WOFATI roof weight?

Does anyone have some estimates for the per square foot weight of a WOFATI roof ?

I want to see what kinds of weight we're looking at as we play with design ideas for our future house.

Hey Penny,

I tried to find the information for you, but my skills in Natural building are low.

Permies has the 48 Hour Rule. If you don't get a good answer to your question in 48 hours, you can post in the Tinkering forum(with a link back to this post) and say you "invoke the 48 hour rule". The staff will then try to track down the answer for you. If they can't find anything, then they will put it out to the Daily-ish email users to try to find an answer. I bet someone has the answer, they are just missing the post.

I think what you'll find is that "it depends" in a big way. But a better way to phrase it is that you'll need to do some engineering to figure that out. You'll need to find the saturated weight of your particular soil, calculate how deep you plan on burying the woofati, and then add the design ground snow load to that. The latter should be available from your county's building department, but the first part may require having some tests done to your soil. Different soils weigh different amounts, have different saturation characteristics, and everyone puts a different amount of soil on top of their buried structure. Snow load can end up being quite significant too. My ground snow load is 316psf, while my parent's ground snow load an hour and a half away is 0psf.

Hmm, I guess what I was asking is what kind of weight per square foot has been used for some of the WOFATI's up at Wheaton labs?

I can figure out the snow load and understand about different soils weighing different amounts but I guess I was looking to see what others were using in their calculations so far.

Thanks

Penny McLoughlin wrote:Does anyone have some estimates for the per square foot weight of a WOFATI roof ?

I want to see what kinds of weight we're looking at as we play with design ideas for our future house.

There is not enough information given to really give you a reply, but I can attest that a cubic foot of gravel weighs 3000 pounds. So assuming you have soil that is 3 feet deep, that equates to about 330 pounds per square foot. It really is not that much to design for, as a monolithic superstructure spreads the weight out over a broad area, and if compacted, creates a lot of briging. Making the soil above with Engineered Earth which is earth fortified with geotextile to get amazing compressive and shear strength, also adds to the overall safety and design.

I believe one of the WOFATI structures at Wheaton Labs experienced a problem with weight, but not in the superstructure, but rather with its columns. Normally a column driven into earth gets its stabalization from the friction surrounding it in the earth. The more a column or post is driven into the soil, the more the surface area to produce friction. A lot of people think it is the underside of the column that allows it to hold a structure above up, but the surface area is so small that it is actually the sides of the column that does the work. In the case at Wheaton labs (I believe, and I could be wrong), the weight of the WOFATI Roof exceeded the friction on the columns and they began to sink. But I believe the posts were driven in sandy type soil too. Ideally they should be bedded onto bedrock, but that is not always possible.

The lesson to be learned here is, a WOFATI can put tremendous pressure on supporting columns. How they are supported is critical to the success of a WOFATI. In this case the superstructure was okay because it had multiple points of contact, in fact almost continuous contact so that the purlins and rafters carried the weight. But when the weight got taken down to a single point of contact (at the column base), failure began in the form of sinking. Fortunately it is a pretty easy fix.

A inch of soil works out to be about 7lbs per foot So wih an estimated 3inches of soil 27lbs.
Check out this link for some more numbers. https://www.soils.org/discover-soils/soils-in-the-city/green-roofs

If you have insulation underneath you would have to include those number too.

I asked around a bit, and it was suggested that the Mike Oehler book has some numbers.

The $50 and Up Underground House Book by Mike Oehler. You can buy the digital version here, but there are also links to Amazon physical versions if you choose.

Mike Oehler's Low-Cost Underground House Workshop & Survival Shelter Seminar - 3 DVD+2 Books Deal. This has both his books and his 3 DVD seminar as a digital download deal.

Travis Johnson wrote: a cubic foot of gravel weighs 3000 pounds.

Hi Travis, I think you might have a number off here.  

Thanks for all your help here.

The dirt weight was what I was hoping for.

I have the $50 Underground house book and have read it but from my understanding, Mike Ohler did a much lighter roof since he was only doing 1 layer of plastic and fewer inches of dirt over it.

I'm reading John Hait's book right now about the passive thermal inertia as I think that his roof loads are probably going to be more comparable to the WOFATI style roofs but I'm not very far into it yet and haven't seen any structural calculations yet.

I really appreciate all your inputs.

S Benji -  yes that is sort of what we are imagining. Although with an actual wall between the living part of the house and the greenhouse part so that we can have better control over how much heat we're letting into the house.

Thanks for the links. I'll check them out.

Yes I also see the greenhouse and living space having a wall.




Instead of just having 2 huge master bedroom.
They could be split in half for a total of 4 bedroom.

Given that we know that 12inch of soil is about 100lbs per square foot.
The question is how thick do you plan on making your living roof.
6inch = 50lbs/ square foot
12inch = 100lbs / square foot
24inch = 200lbs/ft^2
36inch = 300lbs/ft^2

Above the pole/beam/wood support, I understand getting 24inches of rigid insulation for a R90 insulation, topped off with 3inches of soil

But I dont really see the need for 36inches or even 12inches of soil.

I think maybe the question that you are posing to the forum is:
On avg how thick is the dirt that is on top of the pole/beam/wood support system in a WOFATI house vs the usual 4inch in a regular living roof house. And maybe in addition to that how heavy is the beam+poles+wood in addition to the above dirt, that the poles in the dirt under compression have hold up.

S Benji - From reading some more in John Hait's book, it looks like he recommends 24 inches of dirt above the waterproof umbrella but because of the weight that that causes, he has vey little dirt between the house and the umbrella. He did say that the increased soil depth on the plastic kept you from having your roof turn brown from lack of soil moisture and that it also created a sort of transition zone between the outside and the home.

I'm still only in the 3rd chapter though so my understanding may be incomplete yet.

It seems like if I used 24 inches above the waterproof layer, then I would need significant insulation between the house and that since the freezing depth itself would be two feet in the winter in the area I'm planning to build.

You're right though in that I was asking how strong the roofs were designed for on the existing WOFATIs just to get an idea of what kinds of structural strengths might be needed.

I like the longer rectangle design of the Earthship homes though. It seems you would have more natural lighting that way. Where the house is basically only one room deep and all the rooms have one side that opens to the windows to allow light penetration.

If you look at the profile pic, you will see that the back of the house has alot of soil.
And it is not just the back of the build it is on all sides except for the greenhouse front.
And even the front has a 3ft high earth wall.

The floor plan that I posted is only one room deep with two master bedroom with "walk in closet and bathroom"

The entire structure is only 33ft deep. so once you discount the greenhouse.
The master room with the walk in closet + bedroom is only 20ft a bit bigger than usual 12ft by 12ft bedroom without a walking closet.

Alot of the earth ship 1 room deep building are 40ft deep vs the 30ft that this buiding has (20ft if you discount the greenhouse)

But I do like it where every bedroom have access to the the greenhouse/natural light.
What do you think of this 4bedroom. It's 40ft by 110ft

S Bengi wrote:

But I dont really see the need for 36inches or even 12inches of soil.

Paul talks about the roof supposed to be covered with something like 3 feet of soil.  https://permies.com/t/68416/hour-tour-wheaton-labs

I think that the 3inch of soil on the living roof depends on your climate, and species selection.

If I was in the desert/Mediterranean climate I wouldn't plant water loving plants it would be tiny cati and succulents.
In fact, I say if you plant Mediterranean like species/cultivars you will be okay say lavendar, thyme, in addition to some stonecrop/sedum.
Here is a more flushed out plant list for a 3-5inch living roof with no irrigation.
Achillea millefolium
Achillea tomentosa
Allium cernuum
Allium schoenoprasum
Antennaria dioica
Anthemis tinctoria
Campanula glomerata
Centaurea scabiosa
Chrysanthemum leucanthemum
Daucus carota
Delosperma cooperi
Delosperma nubigenum 'Basutoland
Dianthus carthusianorum
Geranium sanguineum
Hieracium pilosella
Hieracium x rubrum
Linaria vulgaris
Origanum vulgare
Petrorhagia saxifraga
Plantago major
Potentilla verna
Prunella grandiflora
Sanguisorba minor
Saponaria ocymoides
Sedum album Cultivars
Sedum reflexum Cultivars
Sedum sexangulare Cultivars
Sedum spurium Cultivars
Thymus montanus
Thymus serpyllum
Verbascum phoeniceum
Veronica teucrium

Looking at the roof in the video it is at least half the width of the door and he does state that he wants to improve the design, so half of a 36 inch doors. 20inch of dirt is no joke.

I do like a 36inch roof WOFTATI for an unheated animal shelter because I can see it being 40F, no wind chill, when outside is way below freezing.
For a heated house that someone wants to keep at 70F, 20inches of insulation+sedum-moss vs 20inches of soil works.
But if the goal is to plant a productive garden with carrots, daikon radish, etc, there is no way 3inch of dirt is going to grow anything to eat, unless it it kept on I.V (irrigation)

Wow, 36 inches is quite a lot of weight to support. And then they will have a snow load on top of that in the winter as well. Quite a substantial roof.

S Benji - I like the second design better. It looked to me like the bedrooms in the first design would have little to no natural light. I wonder if you could arrange to have a hallway between the greenhouse and the bedrooms in the second version so that you didn't have to go thru the greenhouse to get to the bedrooms.

I think the idea of having the really thick earth covering over the roof and berming the walls so thickly as well is so that you would not need to heat (or cool) the house. The idea is to heat up the earth trapped under the large water proof umbrella during the summer so that it keeps the house around 70 deg all winter.

That's why they go to all the trouble to make the umbrella so large and put so much earth on the roof.

John Hait also uses insulation between the layers of plastic on the roof. Does anyone know if Paul did something similar on his WOFATIs?  I don't remember hearing anything about that.

Mike Jay wrote:

Travis Johnson wrote: a cubic foot of gravel weighs 3000 pounds.

Hi Travis, I think you might have a number off here.  

Hey Mike, thanks for pointing out my folly! I just realized my mistake, but have not been online for days. It should have read 3000 pounds per cubic yard. Can I blame it on pre-coffee-grogginess?

Penny McLoughlin wrote:John Hait also uses insulation between the layers of plastic on the roof. Does anyone know if Paul did something similar on his WOFATIs?  I don't remember hearing anything about that.

Mike was a firm believer in using insulation as a stop-gap measure for heat loss, and the University of Minnesota did a lot of studies on heat loss of underground buildings and concluded that just one inch of stryrofoam insulation was more effective then tons more earth, dry or otherwise. The problem was scaling up the structure to hold the weight. It costs more to build the massive structure and free dirt, then it did to buy insulation and have less dirt. But strroafoam is hardly "Green Building".

The permiculture way is use forest dander as insulation, or wood chips.

S Bengi wrote:If you look at the profile pic, you will see that the back of the house has alot of soil.
And it is not just the back of the build it is on all sides except for the greenhouse front.
And even the front has a 3ft high earth wall.

The floor plan that I posted is only one room deep with two master bedroom with "walk in closet and bathroom"

The entire structure is only 33ft deep. so once you discount the greenhouse.
The master room with the walk in closet + bedroom is only 20ft a bit bigger than usual 12ft by 12ft bedroom without a walking closet.

Alot of the earth ship 1 room deep building are 40ft deep vs the 30ft that this buiding has (20ft if you discount the greenhouse)

But I do like it where every bedroom have access to the the greenhouse/natural light.
What do you think of this 4bedroom. It's 40ft by 110ft

It may be a good design, or it may not, I am not qualified to say.

The information to determine that is the height of the front of the structure. A lot of people just assume that because the front of the room is open, light will get all the way to the back of the room, but that is not the case. To get enough light back 33 feet, the angle of the sun has to be accounted for and how deep it penetrates during certain times of the year. The less it penetrates, the less the structure heats up, and it reverts the other way, actually chilling the structure. Thirty three feet is a long ways.

Further north, I have heard that strategically placed reflective ponds can help with interior light levels. They can literally act as mirrors, reflecting light deep into such a structure that would otherwise never make it that far.

Also, the whole one inch of insulation being better than a ton more dirt is all well and good, but I think what those interested in building WOFATIs will want to know is what the effect of the placement of that insulation will be. What will be the effect of an inch thick insulating layer on the outside of that ton of thermal mass?

-CK

I like natural lighting in my living space: Living Room, Family Room, Office/Den/Library, Dining Room, Kitchen, Bedroom.
However I dont see myself spending alot of my daytime in the laundryroom/Walk in Closet/mechroom/utilityroom/bathroom.

The structure is 33ft including the greenhouse. Where the light enters in from.
The 'window' goes right up to the roof of the living space with no eaves.
The dimensions for the living space is 9ft high and 24ft deep, with the 'window' all the up to the ceiling.


Also if I am understanding correctly. The 36ft WOFATI roof actually has layers.
(A) 12inch moist soil
(B) 6mil Plastic
(C) 24inch bone dry insulation but it is natural (strawbale/wool/woodchip/pumice/duff)
(D) 6mil Plastic
(E) wood+pole+beam support structure
Let me know if I am understanding this correctly folks?

Chris Kott wrote:Further north, I have heard that strategically placed reflective ponds can help with interior light levels. They can literally act as mirrors, reflecting light deep into such a structure that would otherwise never make it that far.

I've heard this too but I'm not sold on it needing to be a pond.  When you're fairly far north, most ponds freeze in winter.  Then they get covered with snow.  Snow is very reflective.  So if you want, I think you can skip the pond and just have flat ground with snow on it and you get the same reflection.  

Now if it's a larger body of water with waves on it, in the summer the light bouncing off the waves could give you some nice light despite the passive solar overhangs.

S Bengi wrote:I like natural lighting in my living space: Living Room, Family Room, Office/Den/Library, Dining Room, Kitchen, Bedroom.
However I dont see myself spending alot of my daytime in the laundryroom/Walk in Closet/mechroom/utilityroom/bathroom.

The structure is 33ft including the greenhouse. Where the light enters in from.
The 'window' goes right up to the roof of the living space with no eaves.
The dimensions for the living space is 9ft high and 24ft deep, with the 'window' all the up to the ceiling.

Also if I am understanding correctly. The 36ft WOFATI roof actually has layers.
(A) 12inch moist soil
(B) 6mil Plastic
(C) 24inch bone dry insulation but it is natural (strawbale/wool/woodchip/pumice/duff)
(D) 6mil Plastic
(E) wood+pole+beam support structure
Let me know if I am understanding this correctly folks?

It does not look like your WOFATI will get much light very deeply; not even to the first back wall by a quick look at the plans, but perhaps it does not matter in terms of heat because your greenhouse will act as a solar furnace anyway? You are also right in that the mechanical room, utility room, and walk in closet will not need windows, my current house is above ground and does not have windows in those rooms, but I am not sure I would ever have a bathroom without a window.

In the $50 House book, Mike talks about always having (2) light sourses in every living-type room or a human does not feel comfortable, and I agree wholeheartily with that assessment. A great way to do that on back walls would be to use clerestory windows. That aids in ventilation too.

The only other thing I noticed was your main room colum spacing looked to be about 12 feet. I know from doing the calculations in my own house an 8 x 8 spruce beam 12 feet long has a continuous load rating of 4500 pounds. With a foot of soil, the weight of the insulation, snowloads and then any sort of incidental weight on the roof, you will be at the maximum. You could use White Ash to give you some fudge room, or move your colums to 10 or 8 feet so the span is not as heavy.