Just learned mortar mix ratios actually matter way more than I thought

I've been laying brick for about 6 years now and always just eyeballed my mortar mix. You know, a few shovels of sand, some cement, water until it felt right. Last week I had a wall start cracking after only 3 months and I couldn't figure out why. My buddy who does restoration work in Philly told me to look up the actual ASTM standards for Type N mortar. Turns out I was way off on my sand to cement ratio, like using almost double the sand I should have been. I found a chart on the Masonry Institute website that broke it all down with exact parts per volume. Now I'm using a bucket to measure everything precise and the mix feels totally different, way stickier and holds up better. Has anyone else been sloppy with their ratios and paid for it later?

2 comments

2 Comments

james_campbell121mo ago

Actually gotta jump in on that ASTM point because it gets confusing. ASTM C270 actually lists two different ways to specify mortar: proportion spec and property spec. The proportion spec is what you found with the bucket ratios, but the property spec just says the mortar has to hit certain strength numbers in a lab test. So you can still eyeball it if you know your materials and test it right, but yeah that takes way more experience and equipment. The bucket method is definitely safer for most guys. Also just a heads up, that "double the sand" problem might not be just sand. If your sand is super fine or has a lot of silt in it, the ratio changes even more. I had a load of sand from a new pit once that looked fine but was basically dust and my mix was garbage. A simple silt test with a jar of water saved me a lot of headache later. The cracking wall thing though probably was the weak mortar. Too much sand and not enough cement means the mortar cant handle the movement from temperature changes. Type N is good for most stuff but if its a retaining wall or something structural you might need Type S. Just saying because I made that mistake once and had to tear out a whole section.

sean_torres711mo agoMost Upvoted

Woah hold on. I gotta disagree a bit with the property spec stuff. That lab test thing sounds good on paper but in the real world, nobody's sending mortar off to a lab on a Friday afternoon when they gotta finish a wall by Monday. I've seen guys try to "eyeball it" for 20 years and still get it wrong half the time. That silt in sand thing you mentioned is real though, I learned that the hard way too. But telling people they can skip measuring if they "know their materials" is kinda dangerous advice for most DIY guys reading this. They'll think they're experienced enough and end up with cracked walls like the OP.