Laying bricks with a really thin mortar bed

[WabbitPoo]

Am about to construct brick raised beds in the garden and wish to match the brickwork of the house, which is a red brick victorian job. The mortar gaps in the house are really thin - couple of mm at most I would say.

I am used to laying using a 10mm mortar. Any advice as to using such a thin bed?

 

[Shytalkz]

Imperial bricks are slightly larger than metric ones, so if you're using modern ones your bed joints won't be so thin.

Alternatively, use non-hydraulic or feebly hydraulic lime mortar (NHL2.5) with sand 1:3 mix and reclaims, to get the same size joints.

Preferably don't use soft reclaims with 1:1:6/1:2:9/1:3:12 cement/hydraulic lime/sand mortar mixes as it's too hard, looks shyte and a few years down the line, the faces of the bricks are likely to blow. If you use white cement, that's even harder and more impermeable than OPC.

 

[WabbitPoo]

Thanks for that. Never had to get involved with hydraulic lime. I take it a good builders merchant would sell such a cement? What does the hl do for the mortar which makes such a thin bed possible?

 

[Shytalkz]

Depends on the sand you use. As a rule of thumb, the maximum grain size should be about 1/3 the joint width. Non-hydraulic limes set on contact with air; hydraulic ones with water. The ones from BMs (bagged lime) are usually hydraulic and are the ones used in a cement/lime/sand mortar mix. Hydraulic mortars set more strongly and are more impervious than non-hydraulic ones. They set on contact with water, as they contain more impurities than non-nydraulic ones (but don't ask me the chemistry behind this!) and are mostly imported (from France, I think) and these are the NHL (naturally hydraulic lime) specification ones. Non-hydraulic mortars take ages to go off, but can be speeded up with the addition of pozzolans at the point of knocking up. Addition of pozzolans again makes it more stronger and impervious.

Knocking up lime mortar is more involved than cement-based ones, but you can buy pre-mixed mortars from specialist suppliers around the country, to save you buying quicklime and slaking it yourself (it reacts violently on contact with water) and then maturing it before use, usually a minimum of three months. All in all, more expensive than cement-based mortars. Looks nicer though and is starting to make a big come back these days

 

[WabbitPoo]

Blimey - is all getting complicated now! Dont really understand why a thin bed of "normal" mortar wouldn't work - does it not have enough strength?

Thanks for the info.

 

[Shytalkz]

You'd need smaller maximum sized sand particles to get it to work and it's to do with the way that the sand wraps itself around the binder, which would be the cement element. In principle, however, using more finely graded sand there's no problem with using cement and sand mortar in a thinner joint and it won't affect the wall strength.

As I said previously, if you're using modern bricks, then you'll get a slightly bigger joint anyhow. But, if you use a cement mortar, it won't look anything like the house lime mortar, if that is the look you are trying to achieve. Don't believe those who say using white cement cures all...

 

[WabbitPoo]

With such thin joints I would not think it necessary to match colours of mortar. I was thinking of normal yellow building sand.

 

[Shytalkz]

Here's an original non-hydraulic lime mortar, with Norfolk Reds:

Untitled

• Shytalkz
• 24 Mar 2008
• 1

Original non-hydraulic lime mortar and norfolk Reds

And here's a plinth built in Norfolk Red reclaims using white cement and sand 1:5 mix:

Untitled

• Shytalkz
• 24 Mar 2008
• 1

Reclaim Norfolk Reds in white cement/sand 1:5

I know which type I prefer (clue: it's not the bottom one... )

OPC will give you a grey colour, white cement a yellowy-white colour; the mix ratio will result in some variation to these.

 

[WabbitPoo]

Yes, the first one is so much better. Will look for some non-hydraulic lime at Travis etc.

 

[Shytalkz]

This might be of interest...

 

[WabbitPoo]

TBH, I'm not sure I'll bother now - it all seems overly-complicated! Is there a reason why say, a 5mm mortar gap won't do a job with soft building sand and a bag of Multicem? I'm only going up 4 courses or so.

 

[Shytalkz]

None whatsoever, providing you use more finely graded sand and don't expect it to look anything like the house joints

 

[noseall]

bricklaying whilst using very thin beds is extremely difficult.

georgian 3" machine moulded bricks were commonly laid with a very thin bed and struck pointed with a rounded jointer.

a lime putty would be used for the mortar and the bricks would have be soaked first.

good luck.

 

[WabbitPoo]

Is building sand not fine enough?

The garden wall the beds are going next to have a wider joint, so I will suggest I match those instead! Should keep her happy. Thanks so much for all the help, though. I learned a lot.

 

[Shytalkz]

bricklaying whilst using very thin beds is extremely difficult.

Yep, we've been through that, but he's insistent! He'd probably get away with 5mm though and cement/sand, no? Not that I'm recommending it as a course of action, of course