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Can I use cob as a mortar for a rock house

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posted 7 years ago

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I was wondering if its possible to use cob as a mortar for a rock roundhouse? And then finish the inside with cob or an earthen plaster? Thanks for the help!

Sebastian Köln

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posted 7 years ago

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A mixture of clay and sand (cob) works well as mortar – as long as water is kept away. I have build a small brick wall with it so far and it is quite solid.

EDIT: If your stone does not wick up the water, it may take some time for each layer to dry.

Franak Ostapowicz

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posted 7 years ago

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Ok great. What do you think of the possibility of using it as a mortar for a house? Woyld the inside be weather proof if I was to use an earthen plaster as the inner walls?

Kyrt Ryder

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Location: Graham, Washington [Zone 7b, 47.041 Latitude] 41inches average annual rainfall, cool summer drought

posted 7 years ago

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Franak Ostapowicz wrote:Ok great. What do you think of the possibility of using it as a mortar for a house? Woyld the inside be weather proof if I was to use an earthen plaster as the inner walls?

Depends on your climate. If you can expect little rain during construction and provide very good hat and boots you would probably be fine

John C Daley

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posted 7 years ago

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I am currently using a sloppy mud mortar to lay a double brick internal wall.
The bricks are what we call 'wire cut' with holes in the core, I just dunk the dry brick into the mixture
and lay the bricks.
The wall is very solid.

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Franak Ostapowicz

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posted 7 years ago

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Well that's awesome. What is your mud mortar ratios and ingredients?

John C Daley

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posted 7 years ago

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Naturally occurring mixture of sand and clay with 50-50 ratio, mixed with water so its just thicker than running.
I don't mix it at all, just use it as it came from the ground.
But I did select it carefully from near a creek.
Just experiment and ask more if you need to.

Franak Ostapowicz

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posted 7 years ago

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Really cool. Is this creek on your property or do you know someone that lives on the creek? I do have a small creek that goes by my dads land. It may have what I need. Though I wonder if my subsoil has a decent enough ratio. What do you do to test it? Do you get a glob and put it in a jar with water and shake it up and when it settles you can see the different layers? Thank you so much for your help!

Roberto pokachinni

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When I built with stone, we used concrete, but I don't see any reason why you can not use cob to do it, so long as you have drainage, and a good roof protecting it from moisture in all directions. When I lay stone, each stone is placed on the wall, so that I am sure that it works for that spot, by itself, as if it was a 'dry stone wall' (without mortar). If it needs a small stone placed under it, so that it sits better (a 90 degree plumb angle on the inside or outside face), then add the stone and make it sit proper. Then move the stone off the wall, and add mortar. Then lay the stone (including whatever small stone you might need to sit it right first). The mortar is just filling in gaps, in this case, as the stone stands alone and the wall shouldn't need mortar for it's integrity if it's done this way.

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John C Daley

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posted 7 years ago

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Franak, unusual name, whats the background to it?
I took soil from a hole I dug 4 metres from the creek on my property.
Use the bottle test you spoke of.
Basically the mortar should be the same type of soil that you could build an earth block with.
Jf you decide ro go for cement, think about a lime mortar rather than a cement mortar.

Cristo Balete

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posted 7 years ago

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If you are using expansive clay it will shrink and swell even with just moisture in the air. I live with expansive clay and it surprises me every year how much it shrinks. The eaves on the house have to be really big to keep all rain and blowing rain off of cob and clay-based mortar.

If you are in earthquake zones even the little temblors will cause it to crack eventually.

It just seems like there is an awful lot of maintenance, some of which may have to happen in the rain, because it starts leaking in one of those really rainy years, in which case the cob wouldn't dry in time to fix the leak.

Mediterranean climate, hugel trenches, fabulous clay soil high in nutrients, self-watering containers with hugel layers, keyhole composting with low hugel raised beds, thick Back to Eden Wood chips mulch (distinguished from Bark chips), using as many native plants as possible....all drought tolerant.

Franak Ostapowicz

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posted 7 years ago

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Franak is one way of saying frank in Polish! I will definitely consider digging a bit and using the jar method in a few places on my dads property, that is for the advice John. And thanks for being so frank with me cristo! Ill definitely not be using clay plaster/mortar on the out side of the round house considering arkansas humidity and rainfall. I really appreciate all o' yalls help.

Glenn Herbert

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posted 6 years ago

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Roberto's advice is critical, as clay mortar has no bonding ability, only sealing and stabilizing. Your stone walls must be able to stand by themselves, and resist any stress or shaking they may be exposed to.

Franak Ostapowicz

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posted 6 years ago

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how thick do the rock walls need to be and how can i insulate the inside? wiring in between rock and earthen plaster?

Glenn Herbert

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posted 6 years ago

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That really depends on the character of the stone you have available. The smoother and rounder the stones, the less capable they are of forming a sound wall. You will benefit most from the stone being on the inside of the insulation layer. If the insulation is inside all the stone, the stone mass will stay cold all winter, and any solar heating will be hard to transmit to the interior. If you want the appearance of a stone building and the benefit of thermal mass, you pretty much need to have a double wall with insulation in the middle. This is assuming a climate with real winter, and the possibility of long periods with little sun. If you have a lot of solar gain and little cold air exposure, or cold only at night with sun all day, you can use a monolithic mass more effectively.

Franak Ostapowicz

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posted 6 years ago

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Thanks Glenn. My dad just passed away so I'm not sure if we are keeping the property. Its 12 acres and house that needs some work, and the land goes for 1000 an acre. My sisters want to sell and just split the money and I said yes but I'm thinking that to sell it would be a mistake, twelve acres is worth more than 12000 dollars. What do you think? I'm not attached to the house but I want to farm the land and put a little house on it. The only set back is the money to start and set up a farm garden and its an hour away from any civilization. I may be able to put 4000 down on a little piece of property where I'm living now but that's gonna be a big mortgage payment. I didnt realise the stone would stay cold even in the sun. We don't really get real winters we might get a little below zero for a couple days but mostly 10s 20s and 30s for a spell, north arkansas. I just want to have a piece of land to fall back on.

john Harper

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posted 6 years ago

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A wise man once said,"Never sell land."

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Brian Rodgers

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posted 6 years ago

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Sorry to hear about your father passing.
I've been building with masonry and cob (adobe) for four decades. I would not recommend mixing the two in a structural wall. In fact I would not be able to sleep well at night knowing my family was under a pile of stones only held together with cob.
Alternatively, I have been learning how plaster works and would feel much better using a well researched lime mortar mix, as it has the ability to create a bond with other similar molecules. Plus this building technique has been around for thousands of years and much of it is still structurally sound.
Here are a few references for lime mortar with natural rocks
Lime mortar
I really like this one. I think it fits in well for us Permies.
Natural building with Lime Mortar
Brian

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