Water vs. Structures on slopes

I was reading another recent thread and came across the image below. Not my first time seeing it, as I believe I read the original one or two posts on Wofati.

wofati concept from'build a better world book'

I intend to eventually build a structure whose side elevation will resemble the rendering on the left, so I've thought about this image for a while.

The thing I don't understand is why, in the rendering on the right, water can be simply diverted around the structure, but it can't be on the left--why wouldn't the left-side structure-builder use whatever those same means are to divert water?

Even with the excavated foundation, a well-planned and -executed French drain ought to work. And of course you'd have a gutter at the low end of the roof, with a downspout that channels water off to the side where it can continue downslope unhindered. And there are other tricks we've got up our sleeves as well, like sump pumps. This should all work especially well if the structure is not monstrously large.

And of course there's no reason you need an excavated foundation; you could instead build up a foundation on the downslope side with a retaining wall and backfill, a design which I'm favoring lately:

Am I missing something?

Hi Ned, it is my understanding that the first image of the Wofati is showing how to solve the problem of water intrusion with an underground structure on a slope. Your picture solved the problem by raising the whole structure above ground.

Ned Harr wrote:
The thing I don't understand is why, in the rendering on the right, water can be simply diverted around the structure, but it can't be on the left--why wouldn't the left-side structure-builder use whatever those same means are to divert water? ...

Am I missing something?

I believe this has something do do with how the structure is placed on the lot.

The structure on the left has been place below grade causing the water to run onto the structure basically the roof.

The structure on the right was place higher on grade so that the water runs off before getting to the structure.

When ever we have built a structure we find the highest elevation to build the structure thus avoiding water running into the structure.

Ned Harr wrote:The thing I don't understand is why, in the rendering on the right, water can be simply diverted around the structure, but it can't be on the left--why wouldn't the left-side structure-builder use whatever those same means are to divert water?
Even with the excavated foundation, a well-planned and -executed French drain ought to work. And of course you'd have a gutter at the low end of the roof, with a downspout that channels water off to the side where it can continue downslope unhindered. And there are other tricks we've got up our sleeves as well, like sump pumps. This should all work especially well if the structure is not monstrously large.

I think the point is that the lower the structure, the more and complex are the design features that need to be built in to compensate for the water ingress risk. Simple (minimising the risk) is generally better and will probably end up cheaper.

And of course there's no reason you need an excavated foundation; you could instead build up a foundation on the downslope side with a retaining wall and backfill, a design which I'm favoring lately:

I think if I were building this, I'd be tempted to make the space underneath useful space - root cellar perhaps? rather than backfill, unless I had surplus soil to get rid of.

Nancy Reading wrote:

I think if I were building this, I'd be tempted to make the space underneath useful space - root cellar perhaps? rather than backfill, unless I had surplus soil to get rid of.

So far my idea has been to do an earthen floor on that level with the backfill underneath, acting as thermal mass. I would definitely do this if the site topography is such that there wouldn't be much useful space otherwise (without excavating). But I have had that thought about making it a root cellar or something, in the event there is enough room to justify it, and to use some other means--possibly involving water?--to make a thermal mass under the floor.

Anyway, regarding this:

I think the point is that the lower the structure, the more and complex are the design features that need to be built in to compensate for the water ingress risk. Simple (minimising the risk) is generally better and will probably end up cheaper.

That makes sense to me too but then the same logic applies to Wofati. Why build an underground house at all when above ground is always simpler when it comes to water intrusion?

Ned Harr wrote: Anyway, regarding this:

I think the point is that the lower the structure, the more and complex are the design features that need to be built in to compensate for the water ingress risk. Simple (minimising the risk) is generally better and will probably end up cheaper.

That makes sense to me too but then the same logic applies to Wofati. Why build an underground house at all when above ground is always simpler when it comes to water intrusion?

My understanding of current "best practice" is to build on flat land, then berm up and over the structure for thermal mass. If the structure is an "artificial hollow hill", the water will run off in all directions.

Our house is a badly designed "uncovered artificial hill". My understanding of best building practices is that the ground should always slope away from the building in all directions. Sigh... the north side of our house does this poorly, and we often get water coming into the back workshop, which is an unheated enclosed area.

It's important not to underestimate how far water will travel underground. Our upper field rarely has puddles if it's not actively raining. We don't see the water actively running down the hill. But we know that it's running underground by the wetness halfway down the hill and collecting in every pothole from there, all the way to the winter creek.

Info I got recently does suggest that the type of soil is critical. There are examples of partially, or wholly underground houses dug by human ancestors, but they were in very specific locations, and much of that knowledge has been lost. If my source was accurate, choosing the wrong location resulted in being flooded out, or having things collapse during spring run-off season.

As a Civil Engineer, I would come at the issue differently.
I firstly will ask, what are you aiming to achieve at the end?
Is it a home, do you want a big window, do you want thermal mass, do you want an underground stucture.
I may have some ideas if I know the asnswers.

As Jay has said, water travels underground and your fill area may get very damp.
People tend to not maintain anything and you may find leaves, branches build up against the outside wall, creating moisture and drainage issues.
There is a reason Wofti buildings have not taken over the world.
What size is this structure going to finish up?

Ned Harr wrote:

The thing I don't understand is why, in the rendering on the right, water can be simply diverted around the structure, but it can't be on the left--why wouldn't the left-side structure-builder use whatever those same means are to divert water?

And wrote:

Why build an underground house at all when above ground is always simpler when it comes to water intrusion?

From my reading, there are two big issues, and a bunch of small ones affecting these two questions.

1. Thermal mass frequently makes buildings more comfortable to live in, and more efficient to both cool and heat.

So my last post, where you build on flat land, then cover the building with soil, followed by an "umbrella" to keep that soil dry, followed by more soil that can have plants growing in it, gives the house thermal mass that may mean normal human activities are enough to keep it at a comfortable temperature year round.

2. Water intrusion can be avoided with good engineering. However, all that dirt in a nice mound shape, is great for keeping the house safe in bad weather like tornados. I once saw an image of a community after a hurricane. Almost everything was flattened except a dome-shaped out-building.  If the building is positioned to encourage winds to go over and around it, I even wonder if the wofati at Wheaton Labs would survive a fast forest fire. I'm betting they'd have better luck than stick-built with plastic siding!

3. Specifically the image on the left you refer to, and the modified image you drew.
First post left: All the water coming down the hill, and all the water coming off the roof, *all* of it is going to put pressure on the right hand, built into the hill, wall.
Your modified picture is definitely at less risk, but I once saw a house built in a similar situation. At great expense, the owners had dug out a bunch of the hill, and installed a large retaining wall holding back a large ditch to keep much of the water from approaching the house. I've since seen other properties where I could see that retaining walls and diversion ditches had been placed higher on the hill in stages in an effort to keep the water from sinking in and threatening the wall or basement of the building downslope of it.

4. Lastly, generally, the more earth you're going to need to move, the more expensive the building will be. Sometimes, there's simply no flat land around (much of my Province - "our mountains are very pointy" to quote a song).  In that case, not all slopes are created equally. Some slopes are much less stable than others. Some land truly should *never* be built on (Deltas in earthquake country strikes me as one.) Some areas get minimal rainfall - but be cautious because that 100yr or 1000yr storm could be a doozy.  Some dangers can be managed. I saw a number of houses in Kauii built on tall stilts that provided shade below and was above any Typhoon storm surge. There are specialist engineers whose whole job is figuring out what needs to be done to build safely on a specific piece of land. Please don't assume that something's safe, just because everyone else is doing it. Some people have a very short time focus. They don't think longer than 10 years, which is not what I would want to do if I was building a house to live in.

Earth's climate changes gradually over time. We currently show signs of the weather getting "more enthusiastic" - bigger hurricanes, more tornadoes going further north, more heat domes and atmospheric rivers. If you're going to put the time and energy into building, I would plan for future weather, not past records.

John C Daley wrote:As a Civil Engineer, I would come at the issue differently.
I firstly will ask, what are you aiming to achieve at the end?
Is it a home, do you want a big window, do you want thermal mass, do you want an underground stucture.
I may have some ideas if I know the asnswers.

As Jay has said, water travels underground and your fill area may get very damp.
People tend to not maintain anything and you may find leaves, branches build up against the outside wall, creating moisture and drainage issues.
There is a reason Wofti buildings have not taken over the world.
What size is this structure going to finish up?

Strictly speaking I think my original questions were answered (mainly by Jay Angler*) and I consider the thread "done", but answering your questions will be fun so I'll go ahead:

"what are you aiming to achieve at the end?"
A small (but not "tiny") home, which I will build in around 10-15 years from now, and take full-time occupation of when I'm retired. I want it to have great views, and feel secluded. I had a dream once about a house built onto a slope and although I don't think I'm chasing that dream exactly, the feeling of being in that house has stuck with me.

"Is it a home"
Yes.

"do you want a big window"
I was thinking, a big south-facing wall with multiple normal-sized windows in it. If the windows take up only 33-50% of the wall area that's fine.

"do you want thermal mass"
Yes, but earthen floors are only one possible way I was planning to get it.

"do you want an underground stucture."
No.

"What size is this structure going to finish up?"
I envision it as being in the range of 750-1250 sqft.

*Jay, thanks for your comments. Some of them I have already thought about, some I will now be thinking about. Very helpful!

Thanks NED.
Some ideas;
- cut a wide swale up the hill say 10 feet wide that will not fill with leaves etc quickly.
- have its low point below your proposed floor level, say 12 inches if possible.
- drain the roof to one side of the building to a water tank, 5000 gallons. So it does not add to the amount of water at the rear,
- build the retaining wall either as a dry stone wall or poured reinforced concrete wall with a heel and toe.
- install a membrane on the floor of the filled area, with drains under that leading sideways on a slope.
- insulate the space if need be
- fill that volume with aggregate, plastic drums of water or broken concrete and larger stones and rubble. That may work as a heat bank as well.
- pave the ' floor' to draw in heat to the bank.

Actually Jay, now I'm thinking more about what you wrote in your latest comment and wanted to respond to each numbered point you made:

1. I like thermal mass too--it's been an integral part of my planned design since I started dreaming it up--but I want to insulate the envelope of my structure very well, so I have come around to wanting that mass inside, underneath, or otherwise "enclosed" by the structure.

2. I like your thinking on tornadoes and such. I want to build this house in a place where I believe the most likely natural disaster would be a forest fire. But for reasons discussed upthread (and below, see #4), I don't want to build underground or pile up earth around it until my house is effectively underground. So I am thinking about perimeter earthen walls (at a distance from the house itself) with ceramic tiles embedded in them. These would absorb heat (just like the Space Shuttle's heat shield) and block flying embers. Meanwhile I would reduce the amount of flammable vegetation growing near the house, probably by hardscaping or something like that. These ideas came from some post-analysis of houses that survived the recent fires in Los Angeles, and another house that survived a forest fire in the PNW.

3. Yeah, this is what I think about too; the horizontal movement of water outward from the slope toward the retaining wall. I wonder if a waterproof barrier between the grade and the backfill would help with that? And/or weep holes in the retaining wall so that if water does get behind it, it can easily drain out?

4. You mention that moving earth is expensive, and I agree; but that seems like a huge mark against the Wofati structure. Moving enough earth to backfill a short retaining wall has got to be way less expensive than moving enough to berm the whole house including its roof, not to mention the extra materials and care required to allow the walls and roof to withstand constant direct contact with earth.

John C Daley wrote:Thanks NED.
Some ideas;
- cot a wide swale up the hill sau 10 feet wide that will not fill with leaves etc quickly.
- have its low point below your proposed floor level, say 12 inches if possible.
- drain the roof to one side of the building to a water tank, 5000 gallons.
- build the retaining wall either as a dry stone wall or poured reinforced concrete wall with a heel and toe.
- install a membrane on the floor of the filled area, with drains under that leading sideways on a slope.
- insulate the space if need be
- fill that volume with aggregate, plastic drums of water or broken concrete and larger stones and rubble. That may work as a heat bank as well.
- pave the ' floor' to draw in heat to the bank.

Thanks! These ideas were exactly what I was thinking of doing.

Ned Harr wrote:1. I like thermal mass too--it's been an integral part of my planned design since I started dreaming it up--but I want to insulate the envelope of my structure very well, so I have come around to wanting that mass inside, underneath, or otherwise "enclosed" by the structure.

Absolutely! My son's house plan has a fireproof exterior layer, mineral wool insulation then thermal mass for the walls. There are special standards for this sort of building which they're using for guidance. There are pros and cons of the material they've chosen, but the general goal is for it to be a multigenerational home that will last a minimum of 100 years. Let's hope it works out!

2 ... I want to build this house in a place where I believe the most likely natural disaster would be a forest fire. ... So I am thinking about perimeter earthen walls (at a distance from the house itself) with ceramic tiles embedded in them. These would absorb heat (just like the Space Shuttle's heat shield) and block flying embers. Meanwhile I would reduce the amount of flammable vegetation growing near the house, probably by hardscaping or something like that. These ideas came from some post-analysis of houses that survived the recent fires in Los Angeles, and another house that survived a forest fire in the PNW.

Yes - there's some good information from California recently, and also from Australia. I can remember when large parts of California burned back in the '70's and yet they kept building using the same old, same old. A community near us *finally* acknowledged that cedar shake roofing was a bad idea in high fire risk areas, but I'm not sure people are paying attention to how bad plastic siding is.

John C Daley will always suggest *very* large water tanks. If you think you can fill one, and fire is your greatest risk, just think of how being able to wet down your property all around your house will help your sense of security! That is definitely part of the plan for my son's house also.

Jay Angler wrote:John C Daley will always suggest *very* large water tanks. If you think you can fill one, and fire is your greatest risk, just think of how being able to wet down your property all around your house will help your sense of security! That is definitely part of the plan for my son's house also.

VLWTs are part of the plan. But even if for whatever reason I don't build them, I also hope to (eventually) build a swim lane-shaped pool on the south (downslope) side of the house.

See Mike Ohlers 50 dollar house info
https://thehomesteadsurvival.com/mike-oehler-and-his-50-dollar-underground-house-take-the-tour/
His book is inexpensive. He explains the water drainage issues.

I don't really know anything much about building, but I do live in the high elevation areas of the Appalachian mts. and I observed what happened during Hurricane Helene.  I saw way too many places where the saturation of water was at its max, and the mountains answer to this was to simply lift all of the topsoil and vegetation clinging to it up off the bedrock of the mountain, which in this case was absolutely beautiful green and pink rock called ukanite, and slide it down the mountain.  This left gorgeous little creeks running over beautiful rock, but whatever, including buildings, was in the way of this lifted mass, just went with it.  So, if you hit a wicked crazy rainy season, would having those ditches that run horizontally across the mountain--  I know they have an official name, I have one across my pasture that helps keep too much water from running down and causing a deep rain creek right outside my back door, but I know they're a thing up here in these mts.  People (like me) have them across their driveways and wherever they think water might run and take the land with it.  So, are you planning any of those up above your structure?

Brenda, I just dont understand what you are saying?
Did buildings and vegetation get shifted and blown down the mountain?
As for the drains, are you talking about ;
- culverts
- swales
- ditches please?

My take on what Brenda is saying is, "don't build on mountainsides". I am not seeing how the type of construction or water mitigation techniques would matter, it's all gonna get washed downhill if there is a once-in-a-millennium rainstorm.

Ned Harr wrote:My take on what Brenda is saying is, "don't build on mountainsides". I am not seeing how the type of construction or water mitigation techniques would matter, it's all gonna get washed downhill if there is a once-in-a-millennium rainstorm.

Just about anything can get taken out in a 'once in a millennium' event if you happen to be in the wrong place.

Humans *love* valleys by creeks. They get water and shelter. They built houses in Ravines in the Toronto area and were fine for decades. Then Hurricane Hazel picked up strength over Lake Ontario and hammered the area. The government of the time, banned any future builds in those ravines and turned them all into public parks. The ravines were created by exactly that sort of high water event.

Hurricane Helene dumped a lot of water the mountains weren't used to. If the water found a weak spot - generally an area disturbed at some point by logging, the water loosened the soil which loosened the rocks. "Mud" thundering down a slope will seek the same low spots that creeks do and it is waaaayyyyyy more damaging, due to mass alone, than a flood. Mountain "flood plains" are often more subtle than on flatter ground. My province lost tons of housing during a huge "atmospheric river" event because the water had no where else to go. Many people didn't grok that they were, in fact, on a mountain flood plain and I'm amazed you didn't loose many more lives than you did during H Helene.

I think we need to have better geography teachers in our schools. I got lucky, and remember some of the features ours taught us about soil creep, river building, etc. There's lots of geology information on the web. Nick Zentner does some good stuff.  

John, it may sound alien to someone from an ancient, mostly dry continent, but many areas such as Appalachia in the eastern US are full of steep rugged terrain, and when an extreme rain event hits, it can percolate to bedrock, loosen the bond of many feet/meters of soil, and let the entire hillside (sometimes with trees and buildings intact) slip downhill to crash into the valley below and jumble everything into a heap, or bury houses in the valley.

The original question about facing of the roof slope on the side of a hill, along with Mr. Daley's presence here, made me think of a Simpson's episode. The family travels to Australia and is forced to seek refuge in the US embassy. A diplomat proudly shows them the evidence of US ingenuity used to bring the comforts of home to down under with a highly complex, powered toilet that makes sure wastewater is reversed to exit the unit in an anti-clockwise direction.


Given access to enough resources, practically any 'design flaw' can be compensated for. But if designing from scratch, why not utilize the best ideals from the start?

I'm guessing at the motivation here- is this a south facing slope where you are seeking greater solar input to reach the taller wall under the high point of the roof with?

Coyden thank you for that insight!
Also to Glenn and Jay.
Glenn, not many have insight the the geological details of Australia, how did you get them?

John, I read. A lot. Anything that comes into my hands. Especially anything to do with history, geology, archaeology, paleontology...

should add that not many here understand what you are taking about. We have one town that is 195 years old, thats old for here, and its been flooded
196 times, sometimes to 40 feet!!

Coydon Wallham wrote:I'm guessing at the motivation here- is this a south facing slope where you are seeking greater solar input to reach the taller wall under the high point of the roof with?

Partly that, and partly so one can look out from inside and see lots of outside instead of lots of ceiling. There are a few other considerations in there as well but those are the two main ones.

Given access to enough resources, practically any 'design flaw' can be compensated for. But if designing from scratch, why not utilize the best ideals from the start?

I hear ya. I think all architecture is gonna fight physics in some way; nature wants to put your materials back the way they were, but humans want to put those materials into an arrangement that lets us feel like we're taking a break from nature or at least from part of nature. Even the most aikido-ed architecture is going to eventually leak and crumble. Thus all architecture is bound with technology--that is, the expression of intent in opposition to the received environment. The question is how hard are you willing to fight to get what you want.

In this case, I don't think it would be all that much harder to keep the lowest levels of a slope-built house dry, it just requires the application of a few time-tested engineering tricks. We're not making toilets swirl backward, and we're not trying to support a space station, we're just building stuff like retaining walls, putting down barriers under the backfill behind those walls, and maybe installing some french drains or digging some swales. These aren't costless and they aren't guarantees but they're known to work, they aren't difficult or super expensive, and they'd likely be needed no matter which way the roof is sloping, given the same building site.

Ned Harr wrote: In this case, I don't think it would be all that much harder to keep the lowest levels of a slope-built house dry, it just requires the application of a few time-tested engineering tricks. We're not making toilets swirl backward, and we're not trying to support a space station, we're just building stuff like retaining walls, putting down barriers under the backfill behind those walls, and maybe installing some French drains or digging some swales. These aren't costless and they aren't guarantees but they're known to work, they aren't difficult or super expensive, and they'd likely be needed no matter which way the roof is sloping, given the same building site.

Exactly - it's a matter of admitting to the dangers and working *with* Nature to avoid your home being one of the casualties. The very first image you posted, shows what happens to people who gleefully ignore the water risk and have rain pool by their back wall and then infiltrate. My engineering family has taught me that water is *very* strong. Strong enough to crush swimming pools in the winter if they aren't "winterized" properly. Strong enough to wear through copper pipes if you give it 60 years to do so. Strong enough to move boulders down river under flood conditions.

The exact building site and the "storms of the millennium" are the things to guide you. If it were me, I would want the solutions to be multi-layer and robust. Many slopes can be stabilized just by choosing the right trees to plant for the ecosystem, and making sure that the water has safe routes to follow.  Some famous person said, "Belt and suspenders - I like your thinking!" Where my house is concerned, I'd be thinking belt, suspenders, double buttons, and a back-up chunk of rope. I've lived through at least 3 'storms of the millennium' since moving to my current house. I am well aware of the assumptions the original owners made and how it's not worked out. Hind-sight 20/20 here. Scientific studies show that our planet has gone through many "eras" and Earth's climate has changed significantly from era to era. I trust the man who told me 40 years ago that we are entering an era of the weather being much more energetic. If I was in a position to build my own home from scratch, I would be basing decisions on that information, because I've seen with my own eyes, that the man was right.