Wofati Earth Roof

As outlined in the building better world book, a wofati earth roof is a tremendous and cheap technology composed of membrane, dry dirt, wet dirt, wood duff...

my question is how do we keep the dirt from sliding off the roof?

From what I saw at Wheaton Labs, the shallower the slope, the better.  I'm not sure what the ideal maximum is and/or if it changes in different climates/soils.

also how do you get all that dirt up onto the roof? and there must be a process for getting the dry dirt to stay up there especially near the edges? the shallower the slope the better, for sure, but near the edges, no matter how shallow the slope is, it would definitely still slip off i'd imagine...

I think it would be interesting to copy the Mississippian mound layering methods.  The engineered layering of the mounds resisted erosion for 100s to 1000s of years in areas with lots of rain....fascinating.  I want to do a bunch of reading about how they did this.  Would be nice to be able to use clay layers to shed water instead of polymer membranes.
The Miamisburg Mound shown here is 70' tall and is estimated to have resisted erosion for the last 2500 years.

Green roof design says that the maximum slope without special methods for holding the soil from slipping is 1:12, or one inch per foot.

Of course they are generally talking about a thin layer of soil on an isolated roof, while a wofati has a several-feet-thick layer of soil that continues all the way to the ground. Slipping off is not possible, though slipping down from the peak would be a concern if the slope is much greater than recommended.

There are no exposed edges to a wofati roof except the upper eave, which would generally have a short vertical wall frame attached so the soil can still be a couple of feet thick up to the edge.
Wheaton labs uses an excavator to move dirt. Whatever machine you have that can reach to the center of the roofed area from its edge would be fine, or if you make the structure strong enough, you can drive right up onto the roof. That would reasonably be limited to a small tractor loader, I would think.

another question i have then glenn would be how are the walls of a wofati built? how is the dirt lined up the wall, especially the dry dirt? wouldn't gravity cause the dirt to just keep mounding up at the bottom of the wall, effectively causing the bottom to have many feet wide of dry dirt while the top only has a few inches wide? how is the dry dirt kept the same width at the bottom and the top?

i can see how if the walls are built up, it is harder for the dirt to slip down/off the roof, but then hence this question about the walls...

... is the dry dirt compacted or something?

also just for reference i'll include the photos in the book to make sure we are thinking of the same design

Angle of Repose.

Soil isn't liquid, it has a lot of friction. On a low slope roof ... it will just sit there. Design or structural elements can be added to the roof to increase the friction, but still allow any water to flow down.

There are tricks to applying the soil in an even layer (one is making little supported sticks that rest on the top layer and stick up vertically a preset height... just add dirt till he top of the sticks are buried).

Mike Oehler recommends sticking to 1:3-4 slope or at least 1:5, to be sure there is a good slope for water removal. If you can either use a large enough sheet of poly or glue pieces properly then the 1:5 should work well enough. I would definitely avoid going flatter than 1:5 pitch though. The soil should be tamped along the side walls as you go, so the soil doesn't settle later and leave cavities. Angle of repose will vary based on your soils, so you may either be adding a few inches between the wall and the edge of the original excavation, or a wider space if the native soil collapses easily.

If you can get a proper ground cover installed and growing on the roof, that can help a lot as the roots will hold the soil together. Certainly there's some settling especially after it rains a bit, so you add soil back up at those spots which get exposed or are pretty thin. A wheelbarrow should work for patching spots, and Mike recommends you wait until the roof is ready for soil before you excavate the uphill patio, so you can shovel out that space and toss the soil right on to the roof. The fewer times you move a given shovel of soil the better.

There's a sort of fascia board at the top edges, where the poly sheet wraps over and then the soil is piled up to that edge. So other than sliding down the roof slope, soil shouldn't be falling off any edges. That said, the thumbnail of Mike's place below shows the issue where you have a vertical exterior surface without a berm, and the soil will settle down and away, exposing that poly underneath. So little wing berms to either side of such a window would help that, as would having that top trim piece stick up a few inches so the soil is held in place. Otherwise rain will wash it right off the top no matter how much you put there.

mark, thanks for your reply, but i think it again illuminates a confusion — the video shows a house almost completely covered by soil on all sides but compare that to the model paul talks about in the book which appears to be built into the ground on none of the sides — and in fact he explicitly speaks against this idea in the books saying to face the front of the house into the hill instead of the back of the house — look at this picture and tell me what you think

doesn't it look like a house sitting on a flat space in the middle of a downslope which is not built into the hill at all? and on the back side has a trench cut out to divert water around the house?

michael beyer wrote:mark, thanks for your reply, but i think it again illuminates a confusion — the video shows a house almost completely covered by soil on all sides but compare that to the model paul talks about in the book which appears to be built into the ground on none of the sides — and in fact he explicitly speaks against this idea in the books saying to face the front of the house into the hill instead of the back of the house — look at this picture and tell me what you think

doesn't it look like a house sitting on a flat space in the middle of a downslope which is not built into the hill at all? and on the back side has a trench cut out to divert water around the house?

Hi Michael,
From my understanding, I think the house in the video was built close to the ridge, and then the roof and any wall that was not window or door was earth integrated (covered in earth). So I guess when the wood/pole part of the structure is being built, it is above ground, then when it is covered in a membrane and then covered in dirt, then it is ‘below ground’. However, it is never really ‘underground’ because it’s not dug in and down and lined with wood, it’s more built up on top of the ground with wood and then integrated with dirt. The required dirt comes from immediately up slope, creating the sun patio.

Paul has always said his wofati design is above grade and you add soil as needed from a nearby spot. Mike’s original design was to dig into the soil and be below grade and you are using the soil in the excavation to cover the roof and sides. There are other design differences as well; Mike’s design is just a starting point for Paul’s design.

The profile pictures simply show the building walls and roof relative to the grade of the hill. Once the walls go up, you then start berming soil up on the sides and once you reach the roof you now have a small hill of soil with a house in the middle. The term earth-integrated might be better than underground for describing it.

Mark Brunnr wrote:Paul has always said his wofati design is above grade and you add soil as needed from a nearby spot. Mike’s original design was to dig into the soil and be below grade and you are using the soil in the excavation to cover the roof and sides. There are other design differences as well; Mike’s design is just a starting point for Paul’s design.

The profile pictures simply show the building walls and roof relative to the grade of the hill. Once the walls go up, you then start berming soil up on the sides and once you reach the roof you now have a small hill of soil with a house in the middle. The term earth-integrated might be better than underground for describing it.

ok, so what you are saying is that if you begin above grade, then piling soil around the house is ok?

it still is confusing and i've included another picture from the book which seems to indicate that any soil banked up against the house is bad and also that the water is diverted around the house, so it isn't really clear how that would be possible if dirt was piled up against the house...

Michael:

Water is a big issue!  The key is the layering ... you can pile up all the dirt you want against the walls so long as it is then covered with a durable, water proof (or at least highly resistant) layer, then covered by more dirt, largely to protect that water resistant layer.   Think of it as a giant umbrella or hat over the structure ... it keeps both the structure dry, but also moves the water away so that it is below the level of your floor (probably...) much like eaves and gutters perform on a house.  The difference here is that the water moves through the outer layer of dirt.

if water is piled up all around the house then how can we make sense of the fact that paul says in the book that we will be looking up at a hill? is the backside of the house not covered in dirt? read the paragraph above the picture to see what i mean...

So you can see out the back of the house (the sun patio and up the hill) through the windows at the back. And you can see out the front of the house (through the windows in the gable) down the hill. But the sides and the roof are encased in dirt, which kind of hugs around the house almost everywhere that isn’t door or window.

This picture from Mike's book:

as linked at Paul's wofati page I think explains the idea, as does the video I posted previously and the video linked below. Both designs feature an "uphill patio", where soil is excavated from the uphill-facing side of the house to create a greenhouse/patio, so that you interrupt any water flowing down the hill before it reaches your structure. Many windows will face out to this area, and Mike recommends additional windows facing other directions for more than 1 light source and balanced light. Meanwhile there is soil piled up and over all the walls and over the roof. Any water coming from higher up the hill never reaches the structure, and rain that lands on the roof has a direct path down slope and away, so there is never a spot where water can pool and work its way into the structure.

I realize this is an old discussion, but if people are still looking at it, it is worth clearing up a misconception I see.

There is mention and drawings of diverting water from the uphill porch around the house to get downhill; if this is by a surface ditch as appears to be indicated, there would be next to no scope for burying the sides in earth. The porch surface (which should be protected from uphill water by a diversion ditch) needs to be drained by an underground pipe running out to daylight downhill of the house. If well built, only the actual porch area will collect water that needs to be drained in this way.

Possibly the very first Wofati roof:
www.smithsonianmag.com/smart-news/colossal-stone-monument-build-1000-years-before-stonehenge-shows-neolithic-engineers-understood-science-180984975/

Any earth homes have one significant risk you should be aware of.
Before you do any steps check the soil for Radon Gas and maybe also for other dangerous gasses..

Timothy Murray wrote:Possibly the very first Wofati roof:
www.smithsonianmag.com/smart-news/colossal-stone-monument-build-1000-years-before-stonehenge-shows-neolithic-engineers-understood-science-180984975/

This image from your link sure looks like a Wofati.

So, for a WOFATI earth roof, keeping that dirt from sliding off is all about the layers and proper drainage. The key is to use a combination of materials and techniques. You’ll want a solid, waterproof membrane at the base to prevent leaks, and then a layer of coarse material like gravel or broken bricks to help with drainage and prevent erosion. Above that, you can add your dirt and wood duff. Make sure you create a slight pitch or slope to encourage water to drain away rather than pool. Adding some erosion control mats or planting ground cover can also help stabilize the dirt and keep it from sliding off. It’s all about layering and making sure everything’s properly secured and drained!