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I'm currently mixing render to cover ugly brick walls in my front garden.

Let's say I want to use this recipe for the top coat, just for an example:

  • Cement 1 parts
  • Lime 1 parts
  • Sharp Sand 2 parts
  • Soft Sand 2 parts

The exact recipe isn't critical but my question is ... well a 25 kg bag of lime is much bigger in volume than a 25 kg bag of cement or sand, about twice or close enough, so are the recipes I'm finding online by weight or by volume? None of them say.

Now when I'm mixing mortar I just use shovels (Volume), and that seems to be the advice I'm finding online and cement and sand are pretty close in weight/volume so it's fine, but now I'm seeing this huge difference in the weight/volume of lime I am concerned that the recipes will be way off if I don't break this question once and for all.

Also, I've read all the articles that say it doesn't matter because sand and cement have similar density, eg: Understanding render mix ratios but please remember that this won't cut it when lime is involved because the densities are definitely different.

spl
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2 Answers2

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Disclaimer: I am NOT an expert on cement, mortar, render or other brick laying techniques/technology.

I think the confusion comes from the fact that you're looking to mix in small quantities.

If you were mixing a large quantity, you'd mix by the bag:

  • 1 bag of cement
  • 1 bag of lime
  • 2 bags of sharp sand
  • 2 bags of soft sand

and you wouldn't think twice about, nor would you care, that the bag of lime was much bigger (other than grumbling as you tried to heft it into the mixer).

If you need to make a "half recipe" because you don't need that much, you'd use 1/2 bag of cement & lime and 1 bag of each of the sands.

Since you seem to be interested in making even smaller quantities, you now need to measure, somewhat accurately, your materials. Since you're simply working in "parts", and you now want to measure small quantities, you need to know how to measure the parts.

I think it stands to reason that you'd continue to measure in fractional bags. Since the bags are labeled by the manufacturer by weight, not volume, it would stand to reason that you want to continue to measure your material by weight and not make the switch to volume.

So, a small recipe would now be:

  • 1/25 bag (or 1 Kg) of cement
  • 1/25 bag (or 1 Kg) of lime
  • 2/25 bag (or 2 Kg) of sharp sand
  • 2/25 bag (or 2 Kg) of soft sand

There's no switching of units of measure because your original "bag" measures were all 25Kg units.

NOTE: If the bags are labeled by volume not by weight, then the exact opposite is true: You'd need to make your partial recipe by volume, but it would still be a "fractional bag".

Disclaimer: Again, I am NOT an expert on this matter, I've just worked through it in a way that seems logical to me.

FreeMan
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It's nearly always volume with building materials. The reason this is true is mostly one of practicality. It's much more of a hassle to weigh raw materials on a jobsite than to measure or estimate volume. While recipes depend on density to some extent, they're set up to be applied by volume.

Some examples...

  • Masonry is mixed by shovelful, bag, or cubic yard (sand, gravel, cement). While bags are filled by weight, the quantity of bags in a recipe is more about volume.
  • Tile mortar is mixed by volume parts (dry powder and water), though some products imply both (quarts of water to a bag of powder weighing N lbs.). Really you're dealing with volume in either case.
  • Paint is colored and thinned by liquid volume, irrespective of density (base, colorants, thinners).
isherwood
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