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Microhome Addition

 
pollinator
Posts: 161
Location: South Central Kansas
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I’m building a 12x27 addition to our 320 sq foot microhome to provide a bedroom, bathroom, and living area. This is just about the bare minimum for us to live through the cold winters while we build a new home.

My desire is to use site-harvested materials as much as possible. The current design is comprised of roundwood Eastern Red Cedar post and beam supporting a 1:12 living roof. I’m leveraging wafati technology, included post-in-ground, rebar pins, wide eaves, and umbrellas of billboard tarp. Walls, I plan to use straw ale and cob, as well as ERC siding. To support the walls I am digging a rubble trench with some sort of cap.

I have two major questions - the posts are going lower than the rubble trench - 9 out of 10 post holes stay very dry, while one seems to be more damp. Although I am providing drainage in the trench, I am concerned that moisture will follow the post hole and hasten deterioration of the post. The base of the trench and the posts is in dense clay subsoil. I have redundant structural support in the post and beam and strawbale cob, so I am not concerned about collapse, but I would like to do all I can to preserve the structure for the long haul.

My other question is if there are site-harvested materials that can be used in place of gravel or road base, both for the post hole drainage base, and for the earthen floor base layer. I have almost unlimited sand at my disposal, and a number of 100-year-old cinder blocks. Is there an alternative to gravel for a long-term, stable drainage base?

Other info: site is nearly flat, depending maybe 3 inches in the 27-foot span east-to-west. Addition is on the north side of original slab-on-grade construction. Top-soil is compacted sandy soil. We have a diverse woodlot and sawmill available. I am focusing on ERC because of its merits as a rot resistant wood, it’s ease of use, and the fact the we have many, many standing dead or recently fallen due to crowding.  My main priorities are low cost, self-built, lasting, and fast. I know that mix of factors doesn’t really exist (per fast:cheap:quality) but you get the idea. It has to be usable by November, and I can continue working on finishing detail and interior in the winter.

Any thoughts appreciated! I’ll post progress.
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Excavation with the tractor bucket
Excavation with the tractor bucket
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Some small standing dead ERC coming out of the wood lot.
Some small standing dead ERC coming out of the wood lot.
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Example of where trench meets post hole in clay subsoil. This is the low point of the site.
Example of where trench meets post hole in clay subsoil. This is the low point of the site.
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Post hole. Upper 18 inches are soil, lower 18 inches are clay.
Post hole. Upper 18 inches are soil, lower 18 inches are clay.
 
pollinator
Posts: 3585
Location: Kansas Zone 6a
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I am NOT a fan of putting wood in contact with the ground in most situations, especially clay, unless you have perfect drainage.  The easy answer is a concrete pier.  If you are opposed to concrete, you can cut stone posts to go in the ground or build a rubble trench and build it above grade.  At least for the troublesome one.  
 
pollinator
Posts: 1293
Location: Bendigo , Australia
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I agree with R Scott.
I would concrete a steel stirrup and mount the posts to each stirrup.
The stirrups can be fabricated to suit the posts.
What floor are you going to have?
 
Beau Davidson
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Concrete piers were my original plan, until I got inspired by the bermshed etc. but I think you’re right, that this site is not a prime candidate for wood-in-ground. Piers it is!

I’m planning an earthen floor, either over gravel or road base, unless I discover another lower impact and local base layer alternative that promises good drainage.
 
John C Daley
pollinator
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You may need to lower the floor area if an earthen floor is used, they need a fair bit of depth for the layers.
 
Beau Davidson
pollinator
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John C Daley wrote:You may need to lower the floor area if and earthen floor is used, they need a fair bit of depth for the layers.



I’m currently 14 inches below the floor of the existing structure. My plan is to build up 4 inches of gravel or other drainage/stabilizing media, then add 3 inches in subsequent layers of earthen floor, to ultimately arrive at a plane 7 inches lower that the original floor. I’ll incorporate some wooden threshold at the connecting doorway to step down. That’s the plan, at least.
 
John C Daley
pollinator
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It does not look that deep!
Can you fabricate the stirrups yourself?Some hoop iron diagonally across the roof frame both ways should be a great help in creating lateral stability of the new posts.
 
Beau Davidson
pollinator
Posts: 161
Location: South Central Kansas
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John C Daley wrote:It does not look that deep!
Can you fabricate the stirrups yourself?Some hoop iron diagonally across the roof frame both ways should be a great help in creating lateral stability of the new posts.




Hoop iron reinforcement is an interesting idea, new to me. After a brief internet search - Is there a strong advantage over timber cross-bracing? I suppose they are easier and quicker, for one. And can be embedded into the wall and hidden if placed properly. I have some learning to do.
 
John C Daley
pollinator
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Location: Bendigo , Australia
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The flat metal stip lays flat and is quick to be installed.
We even can use a small device to tighten the strap, its 'U' shaped with a bolt and the 2 'U' shaped parts are pulled together by the bolt, very clever.
 
Beau Davidson
pollinator
Posts: 161
Location: South Central Kansas
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Made a little progress last week before heading out of town. 7/10 posts up on concrete piers with rebar pins. Excavated another 3-5 inches to get down to 14 inch minimum depth so I’ll have more to work with when layering the floor. And lots of thinking and figuring.

My favorite idea for roofing method is to make structural panels with straw and wood chip infill, inoculated with reishi spores. The goal would be to foster the complete colonization of the substrate with mycelium, then dry it out and install, creating a biological insulation layer. But alas, I will probably have to wait on this until a future project, with time to experiment and develop. As it is, I’m still considering making insulated panels for ease of installation over the structure. To maximize headroom I’m thinking of a 1/2 in 12 pitch. I found some rolls of that durable, dimpled foundation barrier that I’m thinking of using over the insulation as a redundant moisture barrier atop my billboard canvas. It will add a pocket of air which I hope should solve for condensation. Over that, I’m considering options for UV protection. We have limitless creek pebbles/sand, so I’m wondering how suitable that would be as a living roof substrate - might welcome some very hardy perennials, but deter most other volunteerism.

Have any of you had any experience making your own insulated panels? Any insights welcome.
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Peeling logs.
Peeling logs.
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More peeling logs. Went for the chainsaw attachment rotary planer. Pretty pleased with speed, although it is very aggressive.
More peeling logs. Went for the chainsaw attachment rotary planer. Pretty pleased with speed, although it is very aggressive.
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Peir with 1/2 inch rebar pin
Peir with 1/2 inch rebar pin
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Log base, concave for good contact with concrete pier.
Log base, concave for good contact with concrete pier.
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Top shot of one post. These are minimum 7 inches diameter at their most narrow.
Top shot of one post. These are minimum 7 inches diameter at their most narrow.
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As it sat before I had to leave town last week.
As it sat before I had to leave town last week.
 
Beau Davidson
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Location: South Central Kansas
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Here’s a quick sketch of the current plan. Still flexible, some aspects very much a work in progress.
image.jpg
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master pollinator
Posts: 1524
Location: southern Illinois.
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I realize this is pretty late in the game.  When I lived in 12 x 32, I decided to go up with thec12 x 16 addition.  I avoided all the foundation questions. The ceiling joists of the original house were more than adequate to serve as the floor.
 
Beau Davidson
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The biggest questions I have at this moment are regarding the roof cladding and insulation materials.

1) Is making my own SIPs a feasible and economical method? I’ve started trekking with Peter’s thread (https://permies.com/t/120/141249/Tiny-house-build-Hokkaido) and y’all have helped come up with some very cool ideas.

I don’t need them to be as high r-value as polyiso as we have a relatively short cold season and abundant wood. I also don’t need them to be water tight as they’ll be under a membrane. My father-in-law manufactures metal-clad panels that are very high performing and form a suitable roof in their own, so he is laughing at me right now. Different goals and philosophies.
2) If panels are feasible, what are some suitable materials (points for stuff I have or can harvest or obtain locally). Some possibilities are straw, wood shavings, and maybe millet hulls.
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passionfruit in bloom in the garden.
passionfruit in bloom in the garden.
 
Beau Davidson
pollinator
Posts: 161
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John F Dean wrote:I realize this is pretty late in the game.  When I lived in 12 x 32, I decided to go up with thec12 x 16 addition.  I avoided all the foundation questions. The ceiling joists of the original house were more than adequate to serve as the floor.



There’s an idea.

So are you saying you went up, adding a 2nd level?
 
John F Dean
master pollinator
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Yes.  It was cheaper in the long run. Heck, for me, it was cheaper in the short run. Yes I know the roof is involved, but how long do you expect your current roof to last before it needs work? As for your current efforts. Make it a porch  designed to be enclosed in the future. But, much depends upon the ceiling joists.  If you have trusses the game may change.  Of course, you may be able to reuse the trusses, but the labor has to be considered.  And unlike a horizontal addition, you have to get it enclosed quickly.  Oh yes, I went with a spiral staircase to save space.  As a precaution, installed a large upper window that furniture could be moved in and out of ..... though I never used it.
 
Beau Davidson
pollinator
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John F Dean wrote:much depends upon the ceiling joists.



I can see how that would be a good way to go in certain situations.

My 2x4s at 24 centers spanning 12 feet are not interesting in becoming a floor, and I’d rather not in displace my family while I’m doing the work.
 
Beau Davidson
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Minipost of today’s progress. Just one beam remains and some temporary bracing, then I’m thinking of moving on to the perlins.

Also, I finally got ahold of our quarry guy, so I can move into gravel and stem wall at some point.

Question of the day:
I’m wondering if 2x4 perlins on 16in center, spanning 7 feet at the max will be:
A) enough to support a modest living roof
B) enough to tie the timber frame together, with or without the aide of metal strapping or angular timber braces.

Anyone have 2 cents?

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John C Daley
pollinator
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I cannot remember what your original plans for the wall cladding was. What ever you use that is conventional may clash with the round posts.
 
Beau Davidson
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John C Daley wrote:I cannot remember what your original plans for the wall cladding was. What ever you use that is conventional may clash with the round posts.



Straw bale and earthen plaster is the plan.
 
Beau Davidson
pollinator
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Pic post. Had guests in town for a week so just getting started again.

I did take some video of the final post and beam. I’ll upload and embed when I have reasonable internet.
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Thanks for the YouTube tutorial, Paul.
Thanks for the YouTube tutorial, Paul.
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Mysteriouslyfound an old cart to help dolly logs.
Mysteriouslyfound an old cart to help dolly logs.
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Moving forward with roof support. Reclaimed 16 foot 2x4’s from a civic road project. Power washed and good as new.
Moving forward with roof support. Reclaimed 16 foot 2x4’s from a civic road project. Power washed and good as new.
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Just the other day, I was thinking ... about this tiny ad:
BWB second printing, pre-order dealio (poor man's poll)
https://permies.com/t/147624/BWB-printing-pre-order-dealio
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