Lessons Learned from Building WOFATIs

I am looking to build wofati/psp type structure this summer and have read through the majority of the threads in this forum. I would like to start up this thread to consolidate all of the lessons learned from the builds thus far into one place. Lots of ideas were discussed prior to building the wofatis and lots of pictures have been posted of the wofati construction, but much of the proposed ideas have not been talked about since construction and some key information is scattered between many threads. Let's see if we can't answer some questions here and make this an essential thread to read before starting a wofati build.


First I would like to discuss the difference in the construction of the roof of the two wofatis.

In wofati 0.7 there are girders set on the posts running front to back, then beams set on the girders running side to side, and on those was set the "decking" logs which will hold the plastic/dirt.


In wofati 0.8 the girders are placed on the posts running side to side and the beams are completely left out of the construction. The "decking" logs are placed directly on the girders running front to back to enclose the space.


Can anyone comment on why the method was changed between builds? The 0.7 version seems more structurally sound with the extra row of beams. The front overhang also seems more solid with the girders protruding out to carry the load, where 0.8 has only the decking logs extending out as the overhang.


Next I would like to talk on the front edge of the roof. Pual brings up his concerns in this thread about what to do with the plastic at the front edge. He also talks about the layers of dirt/plastic/newspaper at the edge on this thread. So I am wondering what the outcome was when deciding what to do about this hard edge? How is the plastic wrapped to keep water out? Does the dirt layer taper down to be thinner at this edge? Does water get in contact with plastic here and flow along the plastic under the dirt? I think this is a critical part to get right when building. Here is a picture of the completed 0.7, but it is not clear as to what happened with the plastic at the edge.




Next I would like to talk about the different layers of materials used in the covering of the roof. Paul talks about layers of plastic, newspaper, dirt, and sawdust in this thread. From the construction pictures of 0.7 it seems like there was a layer of plastic placed directly over the logs, with a layer of dirt over that, then sawdust, another layer of plastic, and finally more dirt. Was newspaper ever used? How many layers of plastic were put down on the initial layer to prevent puncturing from stones/logs? How thick is the first and second layers of dirt? How thick is the layer of sawdust? Is there only one layer of sawdust?





Next is the cob fill that was used between some logs. In Kristie's weekly pics thread there is mention of cobbing between the logs on the construction of 0.8. Here are a couple pictures.



Was this cob used to cover all cracks between logs or just some? If some, which ones and why not others? Was there an issue with 0.7 that prompted the cobbing of the cracks? I see in this picture that some cobbing up of cracks was done from the inside of the exterior walls of 0.7 before the second winter to cut down on the draft. Thoughts on the draftiness due to the spacing between the logs? Should all wall and ceiling cracks be filled with cob?



Next is the heat that escapes through the front, exterior exposed wall and where the wall meets the ground. It is discussed in this thread.
Here is a picture.


Was this issue addressed in 0.8? Paul talked about putting in a straw bale wall in front like this. Is this still being considered or what about insulating below ground to the frost line along the front wall?


I see from this picture that the front wall of 0.7 was insulated. How much insulation was used? Was the entire wall (except the windows of course) insulated?



Next; what was done to keep the buried post ends from rotting? Paul talked about a mix of borax and DE to deter microbes. Was this done, if so how was it applied? Was any gravel/stones put in the hole before setting the posts or are they sitting on dirt?


This is what I could think of right now for points of clarification and discussion. Any talk on these or other issues to consider would be excellent.

Maybe we could get Paul and/or some of the folks who helped build these to shed some light on these details?

I held out answering for a while because I thought surely a bunch of people would jump in and answer. I guess it has to be me.

In wofati 0.8 the girders are placed on the posts running side to side and the beams are completely left out of the construction. The "decking" logs are placed directly on the girders running front to back to enclose the space.

Note that the roof wood is also much thicker.

I think in the future we will want to use smaller poles where possible, in which case the technique in 0.7 would be preferable.

Pual brings up his concerns in this thread about what to do with the plastic at the front edge. He also talks about the layers of dirt/plastic/newspaper at the edge on this thread. So I am wondering what the outcome was when deciding what to do about this hard edge?

This is one of those things where I say what I want, but leave the actual implementation to the person doing the actual work. My understanding is that what was done was a bit different and I don't fully understand why those choices were made. Frankly, I'm a bit concerned about it - but I will wait and see what comes of it - maybe I was wrong and they have something much better.

Was newspaper ever used?

Nope.

Not only did we have a hard time sourcing newspaper, we found an excellent source of free billboard material.

How many layers of plastic were put down on the initial layer to prevent puncturing from stones/logs?

I'm not certain, but I think it is five.

How thick is the first and second layers of dirt?

The first layer is at least 8 inches thick. Of course it is much thicker in some spots - because that defines the umbrella.

The second layer is at least two feet thick.

How thick is the layer of sawdust?

Six inches over the structure, three inches for the rest of the umbrella.

Was this cob used to cover all cracks between logs or just some? If some, which ones and why not others? Was there an issue with 0.7 that prompted the cobbing of the cracks? I see in this picture that some cobbing up of cracks was done from the inside of the exterior walls of 0.7 before the second winter to cut down on the draft. Thoughts on the draftiness due to the spacing between the logs? Should all wall and ceiling cracks be filled with cob?

covered here: https://permies.com/forums/posts/list/80/40371#330160

Next is the heat that escapes through the front, exterior exposed wall and where the wall meets the ground.

I'm told that there is a little something done at 0.8. We have plans for 0.7, but we have not yet started on those.

I see from this picture that the front wall of 0.7 was insulated. How much insulation was used? Was the entire wall (except the windows of course) insulated?

4 inches. Full wall except windows and doors.

Next; what was done to keep the buried post ends from rotting? Paul talked about a mix of borax and DE to deter microbes. Was this done, if so how was it applied? Was any gravel/stones put in the hole before setting the posts or are they sitting on dirt?

Way, way, way, way too much borax was used. When I bought more borax, I made sure they were labeled to prevent that again.

I heard that a few rocks were put into the bottom of the holes with the posts - especially on 0.8 since green wood was used.

paul wheaton wrote:

Pual brings up his concerns in this thread about what to do with the plastic at the front edge. He also talks about the layers of dirt/plastic/newspaper at the edge on this thread. So I am wondering what the outcome was when deciding what to do about this hard edge?

This is one of those things where I say what I want, but leave the actual implementation to the person doing the actual work. My understanding is that what was done was a bit different and I don't fully understand why those choices were made. Frankly, I'm a bit concerned about it - but I will wait and see what comes of it - maybe I was wrong and they have something much better.

I like the finished natural look y'all accomplished with those little vertical boards, but I'm wondering, in what ways (if any) does it differ from Mike Oehler's description of having the poly go over the edge and then be tacked in on the under side, creating a drip edge?

I've tried to somewhat follow Mike's specs for the drip edge on my incomplete wofati eah, and while I'd like to eventually have a natural look similar to 0.7, so far I just have additional tarps protecting the poly. The tarps are held down by a couple of boards that in turn are held down by lots of snow. Since the walls that border on air, (as opposed to dirt,) aren't finished yet, (lots of cold dry air still blows through,) these tarps also help keep snow from getting inside. The project is kind of frozen until spring, but I go out to observe it regularly and the thermometer inside has been hovering around 20F despite it regularly being maybe 10F or lower outside.

Anyway, I think Mike's idea about the drip edge is solid, but I agree about the poly being butt ugly and photodegrading. Also I don't want to puncture it by just tacking boards up, so what about using the weight of the outer layer of dirt (which hasn't been added yet) to hold up a wooden edge cover that hangs down and covers the poly drip edge? This would break down over time of course, but maybe you could use black locust for the part that's buried kind of like sideways posts.

It seems perhaps overcomplicated, and from what I can tell 0.7 seems to have a simpler solution, but is it working to keep water from running down the girders or beams towards the walls?

Nice looking build there Evan! I'd like to see some more pictures of it in the summer time.

This hard drip edge spot is crucial to get right I think. From your drawing there I can see a weak spot I think. I think where the two wood pieces meet, water will drip through there. I don't think you could seal that tightly enough to keep out the water.

Here is what Paul was proposing a few years ago about the hard edge.

I took that drawing and made a little change to it. Instead of having a flat piece of wood over the poly and exposed plastic to the dirt side, maybe angle the wood so the water runs off and have two pieces joined together so no poly is exposed. I understand the wood will deteriorate faster when in contact with the dirt, but maybe it won't be so bad as the end of this piece which is in contact with the dirt is not super crucial. Thoughts on this?

Simon Johnson wrote:Nice looking build there Evan! I'd like to see some more pictures of it in the summer time.

This hard drip edge spot is crucial to get right I think. From your drawing there I can see a weak spot I think. I think where the two wood pieces meet, water will drip through there. I don't think you could seal that tightly enough to keep out the water.

Thanks Simon. I've got a lot of pictures that I need to compile and post sometime soon.

The idea with the wood edge cover is not to keep out water, that's what the layers of poly below are for. It would break down over time and eventually be replaced but meanwhile protect the poly from sunlight. Water would run down through that spot you pointed out and make its way down to the drip edge where it would drip off. The drawing below shows the idea from above. Those vertical lines represent horizontal "posts" buried in the wet earth layer.

I feel like I'm having a hard time getting my idea across... I wish I had access to a sketchup-compatible computer.

I have lots to say, but don't have time today. I'm really hoping that some of the other builders here at wheaton labs will jump in.

Simon's cap idea, especially made from black locust as Paul suggested, would be the best long-term solution to protecting this edge. Even if the fascia with plastic behind it is solidly built of locust, water seeping down between it and the plastic will make it frequently damp (and stay damp for quite some time after every rain) and tend to corrode the fasteners. Depending on how often it rains there, an angled drip edge base behind the bottom edge of the plastic overlap would keep water from ever soaking into the vertical fascia/retaining edge. The original drawing of that retaining fascia completely wrapped in plastic would hold in any moisture and accelerate its rotting.