Cordwood Shed - advice

Hello.

Brand new to posting on the site but I've been here a few times and liked it so thought I'd join the conversation.

In a nutshell, I'm building a potting shed and ASAP once the ground completely thaws (we live in eastern Ontario). I'm thinking 10x12 with a 4 foot wide patio/deck on the 10 foot end with the roof extending over it. No insulation, wiring or water connections just a shed by the veggie garden. It's not for me per se but I want to make it as eco friendly and beautiful as possible (and inexpensive doesn't hurt) for the woman who will be using it. It will be sited almost in our berry patch so I'd like it to be a bit of a retreat area for her as well.

Anyway, I'd really like to make one (south facing) or all four walls if possible, cordwood and recycled glass. We have a lot of cedars on the property and many I've cut for the raised garden beds but I've quite a few left over and most have been down for 1-2 years.

I've heard lot of conflicting information about doing it though. Some sites say concrete is fine for mortar, others cob only (and where would you get the clay besides digging), some say you need long roof overhangs but others say that's foolish, etc. I wanted to try a gravel bag foundation but seeing as I'd like this completed relatively quickly and it would be my first build like this I'm thinking straight concrete blocks but I'm hoping it will be a learning experience in a number of ways. If it goes well, then ultimately I'd like to try a much larger structure in the future but won't get into that now.

We've a lot of land and I'm sure this won't be my last post so hopefully someone has some experience with doing this sort of building.

A few questions to start with:

1. Are roof overhangs necessary?
2. Mortar, mortar, mortar?
3. Is 6 inch-1 foot cordwood structurally enough for non-insulated walls?
4. Is the weight of the walls a concern being on concrete blocks?
5. Has anyone any experience building with cedar logs as the main support beams this way (just an idea)
6. If cob has to be used any suggestions on where to get clay as we have fairly clayish soil but it's rocky and very difficult to dig. Would the clay content need to be tested first I assume?

Thanks

1) Yes, you need enough overhang that rain or ground splash generally won't hit the wall. In a dry climate maybe you could minimize this, but not in a damp climate.

2) Concrete will make a very rigid, strong structure, until the damp that it attracts rots out the wood behind the face. I understand that cob works well, and it wouldn't require highly refined clay for this application. Cob will dry out completely in warm weather and draw moisture from the wood.

3) Six inches might be too narrow for a wall more than a few feet high. One foot should be structurally sound for a modest shed-sized wall. What do you plan for the corners? Timber posts?

4) A modest shed will not have enough weight to be a problem for any foundation that is strong enough to sustain itself in the ground. You would want a wider footing than 8" blocks by themselves would provide. If not concrete, the footing for this could be rough gravel with drainage to daylight so that water can never accumulate in the trench. The foundation should be at least as wide as the wall, so blocks would need to be 12" or doubled up. A more durable foundation for this would be a rubble trench: dug down to an appropriate depth, and filled with rough rocks stacked (not tossed in) to 6" or so above grade. This would look much more attractive than exposed concrete blocks too. If you have rocky ground, presumably you have a good supply accessible.

5) Cedar is not available here, but it is supposed to be rot-resistant, and would probably make fine framing. You don't have to square it any more than you want, aside from connections and maybe the faces that abut the cordwood for ease and stability of mortaring.

6) Clay for your purposes doesn't need to be smooth and refined. My (upstate NY) glacial till clay has lots of sand and gravel, and rocks, in it. I have found that just removing stones that are around half as big as the thickness of the work I am doing is sufficient. I take out stones bigger than a golf ball in general. For finish work sifting dry clay through a half inch mesh works well. If you want your logs to fit closely, this might be a good idea for you.

Glenn Herbert wrote:
I understand that cob works well, and it wouldn't require highly refined clay for this application. Cob will dry out completely in warm weather and draw moisture from the wood.

6) Clay for your purposes doesn't need to be smooth and refined. My (upstate NY) glacial till clay has lots of sand and gravel, and rocks, in it. I have found that just removing stones that are around half as big as the thickness of the work I am doing is sufficient. I take out stones bigger than a golf ball in general. For finish work sifting dry clay through a half inch mesh works well. If you want your logs to fit closely, this might be a good idea for you.

Thanks. Glacial till but few drumlins here. How long does cob take to dry out fully and can it be used as a building material applied over weeks as I'm not sure how much time I will have (multiple applications) to complete the shed in one go.

The soil nearby is clayish but very difficult to dig and I'm not sure a hole in the berry patch is what she would like. I've no equipment to move anything yet beyond a wheelbarrow. Is there an alternative source? Straw can be found but farther away. I tried last year without success but if I can find it does it need to be chopped or simply added via bales?

Ruy Lopez wrote:
A few questions to start with:

1. Are roof overhangs necessary?
2. Mortar, mortar, mortar?
3. Is 6 inch-1 foot cordwood structurally enough for non-insulated walls?
4. Is the weight of the walls a concern being on concrete blocks?
5. Has anyone any experience building with cedar logs as the main support beams this way (just an idea)
6. If cob has to be used any suggestions on where to get clay as we have fairly clayish soil but it's rocky and very difficult to dig. Would the clay content need to be tested first I assume?

Thanks

My father built a large two car cordwood garage 35 years ago and it looks as good as the day he built it. I can tell you how he built his.
1) He did not build large overhangs on the garage. He did use metal roofing and it has held up beautifully and looks like new.
2) He used regular mortar at each end of each block (about 4" wide I believe) and left air gaps in the middle
3) My father built his with conventional cordwood, ie. 16" wide.
4) He dug out about a foot deep, put in 6" or so of gravel and poured concrete over that. After that he used one layer of chimney block on the concrete and built the cordwood wall on top of the chimney blocks. No issues as far as weight, heaving, etc.
5) Not us.
6) No idea about this as we used conventional mortar.

Once mixed, cob stays usable as long as it is damp, and if it dries out, you just have to rewet and remix it. Especially as you are applying it in relatively separate bits, just leave a rough surface on the last cob you do in a session, and get the surface wet with some muddy clay and water before applying the next batch.

I generally find I need to loosen the clay with a pick before shoveling it into a bucket. If it is dry, it is hard but breaks up easily; if damp, it is more resistant to crumbling and sorting. Is there no place out of the way where you can dig a hole? Purchased clay will be significantly more expensive, and will still need to be worked up into cob with sand and straw.

A garage is not heated or occupied by moisture-generating functions, so might last longer than a house wall in the same conditions. Mortar has less cement and more sand than concrete, so would probably allow wicking and drying better. A 4" thickness of mortar may allow enough evaporation to be safe... I would want the advice of several experienced builders for that.

I'm not sure how it all works together, if the mortar breathes enough, or if the air gaps in the middle help, or ? I know when my father built it, he wasn't real careful to get the amount of mortar right or anything. He just built it. Regardless, I can tell you it's very solid and has stood the test of time well. Maybe the winters being so dry here help too. I'm just not sure.

I realize it is just a shed, but depending upon the length of your spans and size of your beams, you may want to rethink cedar as structural beams. It is very rot resistant, however it is not all that strong. Brittle is what I would use to describe it. Unlike Spruce or Hemlock that can take a load and bend, cedar just snaps...and rather abruptly. There is a reason I know this, I was jacking a building up to move and put cedar skids under it when "wham" 12 inch logs snapped like matchsticks.

Would beams work in a shed...maybe. It depends on the length and load. If you have other woods, I would use that. I love hemlock only because it is semi-rot resistant, much more than spruce, but stronger than cedar.

Good information about cedar in structural applications. I know that people do use it, but maybe they size it larger than if it were another species.

In this case, the cordwood infill would most likely be strong enough that I wouldn't be concerned about beams in a 10' x 12' shed snapping.