earthen floor over wooden sub-floor

moved into a new old house a month or so ago. we've started tearing out a terrible linoleum floor, and the really old weird asbestos tiles underneath it. under all that, there's a really solid ship lap sub-floor an inch or more thick.

an earthen floor really appeals to us, so we've decided to go that route for this room. most of the information I've seen regarding earthen floors describes building them on slabs or the ground, not on suspended wooden floors. so what extra steps will be necessary?

should I lay plastic down over the wood to protect from moisture while the floor dries? or maybe it won't be an issue because there will only be moisture for a relatively short period of time.

should I use the several inches of road base under the final clay/sand/straw top coat, or will the wood sub-floor serve the same purpose that the road base is designed to accomplish?

what else am I not thinking of?

My main concern would be weight.........

it's an old house. pretty stout lumber. I believe the joists are full dimension 2 x 8, and there's accessible space underneath for extra bracing if it becomes necessary. so weight might be an issue, but I'm pretty sure it could be dealt with relatively easily.

any other concerns? I would rather know about potential problems now instead of after the fact.

I would second the concern with weight. While older solid lumber is certainly stronger then many of the younger lumber of today, I would assume it was likely under built structurally, at least by today's standards. I wouldn't span more then 7 feet with 2x8's on 16" centers, maybe 8 or 9 feet with 12" centers. I would put a girder under the joists centered in the room you are going to put the earthen floor in. The stronger your substrate, the thinner you can make the earthen floor, the lighter it will weigh, etc.

Another recommendation would be to run a layer between the subfloor and the earthen floor. Felt paper works good as well as any of the synthetic roofing underlayments. If you wanted a more natural product then red rosin paper is a good choice. The idea is not only about the waterproofing, which with a solid subfloor would be more about rusting the fasteners prematurely, but to actually seperate the wood from the earth. This "bond breaker" allows the wood and the clay to expand and contract at their own rates, without cracking.

red rosin paper: check.

so, you've both convinced me to reinforce the floor. shouldn't be too difficult. the house was built in stages, though, and the room in question spans two different eras and two different methods of construction. looking at it from underneath just now, it's a bit of a tangle of joists. it was built to support a cast iron tub, though, which seems like a good start to me.

which brings up another question: any special considerations for a bathroom apart from the plumbing?

also, there are a couple of layers of floor in there. directly over the joists is the ship lap that I mentioned previously, and on top of that is what looks like 3/4" plywood. should I put the new floor over the plywood, remove the plywood and put it over the ship lap, or remove the plywood and add something else over the ship lap?

while your interest in earthen flooring, over an existing wood sub-floor is interesting, all of your concerns are moot until you have addressed the engineering for reinforcing the floor. You should have someone who knows about that to look at it if you cannot with an objective eye. The engineering is paramount as you could find yourself in a pickle pretty quick with too much weight. One thing to consider is the thickness of the earth floor as it pertains to stability against cracking. This will translate into weight and proper engineering. It could all go horribly wrong if this is not addressed primarily. All your other questions are contingent on the cost of reinforcing your floor. please get that done first. A cast iron tubs weight (300 lbs) is not comparable to tonnage of earth you need for an earthen floor.

dan simon wrote:A cast iron tubs weight (300 lbs) is not comparable to tonnage of earth you need for an earthen floor.

a cast iron tub full of water certainly weighs considerably more, but your point is taken. have I mentioned that I'm going to be sure to reinforce the floor adequately?

i lived in a house with earthen floor on top of wood sub-floor. The floor was applied right over the plywood. the home was newer so had adequate floor joists. however the thickness was only an inch or so and spread out over the joists was not much of a weight concern. The big concern for you should be major cracking. Do focus on separating the floor from the wood as best you can, like rosin paper. The floor we lived on had cracks all over the place, especially in higher traffic areas. Making the subfloor as rigid as possible should be your goal too.

An old standard for isolating tile or bluestone floors from a substrate is a sand bed, maybe a very thin layer would be appropriate in addition to rosin paper. If you deal with the weight issues you will decrease the stress cracking, but, just as with tile, spanning a ceramic floor over an area that was built in stages can also be tricky. Different framing, different foundations under each area, often make trouble. I have not as of yet had an issue with tile laid in these conditions, but I often use kevlar mud or double modified mud as well as a lot of fiberglass mesh when clients will not let me put in an appropriate expansion joint.

An odd thought for a thin-skin mud floor would be to use glass or poly fibers (concrete reinforcing) into the mud in place of the straw that you cannot put into a floor system. I am currently building a floor system for a small loft over the first room of my cottage. As I intend to use a thick earthen floor, the structural elements will be around six times as heavy as standard constuction - really heavy beams, big purlins, then pallets as decking, then mud. The interior room dimensions are not that large either.

Matt River wrote:
An odd thought for a thin-skin mud floor would be to use glass or poly fibers (concrete reinforcing) into the mud in place of the straw that you cannot put into a floor system.

why couldn't straw be used in a floor system?

As regards the finish coat for a floor it does not seem like straw would not work very well if a smooth surface is desired. Also makes it a bit trickier to get a "pour" or leveling effect if the straw is long-stemmed. Manure is referenced by some cob manuals for both strength and stick, this would contain smaller fibers to allow a floor finish. My own floor systems have straw in them but I am building on a very well-drained site over large amounts of gravel.

straw is typically used in a cob floor and leave it long. leave the floor 1/2 to 1 inch lower than you want it finished and then use sand and clay only to finish the top coat of cob flooring. this way you have no straw poking out of the floor.

A rigid floor will help, as the major problem will be differential movement between the subfloor and the earthen layer.
I'd advise you skip the bottom layers and just go with a 1 inch top coat.
Make a VERY sandy, slightly dry mix with horse (or better yet) cow manure in for fiber. I've never seen a floor mix that was wet enough to "pour" work well.. They have all cracked badly and the self leveling theory just doesn't work. The best floors (IMHO) are packed down hard using dry(ish) and VERY sandy finish coats over a tamped road-base bottom coat.
Lay jute over the sub-floor and cover it with clay slip (just paint it on) then trowel down the floor coat. The jute will help the earth to float on top of the subfloor, keep it together and avoid cracking. A little sand under the jute probably wouldn't hurt, but don't use plastic.
Trowel it down HARD, maybe use screed boards to insure flat, even coverage.
When it's layed and dry, you can make a fine slip/sand alis (essentially a thin plaster) and sponge it over the base layer, which will fill odd holes and make the finish quite smooth.

Then oil it using your favorite routine..

I know of one case where this method has been used.. So far, it's good.. My feeling is that it won't be as resilient and wear resistant as normal earthen floors. It's likely to develop cracks over time, though they don't HAVE to look bad.

Kirk Mobert wrote:
Lay jute over the sub-floor and cover it with clay slip (just paint it on) then trowel down the floor coat. The jute will help the earth to float on top of the subfloor, keep it together and avoid cracking. A little sand under the jute probably wouldn't hurt, but don't use plastic.

would old coffee sacks work for the jute?

why do you expect the example you've seen to crack? because the subfloor isn't rigid enough? not thick enough?

and do you see any complications with this going in a bathroom? my best gal is a little worried about moisture being a problem for an earth floor. not having much experience with these floors myself, it's my impression that wax/oil sealing should handle moisture just fine.

tel jetson wrote:

Kirk Mobert wrote:
Lay jute over the sub-floor and cover it with clay slip (just paint it on) then trowel down the floor coat. The jute will help the earth to float on top of the subfloor, keep it together and avoid cracking. A little sand under the jute probably wouldn't hurt, but don't use plastic.

would old coffee sacks work for the jute?

Maybe.. The jute is there to help keep the earth in one piece, floating over the subfloor.Probably best to overlap the sacks a little, maybe even sew 'em together?

why do you expect the example you've seen to crack? because the subfloor isn't rigid enough? not thick enough?

Plywood subfloors are flexible and move quite a bit when you walk on them. Normally, earthen floors are layed on a solid base, like packed road topper with a crushed drain rock layer under that. The layers (on the usual earthen floor) don't move around when you walk on 'em.
It's my feeling that the earthen topper may end up a little too floating and break up over time.. 'Course, the only way to know for sure is try it and see.

and do you see any complications with this going in a bathroom? my best gal is a little worried about moisture being a problem for an earth floor. not having much experience with these floors myself, it's my impression that wax/oil sealing should handle moisture just fine.

Normally, I'd say a well oiled and waxed earth floor will do fine in a bathroom. In this case, I'm not so sure. The problem will show up if water gets down under the finish layer. Oil will only soak in (to an earthen topper coat) about a half of an inch, no matter HOW much you put on, no matter HOW well you thin it.. Under that oiled layer is dry dirt, sand and horse (or cow) crap... If the underside gets wet, it'll turn back into mush pretty quickly.
If your floor cracks and water gets in, or if water weeps down an edge against a wall and gets in, it can make short work of the floor.

I'd try it in a low risk, high traffic area first just to try it out. Put it someplace you can shovel off easily if it fails, try it for a time and see what happens. Could be it would be fine.. Maybe not.

thanks, Donkey.

how about a wacky, and probably entirely ineffective idea? I'm thinking about the moisture issue some more, which it sounds like would be the main problem (cracks wouldn't thrill me, but if they didn't lead to moisture destroying the floor, I could live with them and I would guess they could be made to look nice). what if the entire the floor was permeated with wax?

I keep bees and know a few other folks who do, too, so I have access to a fair amount of beeswax. I'm reasonably confident that I could heat the floor to above the melting point (around 150° F), at which point I would expect the wax to wick throughout the mass of the floor. then follow that with the drying oil treatment for the surface.

I'm trying to salvage this plan for the bathroom, which it sounds like may be far from the ideal spot to experiment with something that's new to me. so what do you all think? doomed to failure? terrible waste of good beeswax? brilliant?

Normally the wax is put on after the oil..
Might work, might not.

I'd do a test on a piece of scrap ply.. Make it around 3 feet square, do the whole shebang, jute, top layer, wax, oil, etc..
When it's all done, see how much of a beating it can take, dump water on it (and more importantly) under it, stomp on it wet and dry.
If it can hold up to an unusual amount of abuse, it'll do fine with the day to day grind.

My feeling?? Thinned linseed oil will only soak in to a half an inch, I doubt wax will do better.. Worth a try.

I'm really happy this thread exists! I value all the advice offered here. I'm planning a similar construction project for my Portland, Oregon home, built in 1982. I just moved in, and there's almost-new carpet, but carpet and my allergies don't mix, so I plan to remove the carpet and lay an earthen floor.

As advised on this thread, I'm probably going to reinforce the floor joists before laying my cob floor.

I have a question about something called magnesium binder. I was sold on this stuff by an earthen builder acquaintance.
The binder is advertised to be 6 times stronger than Portland cement, and that it should cover 100 sq ft at 1:1 concentration at 3/8 thickness. He said to mix in the binder at a ratio of 1:7, binder:cob, maybe even less binder.

After buying this binder, though, it started to concern me. One concern is that the mixture has some fiberglass in it. Of course, that makes the mixing process more of a hassle because of the need for respiratory protection. Furthermore, the binder supposedly makes the mixture set up quicker (only 15 minutes, supposedly.) So, I'd have to mix the cob first and then add the binder at the last minute before application. Having never set an earthen floor before, I'm not sure whether having to work this quickly would be an added hassle, or if it would actually be nice to have the thing set up quicker. I'm imagining dealing with the stuff would be an unnecessary discomfort, and it might take away from the joy of the process. After all, people lay earthen floors without it, right? However, it might be worth it if it were to offer some significant advantage. Anyone have experience with magnesium binder?

The same acquaintance gave me some advice with regard to the layers which I'd like to run by you all.
He recommended:
Layers (from bottom to top)
1. chicken wire on subfloor
2. cob mix with magnesium binder for stabilization
3/4 in thick
3. top layer with a higher proportion of stabilizer. 1/2 in thick
4. 4 layers of linseed oil, dilluted with oderless mineral spirits
Each layer has a different amount of the oil:spirits ratio.

Thanks so much for you feedback about my little plan here.

@tel jetson,

I've been in that situation before & if you must do this, then I would STRONGLY suggest removing the floor & the joist ( reuse them somewhere ) , then creating a proper boundary for your earth or stone infill against the foundation of the structure. Moisture proofing & insulating additions are always a must have too, then bringing the finished surface materials, up to the proper level, to finish an earthen floor with.

It's hard to do, but worth the extra efforts (as wood rots over time, even if you moisture proof the wood it will still weaken and give way) as your combination wood earth floor system will fail over time. Your name will then not be spoken, as the builder with a vision to add an earthen floor to an existing structure , but as something else.

I wish you only the best floor possible & Happy Building !

Joe Woodall, Ecoitect
Georgia Adobe Ecoitecture
http://www.georgiaadobe.com

there's a six-foot deep basement underneath, so infilling isn't really an option. scrapped the earthen bathroom floor plan for now, anyhow. I'm till very much enamored with the idea, but I think I'll save it for a more suitable application.

@tel jetson,

I see. Well everyone should know their own limits. Here in Georgia that's just a normal crawl space amount so, it wouldn't be deal breaker for us; the one we did, was as I recall, 5' deep.

Best Wishes,
Joe Woodall, Ecoitect
Georgia Adobe Ecoitecture
http://www.georgiaadobe.com

wow. that's a pretty serious endeavor. it would mean burying a lot of plumbing to do it in a bathroom, which doesn't seem like a great idea to me, particularly since we may want to rearrange things a few years down the line.

excellent discussion... I would advise as thin a layer as possible on top of a wood subfloor using fiberglass hairs to prevent cracks, and putting in some good old fashioned expansion joints like the cement boys do... and also I would NOT mix in any type of manure binder or anything that smells weird. A friend's place has a very subtle putrid smell after using a manure binder... you don't want that indoors! Best wishes - and I do hope you do your earthen floor. It will give you years of great service.

My earthen floor is 1-1/2" thick on top of 1/2" of sand on top of three layers of roofing felt paper on top of thin cinderblocks for underfloor heating+cooling... ALL this on top of 2x8's spaced 16" with 1-1/2" plywood subfloor and it's held up well for over 10 years now.

I know I am resurrecting this thread, but does anyone have any updated news on what you have tried? What worked, and what didn't? I would like to keep things as natural as possible. Right now I'm thinking I will put down some paper, a small layer of sand, and a 1" dry mix with straw in it and maybe some manure. I am also thinking of putting burlap in the center of the cob layer to prevent cracks and increase strength. I am considering this based off of an experiment which you can read here:
http://www.buildwelllibrary.org/building-materials/earth-1/adobe-1/8-burlap-reinforced-adobe

Any other thoughts?

Thanks
Frank

Kafue what happened then?🤣

We are in the midst of completing our earthen floor over wood. Our natural build is a little different from the norm. (Is there a norm in natural building?)

For a variety of reasons our structure consists of (12) hardwood round piers 10-14" in diameter, charred in the Yakisugi method, supporting (3) hardwood round sills 9-12" in diameter, upon which rough sawn (true dimension) Eastern Hemlock joists of 2x10 or 2x12 span 11.5-12'. 2x6 hemlock boards run diagonally and comprise the subfloor. The earthen floor is as follows:
1) (3) layers of red rosin paper "float" on the subfloor. The pieces are taped at any seams, but are not attached to the floor or walls.
2) ~0.5" of raw earth
3) ~0.5-1.0" of wet cob applied over dampened earth layer; radiant piping installed.
4) ~0.5-0.75" of wet cob
5) ~0.25" grit-heavy (sandy) cob mix top coat. Very little fiber in this mix. We anticipate some cracking.
6) 4 coats of raw linseed oil
Total thickness = 2"

We have completed the dining room and hallway and will be connecting those with the kitchen later this week. I will try to get some decent pictures.

Initial thoughts: cracks are small and add to the rustic aesthetic, as do the goat hoof prints (when they snuck in), the cat's and dog's foot prints, and a certain little girl drawing in the moist top coat. These all create a custom unique floor. It is not hard like concrete, but gives a little. We are trying to be careful with chairs. For now we are not planning to do any more coats of oil or adding wax. Those two small sections soaked up 2 gallons of raw linseed oil at $45 each. We have not thinned the oil, but applied at full strength, after having it warmed in the greenhouse.