostly cob in the front for heat insulation and pebble on the backside to keep it slightly lighter will I be happy?… 😁

Can you do air paths underneath so the air can flow underneath and then up the back through the pebbles?

I absolutely can; and that’s been on my mind— I’m on the 6th dvd / 2nd dvd of the RMH special where that seemed to fail, so I’m still learning about it— & I’m about order the RMH book from Earnie & Erica… you think that might be a good idea for this build?… this is a build in an addition on a modular home so I’m open to free-styling & I plan to be a pro in the next 10 years… thanks Jeff.

One of the things needed for the pebbles system to work is for there to be air flow through the pebbles to heat the room.

Adding air vents/passageway underneath the mass also aids in making sure the heat doesn't travel down through the mass and get the floor too hot.

Jeff Bosch wrote:One of the things needed for the pebbles system to work is for there to be air flow through the pebbles to heat the room.

Adding air vents/passageway underneath the mass also aids in making sure the heat doesn't travel down through the mass and get the floor too hot.

After a ton more research and pondering I am shying away from air vents and pebbles... What if I use about 3 times the necessary pearlite to keep it a bit lighter and to give extra insulation against "combustable" joining walls?

Also... I don't really want to extend the barrel into the room (to create 36" clearance from the wall), what if I heat-shield the wall AND possibly (not sure how yet, but) if I line the inside of the barrel on the wall-side with tightly formed brick like Peter van den Berg did in his "Minnie Mouse"?

(It's standard drywall) with the other adjoining wall being stone..

Thank you Jeff, it's nice having someone to bounce my ideas off of

There are lots of options but are you basing you plans on a J tube?
Have you considered a batch style stove and a hollow bench with a chimney at the far end of the bench?
I love my J tube for cooking and radiant space heating but if I was heating a home lounge  then a batch style seems far more effective.

Glenn Herbert wrote:For a long bench style mass, I would suggest a bell cavity in the bench, with little mass below the cavity. This will give you the best heat transfer and storage with minimal mass, and make supporting it easier. If the bench runs crosswise to the floor joists and along one edge of the span, you can probably support it fine by beefing up the beam or adding another parallel to the existing one under the "room facing" edge of the bench, as David describes.

Glenn, can you expound on how to use this bell cavity concept? I really like the idea and think you’re onto a valuable concept for this build. What do I put in this cavity? Are there any examples you can point me toward?

Thank you!

Fox James wrote:There are lots of options but are you basing you plans on a J tube?
Have you considered a batch style stove and a hollow bench with a chimney at the far end of the bench?
I love my J tube for cooking and radiant space heating but if I was heating a home longe then a batch style seems far more effective.

Yes, I am basing my plans on a j-tube

What exactly defines a RMH as a batch style, or is a batch style not considered a rmh?

Thank you!

A batch box design is a style of rocket stove that burns a batch (a set quantity of wood)  that is loaded in one go.
A batch design will generally have a glass front door more akin to a conventional wood burner and the batch of wood will last an hour or more before it needs re loading
Batch designs are normally more powerful than a J tube simply because they burn a lot more wood at any one given time.
A J tube needs feeding every 15-20 minutes with smaller pieces of wood and the fire box  is much smaller.
However both designs work very well and a J tube is simple to build and operate.
There are hundreds or even thousand of post on this forum showing all designs in detail and equally there are many build guides on Youtube.
Search for Batch box.

Fox James wrote:A batch box design is a style of rocket stove that burns a batch (a set quantity of wood)  that is loaded in one go.
A batch design will generally have a glass front door more akin to a conventional wood burner and the batch of wood will last an hour or more before it needs re loading
Batch designs are normally more powerful than a J tube simply because they burn a lot more wood at any one given time.
A J tube needs feeding every 15-20 minutes with smaller pieces of wood and the fire box  is much smaller.
However both designs work very well and a J tube is simple to build and operate.
There are hundreds or even thousand of post on this forum showing all designs in detail and equally there are many build guides on Youtube.
Search for Batch box.

I’m pretty set on RMH w/ the J-tube; when you say “hollow bench” how in the world would that work with cob?… do you have any resources I could look at with that concept? Would I still need the pass through vents underneath?… I’m considering doing 3 vent underneath the core with brick….

Cheers, ya’ll are awesome & have been a huge help!

One of the simpler ways to build a bell bench is to cut a barrel in half lengthwise, then cover it with cob. Matt Walker came up with that idea and it works well for him. I don't know if this video shows the bench but it does show the way that he converted a j tube into a batch box.

Jeremy VanGelder wrote:One of the simpler ways to build a bell bench is to cut a barrel in half lengthwise, then cover it with cob. Matt Walker came up with that idea and it works well for him. I don't know if this video shows the bench but it does show the way that he converted a j tube into a batch box.

outstanding build & concept, I’ll likely end up doing similar down the road. For now I just want to essentially practice & learn with the RMH style.

I think the half barrel concept is great and I will look further into it before I do my build (t-minus 10 days or so before I start construction.)

Thank you for sharing.

Welp, it’s go time for me. Here’s how we supported the floor.


aaaaaand.. I finally got time to do some editing and uploading.

All feedback welcome!

If anyone could please check out the video and let me know if you think that's sufficient to hold up a 5,000 pound RMH...

Thank y'all!

Hi Chris;
I watched your video.
I think you did a fine job of adding support.
Without knowing the placement of the mass and the exact location of your new supports, I will not say for sure.
But what you added looks properly done and very stout!

For a 5000 pound mass spread over many feet, your support is way more than enough. If each of the piers takes about the same amount of it, that would be 1250ish pounds each, spread over four square feet (500+ square inches) of footing, gives less than three pounds per square inch from the RMH. The beams as you show are similarly lightly loaded. This should last about forever