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Paul Delaet
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My father is giving me some land on which his childhood house used to sit. it sat on large 2 1/2-3 ft long sandstone block that are about a foot thick and a foot wide. the whole foundation is 17ft x 35ft. i was hoping to dig a foundation for a 16x16 (inside diameter) building and use the sandstone block. So basically, id be looking at digging to the frost line and grading it to one corner, laying some drain grade gravel (57s?) down level, and then setting the sandstone block on top of that for my foundation. My question is "can i build cob walls directly on top of the sandstone blocks?" If not, is there still a way to utilize these large sandstone blocks?

Here is some info on the building site and myself.

We are in north central West Virginia where we experience imo the average 4 season year.
Summer- average temps are 60s at night and 80s in the day with a few 100 degree days some years.
Fall-
winter- snowfall varies. out of 23 years of life, ive seen one year where we got 2ft of snow in one say. but other than that its usually an average of roughly 35 inches a year. temps rarely drop below 0. usually average between 25-35 lows and 40-55 degree highs.
spring- Usually warm and wet, but not enough rain to create problem.

There is little seismic activity in our area that i know of. While the state has had them in the past, i cant think of ever hearing of one in our area.

there IS a creek about 20 yards or more away from the site, but it never floods. Its one of the drain creeks for the 22 acre lake that sits off behind us. the drain is constantly flowing at max capacity and the creek is only like 3 inches deep. even with heavy rain, we have only ever seen it raise 4 or 5 inches if that. its not long enough for ground water to really affect its level. it runs under the main road and to a larger creek down hill.

I was hoping to do all cob walls, but after examining the resources on the property, i think cob and cord wood walls would meet our needs of quick construction, materials available, and cost. The 4 acres has a variety of trees in a variety of sizes and i wont have to dig up the whole lot trying to get enough clay for just cob walls.

i was thinking something with a salt box roof. not sure of exact mentions but somewhere around 18ft high on one side and 10 ft high on the other side? wanting to do a metal roof also. but im getting way ahead of myself here.

Thanks for any help in advance!
 
Jay C. White Cloud
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Hi Paul,

I will do my best for what is is worth...and welcome...

We are in north central West Virginia where we experience imo the average 4 season year.


This is a very nice area, and as such "frost" (when and if it occurs) is shallow. Clay soils is more an issue alone than being below frost level so, as such, a "gravel trench" foundation is more than sufficient to lay your sandstone on. The project should be fine with only one course of stone below finished grade and the house floor level 600 mm minimum above finished grade.

There is little seismic activity in our area that i know of. While the state has had them in the past, i cant think of ever hearing of one in our area.


The East coast tectonic plates are actually "past due" for a Richter scale 5 Level or above, so I would not (and don't) recommend any structure that does not have a superstructure of some form. I typically recommend a timber frame of some form. This means not "structural cob" or related wall forms other than an "infill" or cladding system. Often better than this is a timber frame with a "loft insulation" like mineral wool, straw bale, light cob, wool, etc, and then all internal walls/partitions acting as your "flywheel" mass for storing heat or coolness.


i was thinking something with a salt box roof. not sure of exact mentions but somewhere around 18ft high on one side and 10 ft high on the other side? wanting to do a metal roof also. but im getting way ahead of myself here.


This would lend itself well to a cob/cordwood structure with supporting timber frame.

Good luck,

j
 
Paul Delaet
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Jay C. White Cloud wrote:Hi Paul,

I will do my best for what is is worth...and welcome...

We are in north central West Virginia where we experience imo the average 4 season year.


This is a very nice area, and as such "frost" (when and if it occurs) is shallow. Clay soils is more an issue alone than being below frost level so, as such, a "gravel trench" foundation is more than sufficient to lay your sandstone on. The project should be fine with only one course of stone below finished grade and the house floor level 600 mm minimum above finished grade.

There is little seismic activity in our area that i know of. While the state has had them in the past, i cant think of ever hearing of one in our area.


The East coast tectonic plates are actually "past due" for a Richter scale 5 Level or above, so I would not (and don't) recommend any structure that does not have a superstructure of some form. I typically recommend a timber frame of some form. This means not "structural cob" or related wall forms other than an "infill" or cladding system. Often better than this is a timber frame with a "loft insulation" like mineral wool, straw bale, light cob, wool, etc, and then all internal walls/partitions acting as your "flywheel" mass for storing heat or coolness.


i was thinking something with a salt box roof. not sure of exact mentions but somewhere around 18ft high on one side and 10 ft high on the other side? wanting to do a metal roof also. but im getting way ahead of myself here.


This would lend itself well to a cob/cordwood structure with supporting timber frame.

Good luck,

j
Thanks for the warm welcome! And thanks for the help! I forgot to mention that i plan on having a timber frame construction. 12 inch pillars on each corner tied together at the top, with one extra pillar in the middle of the wall on the 18 ft side. The vertical beams that tie the corners together will support the roof. Any thoughts on the sandstone and cob being in contact with each other? what about the logs used for the frame? what prep is there to them before they can be in contact or encased in cob? As for the house floor level, are you saying i need one course of block under finished grade and another two courses of block (2 feet) above finish grade? I dont mean to be a pain, im just slightly confused. my thoughts were to dig the trench, dig the holes and set the corner poles, then lay gravel and set a course of block under finish grade and then set another course on top and begin to lay cob. on the inside of the house i was going to do some sort of earthen floor by back filling and putting the level of the floor level with where the stone and cob meet.
 
Wade Glass
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Morning Paul,
To help make it clear what you are working with on your proposed building site. You may want to post a picture or two. Worth a thousand words as they say.

 
Paul Delaet
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Wade Glass wrote:Morning Paul,
To help make it clear what you are working with on your proposed building site. You may want to post a picture or two. Worth a thousand words as they say.


Morning to you too!
ill draw up a few pictures of what im trying to accomplish. id take a few pics of the building site but its yet to be cut and cleared. grass and brier bushes 5 ft high and what not. just trying to find the old foundation after not being there for years was a fun little adventure =) Perhaps ill make a scale model and post some pics of it!
 
Jay C. White Cloud
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Hi Paul,

Pleasure to help...

Thanks for the warm welcome! And thanks for the help! I forgot to mention that i plan on having a timber frame construction. 12 inch pillars on each corner tied together at the top, with one extra pillar in the middle of the wall on the 18 ft side. The vertical beams that tie the corners together will support the roof.


I would strongly second Wade's kind suggestion. I am "visual" developer. I can glean much from words but not enough to give good guidance. Even something scratched on a peace of paper and a picture taken of it would be great. I understand that most phones today can do this trick really well, as can most computers with there little cameras...

That design (in my head) seems o.k. but could be improved (I think??) but not sure without really seeing what you are seeing...

I glad to read about the timber frame primary structure...Most excellent!

Any thoughts on the sandstone and cob being in contact with each other? what about the logs used for the frame? what prep is there to them before they can be in contact or encased in cob?


I understand your concern, as sandstone has a very open grain structure and some forms of these sedimentary rock from Virginia to Georgia can take up to 25% or more in weight by moisture. Some of these sandstone types are really hard and dense while others much more friable. Its capacity to take on moisture and move it through its interstitial void structure by cohesion is a positive and sometimes a challenge.

I think this is my first opportunity here at Permies to write about DPC in some masonry types. DPC (Damp Proof Course) is a traditional method of arresting the challenge of ground moisture and "splash" from spreading up a masonry wall and into the upper wall voids. There is much controversy about this subject in other countries like the U.K., as well as, a broad range of interpretation of its historic significance, methods of application and even effectiveness in many vintage structures. Sandstone is one of those masonry types where it is considered "good practice" to either have the stone type change, and/or have the sandstone go further up the wall.

Presuming this project has a rather "tight budget" there are some other vernacular modalities for addressing this "damp issue" that can happen with some sandstones. One is to make sure that no soils are used as backfill and that sandstone only sits on stabilized and well drain gravel in the 20mm size range. The next effective "moisture break" (aka DPC) is to take a mixture of flax oil, citrus oil, and beeswax and put three coats of this on top of the last course of sandstone. Now some would suggest that this would (and it absolutely does!!) create a "break" between the cobb and the sandstone. In other words, the cob does not adhere at all to the sandstone. This is really not a probably at all, nor should it actually be the goal to have cob "stick" to anything but perhaps itself. "Mechanical Keying" is the primary structural bonding method between cob and consecutive courses of building material no mater what it is...i.e. wood, cord wood, more cob, more stone, brick, timbers, or whatever.

So no mater the type of sandstone, my first choice would be to put an oil/wax finish to the top course before placing the cob.

As for the house floor level, are you saying i need one course of block under finished grade and another two courses of block (2 feet) above finish grade? I dont mean to be a pain, im just slightly confused. my thoughts were to dig the trench, dig the holes and set the corner poles, then lay gravel and set a course of block under finish grade and then set another course on top and begin to lay cob. on the inside of the house i was going to do some sort of earthen floor by back filling and putting the level of the floor level with where the stone and cob meet.


I am kind'a confused now too...so don't feel bad...

Get us a sketch or something and we can get it all sorted out...

Regards,

j
 
Terry Ruth
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Paul Delaet wrote:My father is giving me some land on which his childhood house used to sit. it sat on large 2 1/2-3 ft long sandstone block that are about a foot thick and a foot wide. the whole foundation is 17ft x 35ft. i was hoping to dig a foundation for a 16x16 (inside diameter) building and use the sandstone block. So basically, id be looking at digging to the frost line and grading it to one corner, laying some drain grade gravel (57s?) down level, and then setting the sandstone block on top of that for my foundation. My question is "can i build cob walls directly on top of the sandstone blocks?" If not, is there still a way to utilize these large sandstone blocks?

Here is some info on the building site and myself.

We are in north central West Virginia where we experience imo the average 4 season year.
Summer- average temps are 60s at night and 80s in the day with a few 100 degree days some years.
Fall-
winter- snowfall varies. out of 23 years of life, ive seen one year where we got 2ft of snow in one say. but other than that its usually an average of roughly 35 inches a year. temps rarely drop below 0. usually average between 25-35 lows and 40-55 degree highs.
spring- Usually warm and wet, but not enough rain to create problem.

There is little seismic activity in our area that i know of. While the state has had them in the past, i cant think of ever hearing of one in our area.

there IS a creek about 20 yards or more away from the site, but it never floods. Its one of the drain creeks for the 22 acre lake that sits off behind us. the drain is constantly flowing at max capacity and the creek is only like 3 inches deep. even with heavy rain, we have only ever seen it raise 4 or 5 inches if that. its not long enough for ground water to really affect its level. it runs under the main road and to a larger creek down hill.

I was hoping to do all cob walls, but after examining the resources on the property, i think cob and cord wood walls would meet our needs of quick construction, materials available, and cost. The 4 acres has a variety of trees in a variety of sizes and i wont have to dig up the whole lot trying to get enough clay for just cob walls.

i was thinking something with a salt box roof. not sure of exact mentions but somewhere around 18ft high on one side and 10 ft high on the other side? wanting to do a metal roof also. but im getting way ahead of myself here.

Thanks for any help in advance!




Paul, great job describing your design loads and environment. Sounds like you have some great intuition too. I have a son a few years older than you that has his own construction company doing very well you can see in my blog below, and we have a management team around your age.

1. You definitely want to dig below frost line as you planned...International (world wide) code requires it unless you set your sandstone blocks on solid bedrock, or have frost protected foundation. Roxul has a Comfortboard IS @ 1.5 rigid mineral wool board I recommend for your sub and above grade foundation if you plan on living in this building. http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_4_par020.htm

2. Your roof design snow loads are ~20 PSF add another 20 for weight of materials and live loads= 40 PSF. Cob can handle 15,000. Erect the COB on your sandstone blocks evenly. COB density should be around 100 LBS/FT3. It is best to get core test samples before the build to verify that.

3. You DEFINITELY want a good bond to the sandstone to take out bending or the weight of the top of COB from uneven snow loads in your case. Sandstone depending on type is low permeable a barrier is not needed. It's basic structures design to know that if a poor bond occurs here at this low level of the wall and the upper level weight and arm distance away to this critical primary structural bond line, will cause overturning moment from high wind gust or uneven snow side loads, the walls can cave in on you, and/or cracks will probagate at this location and will continue around the building causing ultimate failure. A large seismic event would compound this 100 times. You are in Seismic Design Category B, A being the lowest, no concern at all no matter what anyone says. You are in 90 MPH 3 SEC gust, no concern for COB. I would use a siloxane with natural silicone as a sealer. It lowers the pore size to less than a micron to resist water but leaves the COB highly permeable and breathable.

4. The biggest concern you have is transferring the weight of the building and sandstone to the soil. I attached a table to identify the bearing capacity of soils. We can determine a weight and bearing area of the building and it's environmental loads to see if the soil can take it or not. A soil test is best to make that determination. If not the design does not work period, we need to modify the soil or the weight of the building. BTW: Since soil strengths can vary in the perimeter of the building drastically, there is no comparing or using other building's as empirical design data.

5. When it comes to designing: Use code or at least an Engineer. I can get you close but I suggest you find a local structures engineer (PE state licensed best) to look it over for things we have missed from online communications. That should not cost much and should put you at ease knowing that the design has been approved by a professional, not someone that has no design experience off the internet.

Assuming it is a one story building we need a fully dimensioned floor plan to see how the COB is braced laterally by internal walls and whether buttress are required since the weight of the COB does not support itself too well laterally, that depends on the thickness of the wall. The wood in your case will not be required. There are alot of parameters of COB we can adjust to take out side loads, just like concrete that has better mechanical and isotropic properties that are more predictable than anisotropic wood, especially surrounded by COB that needs to be quantified. If you want wood aesthetics save the framing cost and put the money into non-structural beams.

Here are some things we need to know:


COB-Compression-Strength.JPG
[Thumbnail for COB-Compression-Strength.JPG]
COB-density.JPG
[Thumbnail for COB-density.JPG]
Sanstone-Density.JPG
[Thumbnail for Sanstone-Density.JPG]
 
Terry Ruth
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Soil code design allowables table:
Soil-Compression-Allowable.JPG
[Thumbnail for Soil-Compression-Allowable.JPG]
 
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