Round spruce woodshed - floating footers?

Hello. Hoping to tap into the wealth of experience and knowledge here, to help us newbies with plans for a round timber woodshed at our off-grid property in Talkeetna, AK.  We have a plethora of beetle kill spruce trunks prepped/debarked on site, which we are hoping to use for posts and beams for a mono-slope/shed roof open woodshed, roughly 8' x 20'.  The site has just been prepped with gravel on gravel (excavator came in and scraped down to natural gravel layer, and filled in with 12" of gravel on top, compacted), so it will be solid and well-draining.  We're up on a ridge with good drainage/soils overall, and though we do have wind, it's a forested area, with the site itself pretty sheltered.

I'm looking for feedback on foundation/post system.  We're hoping to do minimal or no digging, since it's a well-prepped site, and we've had good luck so far with minimal movement with our yurt platform/deck up there which is on a similar pad (gravel on gravel), on floating concrete footers/deck blocks.  

My first question is if we could get by with some sort of floating concrete footers, with the spruce posts somehow affixed on top.  My leading idea along these lines (to get a wider platform for the posts to set on, than the 8" deck blocks) would be to get hollow core 16"x16"x8" concrete blocks that I've found locally and fill with quick-set concrete, to set the rebar/anchor bolt, which the post would set on (with a vapor barrier and/or standoff spacer between to prevent moisture transfer).  Then I'd need to figure out if we could do an L bracket or something to fasten it from the sides - one part into the post, one into the concrete.  If there was an advantage to partially sinking the concrete block into the gravel (for at least some lateral stability perhaps?), I'd consider doing some digging to do so, potentially even setting two of those 8" high blocks on top of each other (maybe sinking up to 12", leaving 4" above ground).  Not sure if that minimal 'nestling in' would make much difference, or if we should just go with the truly floating concept.  We do have a few ground anchors that we'd bought for the yurt but never used, so I was thinking we could add those once the structure is done, for a little extra protection against uplift (but we don't necessarily have heavy winds to be concerned about in that spot).

The other option that we could consider is digging 2-3' down into the gravel and sinking the spruce poles directly in the ground (perhaps with some pea gravel or crushed stone at base to promote even better drainage), after brushing on a preservative treatment (would need suggestions on that), then backfilling with the gravel.  We've had friends use that approach up here with good success, if in well-draining soils (in this case, would be all gravel).  We know that wouldn't go below the frost-line, but feel confident enough in hearing others having done it this way that we'd be willing to try it, considering it's not a high-stakes structure.

Either way, we plan to do plenty of cross-bracing.  Not sure if it matters, but rafters/roofing system will be dimension lumber, with metal roofing (not sure yet if will put plywood under metal for more stability or not), at a good pitch to help with snow load.  

Any feedback/thoughts/suggestions on the idea of the floating footer system described above (vs. digging a few feet down, which we'd rather avoid)?  Want to get this project going quickly (hoping to get the shed up by snowfall), but don't want to go down the totally wrong path, if this floating system is totally off-base.

Thanks in advance!  

Hey Katie, welcome to permies!
I think your block idea is good, though you could easily bang together your own form,  or use a tire.
You could drive the rebar as deep as possible before  before pouring the cement around it.
That should go a long way from preventing lateral movement.
Setting the form, tire or block into the gravel would help as well.
I dont think an L bracket is needed.

What direction does the prevailing wind come from, and where can storms come from? I presume you would like to have the shed roof open to the south for sun warming/drying the firewood. An 8' x 20' footprint is large enough that I don't think lateral movement of the post bases is an issue, rather uplift on the long open high side of the shed would be the thing I would want to guard against.  Cross bracing on all the walls would keep the structure upright and the post bases from spreading.

I think the calculation of chimney block filled with concrete versus forms filled with concrete depends on the relative cost of the materials. I would certainly use a bracket of some sort that will let the posts be tied to the footer block mass. If you can mound the top of the footer so the post seating is self-draining that would help. Roof overhangs would keep the post bases dryer too.

William Bronson wrote:Hey Katie, welcome to permies!
I think your block idea is good, though you could easily bang together your own form,  or use a tire.
You could drive the rebar as deep as possible before  before pouring the cement around it.
That should go a long way from preventing lateral movement.
Setting the form, tire or block into the gravel would help as well.
I dont think an L bracket is needed.

Thanks, William, for the quick response, and the ideas.  Hadn't thought of trying to drive the rebar further into the gravel before setting the form and pouring the concrete - like it.  Would you recommend epoxy for setting the post on the pin, especially if considering not using additional brackets?

Glenn Herbert wrote:What direction does the prevailing wind come from, and where can storms come from? I presume you would like to have the shed roof open to the south for sun warming/drying the firewood. An 8' x 20' footprint is large enough that I don't think lateral movement of the post bases is an issue, rather uplift on the long open high side of the shed would be the thing I would want to guard against.  Cross bracing on all the walls would keep the structure upright and the post bases from spreading.

I think the calculation of chimney block filled with concrete versus forms filled with concrete depends on the relative cost of the materials. I would certainly use a bracket of some sort that will let the posts be tied to the footer block mass. If you can mound the top of the footer so the post seating is self-draining that would help. Roof overhangs would keep the post bases dryer too.

Thanks for the input, Glenn.  Good point about the prevailing winds, and being mindful of that higher front side (which yes, will be facing south).  That's where potentially using the ground anchors that I already have, just to be safe, might make sense.  You're right about the forms - likely about the same cost, and I like the idea of round vs. square, so might go that route (think 12" would do it, and might cut into 2' sections, to be able to sink them just a bit and still keep the posts 12-18" above ground).  Yes, will have overhang (and hopefully gutters off the back, for rain catchment - no water on site), so that should help keep them dry as well.  Haven't ever poured forms before, so not sure how much to mound to help drain - but makes sense, will look into the technique.

A couple of things to think about;
- instead of plywood under the roofing iron for cross bracing, hoop or strap irn could do the same job for much less cost.
- What forces are you dealing with regard to need to fix the posts to the ground?
uplift or sideway stability.

- What height are the posts planned to be?

Here in Australia I would use 2 inch water pipes as posts, set into holes at least 2 feet deep x 2 ft diameter
and filled with concrete.
The foundation has to be heavy enough to hold the structure down if big winds are about.

If the posts are set in this manner, no cross bracing of the walls would be required.

Given your location, you can float things as you originally describe. Everything else is overkill (but, overkill is good). Your worse enemy will be rot.  I helped put in a barn in the 80s? that floated.  It is still standing.

For connecting the posts to the footers, look into Simpson Strong Ties. Engineers called for two versions to hold our round wood timber frame to the foundation. One hidden for interior posts, one that straps on the outside of the post for exterior posts where appearances were less of a concern.
I'm with the folks suggesting rebar driven into the gravel, then concrete poured into forms directly on the gravel. Seems like the best option for.a secure bond to the ground.

Hi all, thought I'd give an update - got started this weekend, thanks to a friend with more experience than we, who was willing to help. We decided to go with the deck blocks after all, due to the posts fitting better than I thought, and it saving us time digging and mixing concrete (once I did the calculations, I realized that was a LOT to mix, with no water on site, and being short on time/weather).  We epoxied rebar into the block and set the posts on top (pre-drilling holes into posts).  Got the 6 posts and 2 beams up (notched then pinned with rebar), with posts creating about 6' by 16' footprint (built to the lengths of the good beams we had - there was more punk than we thought on the other ones we had down). Front posts 8', back 6'.  Attaching a few photos of our progress - including the AK range which was pretty amazing (took down a few more dead spruce this weekend to open up that view, couldn't help ourselves).

Next weekend we'll start on the roof system.  Thinking 2x8" rafters (could probably get by with 2x6"?), 16" on center.  Not looking forward to notching and trying to make something resembling square, on the very irregular framing below - any tips/tricks to keep us sane and making progress (more experienced friend won't be there this time), would be greatly appreciated.  Then either 1x or 2x purlins (thoughts? 2" would allow for more overhang?), fascia, and metal roofing panels on top.  Want a good 16-18" overhang, and probably add drip edge at least at the back too.  And bracing on those posts, of course, and open slats (not sure what yet- cedar fence boards?) on back and sides.  Next spring will add gutter system on the back for rain catchment.  Then hope to get started on the camp kitchen shelter, overlooking that view, next summer (round timber framing, part 2).  Thanks for the input so far; welcome any additional thoughts to help us along.