Those dry-stack concrete blocks - what am I doing wrong?

20 years ago

I'd like to build a circular planter, around 6' wide, in a corner of my garden. Last week I had delivered a pallet of those wedge-shaped concrete blocks with the lip on the back for easy stacking.

It seemed simple enough; scribe a circle in the mulch, arrange a single row of blocks following the line, and if the last block doesn't quite fit, move all of them in or out just a bit until they're arranged in the right-sized circle. Then add additional runs of block on top until it's four blocks high.

Well, this morning was my 5th completely unsuccessful attempt at this, and I'm getting really frustrated with myself. By the third run (and only the second this morning), invariably the blocks are off enough that I can't complete the circle.

Because it occurred to me that each circle is going to be slightly smaller than the one below it, I'm hoping there must be some geometry rule I can follow so that I can stack these babies four-high. But... what IS that rule? How does this math-impaired person start out?!

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Comments (6)

pls8xx
20 years ago
What is the width of your blocks at the widest point and how much is the set back for each row?
deeje
Original Author
20 years ago
pls8xx, they're 12" at the widest, 9" at the narrowest, and the lip and set back is 1".
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pls8xx
20 years ago
deeje,

Go with 20 blocks on the two bottom runs, then 19 blocks for the top two. Establish a center radius to measure to the edge of the block at it's widest point. On some blocks the widest point will be back a bit from the face. I suggest that you make up a jig to help with uniform measurements. In the graphic below I show a board laid on top of a block so that the edge of the board is at the widest part of the block. a second board is set under the first against the block face. Clamp the boards together and then nail them. Reset the boards back on the block and make a mark shown red on the board at the block edge. Check several blocks to see that the board's back edge fits for the widest part of the block. You will want to make all your radius mesurements to the board at the red marks.

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The radius for the bottom run should be 39 and 5/8 inches. There should be about a 3/8 inch gap between each block. Lay a few blocks and stop. Set a block on top of the first run and check the setback. Because you're doing a curve, it's possible that the setback may be more than 1 inch. If so, take the amount over 1 inch, multiply it by three and add to 39and 5/8 radius for a corrected radius to use on the first run. Note: the fourth and final run must have a radius no less than 36 and 5/8 inches.

The first run will have about 3/8 inch gaps, the second about 1/16 gaps, the third about 3/8, and the last one should be tight.

Since the 2nd run has 20 blocks and the 3rd run only 19, there will be a place in the circle where the joints will almost line up. You might want this to occur on the backside.
deeje
Original Author
20 years ago
Wow, pls8xx, I'm SO glad you were reading the forums this weekend! I think you've just saved the project for me. Hee hee, and as I was beginning to type this post, the sun peeked out for the first time today from behind the clouds. It's a sign!

Seriously, thank you SO much for breaking this out into manageable and understandable steps for me!
antra786
18 years ago
Trees have alot of importance in our life.
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antra
Wide Circles
markalan
17 years ago
If your going 4 high, your first course needs to be in the ground. If your filling with dirt I wouldn't have a gap anywhere. For the second course and on up, finish where there is no view or limited view use a masons chisel and cut to fit. If there loose they will move over the years. If you finish with a course and have a gap almost big enough for another block trim the tapered edges on other blocks in the course to push them closer together. I have stacked quite a few of these and that's my 2 cents.