Wofati engineering questions for Paul - real world applications and experience

The purpose of this thread is to prepare for an upcoming consultation I have with Paul, and it is my hope others who are interested in this type of building will find the information useful. I am in the process of building a Wofati inspired home on my land in central Missouri and ran across an engineering question/problem and needed to talk to one of the few people on the planet who understand and have experience with this type of building. I have been to Wheaton Labs and visited the Wofatis and was truly inspired by them.

I say "Wofati inspired" because my building does not exactly fit Paul's definition, however it is heavily influenced by Paul's Wofatis and the same builders/authors that informed the Wofati concept. My home is a pole structure made from trees cut off my own land (mostly) and it will have an earthen roof and here is where my engineering problem creeps in. I did try to contact local structural engineers but despite multiple phone calls and emails I couldn't get any of them to call me back. Its probably for the better, I suspect they are super busy and are more interested in pressing the easy button (copy/paste) for another stick frame house garage door header than figuring out what I've got going on.

My design calls for posts on a 8x12 pattern for most of the building, so each of these posts are carrying 96 square feet of roof. I am doing my calculations with a 200 lb per square foot roof load. That was figured by adding 1ft of dry dirt at 80 lbs per sqft, then insulation and waterproof membrane, then 1ft of wet dirt at 100lbs per sqft. then adding central Missouri's snow load of 20lbs per sqft. Total is 2 feet of earth on my roof and 200lbs per sqft of a roof load. For a single post carrying 96 sqft of roof, multiply 96 by the 200 lbs and you find that that post is carrying 19,200 lbs of weight. I have read Mike Oehler's book on the subject, and John Hait's on annualized solar. I had done all this engineering about 5 or 6 years ago when designing the structure to figure out how big the horizontal logs had to be to carry this roof and I feel really solid in those numbers.

Like I said I am in the process of building, I've got 13 posts in the ground and am trying to make progress. My conundrum arrived last week as I was putting in a vertical post that was a little smaller in diameter than some of the others so I thought I would double check it could hold the weight. I turned to my new structural engineers Google Gemini and Grok. These tools were not around when I first started working on the design but I now use them to double check stuff. I asked the AI how much a weight 7 inch diameter white oak post could support, and after a few back and forth questions arrived at the conclusion that the post could indeed carry the weight. THEN it asked me if I wanted it to design a footer for this post to sit on. This had not occurred to me at all. Oehler's book doesn't mention it other than (paraphrasing) "if you want to, toss a large rock in the hole". I've scoured the Wofati build threads on permies and it seems that they just put posts in the ground. But now AI is asking if I want a footer designed, so I say "sure, design a footer for 19,200lbs" This thing comes back and says I need 42" by 42" concrete pad 10 inches thick under every post! What?! No! My soil is basically rock and clay so I tell it that and it amends it's recommendation to a 31" by 31" concrete pad under each post. Gemini claims that without the footer to spread the load and this kind of weight pressing down on that small of an area, that my post is just going to sink into the ground like a giant nail. But Mike and Paul don't have footers so what gives? So I stop working and decide to ask someone who has experience in this arena.

I've since revisited the threads on the Wofati builds and discovered that 2 of the posts at Allerton Abbey had indeed sunk but there's a lot more than 2 posts in that building, what was it about these 2? No mention of the posts at Cooper Cabin, everything there seems fine. So I decided to get Paul involved and talk it out and see what he has to say about this. Follow along if you're interested.

Here is a picture of one of the posts, you can see the rocky spoils from digging the hole to the right of the post. I'm digging with a 4 ton mini excavator and it has a hard time getting through this stuff. My process so far has been digging a hole, then putting in a few inches of crushed gravel and compacting it, then placing the post on the compacted gravel, then backfilling around the post with more gravel for drainage.

Another picture shows some progress of the build.

Interesting question. I know that for pole barns, they calculate the weight per post and then often times pour concrete piers of bigger diameter than the posts, depending on weight and soil type.

Mike Oehler, in his video series, talks about putting the posts 6-8ft apart, using 6ft as the default. And he only puts 4 inches of dirt on top, as the roof. So in his designs this doesn't seem to be a problem to worry about. But adding way more dirt, and spacing the posts farther apart..

Join us Thursday September 10 at 2pm mt time!

We are about to begin!

Very interesting discussion. I would love some updates or youtube videos of the build! Super cool to see someone reproduce the design in another climate/geography. I'm excited to see how it goes.

Beyond the sinking effect, are you doing anything to the posts to protect against moisture/rot over time beyond the gravel sock? Any charring/treating?

Beyond the sinking effect, are you doing anything to the posts to protect against moisture/rot over time beyond the gravel sock? Any charring/treating?

I am. I 'm treating the base of each post with a solution made from borax and glycol. Borax is anti fungal, toxic to insects, and available in the laundry soap aisle at the grocery store. The glycol helps the borax penetrate deeper into the wood than it would with just water. Mostly relying on keeping them dry under a roof with 20ft overhangs. I first encountered this recipe when I took a log home building class from the LHBA and I have since found out there are some commercially available wood preservatives that are essentially glycol and boric acid.

Summary of the consultation as it pertains to posts in the ground and whether or not they need a footer under them to spread the load.

Typical structural engineer assumes you are installing the post at the surface not in the ground, also assumes the acceptable amount of settling movement is zero. To that end a very large concrete footer is required to spread the load out over a large area. Paul doesn't like concrete. I also would like to avoid it.

Paul's position on pole structures is that once in the hole, the post can only move down two ways. One if it compresses the soil beneath it (not likely in that case of a well compacted rocky subsoil) or two, if the material beneath the post is able to move out of the way to the sides allowing the post to sink (again not likely in a well compacted rocky subsoil). Paul was asking if there were tree roots down there that could potentially be air pockets in the future which would create the opportunity for settling. I haven't seen any in my digging at this depth. The house site is an already excavated hillside cut to make a level spot and uphill patio and then I am digging further beneath that to set my posts. I am also okay with some minimal settling, like 1 inch.

They are currently installing posts at Wheaton Labs the way that I am doing it here. Dig a hole, a few inches of crushed/angular clean gravel in hole (no fines), compact the gravel, install post, compact post with excavator, refill remainder of hole with gravel and compact that around the post. They refer to it as the gravel sock.

Paul suggested I watch the Wofati greenhouse movie since it contains everything they have learned from the first two Wofati's and he feels really proud of that build quality. Good news, I supported the kickstarter for it and already have access to it! Only reason I didn't watch it already is because I'm not building a greenhouse yet, but I'll watch it now.

Other stuff was talked about too but that's the basic nitty gritty on posts and whether or not they need a footer under them.

When the large footer was suggested I got nervous about how to proceed but I feel confident now about how I've been doing it and moving forward with that plan.

I wonder if it makes sense to put a small mark on the posts. That way you could get an objective & precise measurement of any settling?

Yes good idea. I will set up a laser level and mark all the posts before I put the dirt on the roof and we'll see what happens.

As I'd alluded to in the comment on the Livestream YT video, here's the scan of the table of soil bearing capacities from Seddon's 1985 book "Practical Pole Building Construction".

On further edit, here's the link to the livestream, for ease of access:

My comment shouldn't be too difficult to find there , but here's the copy/paste of what I wrote there, to make this thread more stand-alone, should the YT comments evaporate or something:

"I think there's likely a units problem in the soil bearing capacity specs being quoted, though I don't know where the error crept in.

I checked my copy of "Practical Pole Building Construction" by Leigh Seddon (mine is the first edition 1985 copy write).  In Table 5-2 (page 57), the soil bearing capacities range from 1500 to 6000 pounds PER SQUARE INCH, depending on soil type.  For the rocky soil shown in the photos, I'd think the bearing capacity would be close to the higher limit, but a soil analysis - even just shaking up some of the soil with water in a glass jar and letting it settle for a few days - might give a more definitive answer regarding soil composition and thus allow better inferences to be drawn regarding the safe bearing capacity value for this design.

In any case, I think the 1500 pounds PER SQUARE FOOT is off by a factor of 144 (that's how many square inches are in a square foot), and probably by some additional factor due to the local soil having higher bearing capacity than poorly drained plastic clays and silts to which the lower 1500 psi value applies.

Seddon says that typical pole loading is usually between 6000 and 12000 pounds.  So, the 19000-ish pounds is high, but not crazy high.  A large enough diameter pole of a decent (structural) grade should be able to safely take that load.

Many historic New York City high rises are built on wooden pile foundations, "driven to refusal", somewhat like pushing down on the poles with the excavator, though a pile driver is typically used when driving piles for foundations.

I'll try to post a scan of Seddon's Table 5-2 on the Permies forum in the thread, for reference."

Hopefully this is helpful to someone, whether building a wofati, boat house or pole barn.