Cob House Number Crunching

integrated cob seating ideaThis will be the first in an ongoing series of posts about the cost and material volume of our planned cob house. As our plans are fluid you might find a lot that these numbers don’t match up down but we’ll explain what they mean each time so that should help anyone who is using the info we are putting together for their own project or to try to figure out how to do a similar project.

Right now the ideas center around a mostly round structure with a reciprocal roof sitting on two city lots near the crest of a south-facing hill. The picture on the left shows some of the design idea which is two floor, open, and will feature passive solar designs.

Architects & Structural Engineers & Inspectors! OH MY!

First things first – let’s talk about people that we’re sure to encounter (since we’re building in the city). We have yet to hire an architect or structural engineer which is almost certainly going to have to happen. For those that are unfamiliar with what an architect will do or those that think all they do is draw pictures of houses or building you should know they also  provide  a list of materials and estimated costs of those materials (if they don’t you should probably not be paying them what you’re paying them) in addition to reviewing and/or reworking your plans. Architects are (usually) really cool people and you should not look at them (or at engineers or inspectors) as evil people that you need to pay a toll to. They should be viewed as collaborators – find someone you like, respect, and want to work with. We’re looking for just that right now.

Structural engineers are people who love to test stuff and look at data. Don’t think of them as a hurdle – think of them as not only making sure what you’re doing is safe but as providing you with solid information that will inform your process. Again – find someone who you like and that is fun to work with. An engaged structural engineer should be excited to test your materials, not standoffish, and they should be interested in the process you’re undertaking. We’re looking forward to finding such people.

You can’t choose your inspectors (well you can by choosing where you build) but for us – since we really really want to stay in Pittsburgh and to build here we kind of can’t. Though we can make sure they are at least comfortable with what we plan to do and we can build consensus and excitement in the community (which we are doing). This way even if the inspectors are not hot on the idea we will have a large support group that we can turn to in order to help influence their final decisions.

Cob by Numbers

So we have begun to think that including pumice is the way to go. Pittsburgh is not known for it’s wealth of volcanic material so we sadly will need to look outside of the area to find pumice and will have to pay for it’s transportation but it will make the walls stronger, lighter, and provide a higher R value as the pumice is more insulating. Since we’ve yet to order any materials we don’t know what the final mix will be but we aim to start with 30% clay, 30% pumice, and 40% sand Then we’ll test different amounts of straw to find a good mix of compressive strength and tensile strength. The straw decreases compressive strength but adds tensile strength.

We estimate that we will need approx 120 cubic yards of material for the external walls not including the urbaite that we plan to build the stem walls out of. Urbanite is recycled unreinforced concrete. Click on the link for more info.

What does 120 cubic yards look like?

An American football field is 100 yards plus 10 yards for each goal area – totaling 120 yards (flat). Set two fields next to each other (include sideline room) and stack 3 feet of material on top of them or stack 6 feet a material on top of one of them plus their sidelines and you’ve got the amount of raw material we’re going to need to use to build the exterior walls of our home.

Estimated costs of Sand & Pumice

We assume we’ll buy that much sand and pumice and dig up that much clay even though we’ll be adding significant volumes of straw to the mix since we’ll also be building interior walls out of cob. Sand costs anywhere from $25 to $40 a ton. With 120 cubic yards as our estimated starting figure and 30% of that being sand we’ll calculate that out to about 64.8 tons of sand which should cost between $1,500 and $2,600. Pumice is lighter so the tonnage should be less but we have not done the conversion or found prices yet. Even if it is equal to or slightly greater – 70% of our (non-straw) cost should not exceed $6000.

Estimated Cost of Clay & Straw

Since the clay will be harvested onsite (with the help of a small front loader) or if need be can be bought at a cost of less than $12 per ton. We plan to harvest a lot of the clay, if not all, onsite but needing as much as or more than 36 cubic yards could result in needing to buy some. A 36 cubic yard hole that is 6 feet deep must be 54 feet by 54 feet long. If it’s 9 feet deep we would need to dig a hole that is 27 feet wide by 54 feet long. 9 feet deep is deep. That’s the deep end of a swimming pool.

The good news is that clay is significantly denser than sand the tonnage should be much less though we have yet to do an accurate conversion or find a supplier to know the cost. Assuming the 36 cubic yards of clay is moderately dry there should be 8 1/3rd cubic yards per ton so we’d need less than 5 tons. Even paying $30/ton and buying the full supply that is only $150. I’ve seen bulk clay sold for less than $12 per ton but not  in the US. It is literally dirt cheap so finding someone who sells it is difficult (since the prices are so low).

Straw is cheap too – it’s a waste product of farming. Straw (not to be confused with hay) has zero nutritional value and is similar in organic makeup to wood. It costs anywhere from $0 to $10 or even $15 per bale. We won’t know how many bales we’ll need until we test our mix but assuming 300 bales at $5 each – that’s $1,500

This brings our total estimated material cost (not including roofing) to $7,650. We plan to buy the land from the city and we hope that it will not cost more than $5,000. All told that will bring the cost to somewhere under $15,000. We plan to buy used windows, doors, and as many fixtures and things as possible. Our goal is to spend less than another $10,000 on those items and the installation of electric, gas, and water. All told (if we add a cistern and some other materials) we should be  able to build the house for under $30k and hope to control costs and bring the total closer to $18,000 but that might be difficult. With both floors we plan to have approx 1,600 square feet of space. That would means our cost should fall between $11.25 and 18.75 per square foot. Even on the high end that is less than a 1/4 of the cost of a conventional home build in the US in 2011.

If you’d like more information or are interested in helping drop me a line on the contact form or leave a comment. We’re dedicated to helping anyone in the Pittsburgh area to build their own home this way that helps us to build ours!



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