(03/30/26) Half block length clarification

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Some have commented about an apparent discrepancy concerning installed half block length determination. The following explanation should prove valuable for better understanding of this topic.

Normal half block = 7-5/8” (7.625) length when 2 each equals 15-2/8 (15.250), not 15-5/8 (15.625) which is the length of a full block. This is because 3/8” mortar was expected for each of the half blocks.

So the normal half block installed length in drystacking is actually 7-5/8 +1/16 = 7-11/16 = 7.6875 which rounds up to 7.7”. So much for the actual physical real world.

For planning purposes a half block is considered ½ of the planned whole block installed length which is 15-5/8 + 1/16 = 15-11/16 (15.6875)” and half of that is 7.84375” which I rounded up to 7.9”.

Remember in drystacking, you can hide small gaps by spacing previous blocks in that wall, but you can’t cram a block into a space that is too tight. Also, when you have two parallel walls one with one half block and the other with two half blocks, then those walls would have different lengths.

So the solution was to use 7.9” for planned half blocks and adjust the gap accordingly. This is why the text specifies physical half block length as 7.7” whereas the installed length is 7.9”. This may appear as a discrepancy but it’s not and it actually improves both planning and build quality.

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