Imported slate - bedding mortar sand?

[mrrodge]

Hi all,

Never actually got my patio off the ground so to speak, but now I've started it again. I've built a concrete block shuttering and filled & compacted the middle with 200mm of 40mm crusher run. I'm ready to lay my imported green slate on top.

I have SBR & Cement to paint on the back of the slabs, but what sand should I use for the bedding mortar?

This link clearly states that grit sand should be used.

However this link states I should use a 'very stiff' class IV mortar which is described on here as being strictly building sand.

I understand building sand is weaker but sticks to the slabs, which from what I gather is essential with slate. It's just that the slate page also states that a 'very stiff' mortar should be used, which sways me towards grit sand.

Is all this irrelevant when I'm using SBR paint? Does it really matter?

Cheers!

[London Stone Paving]

Hi Rodge

You have done the right thing with the bond bridge. Before I discovered this site I didnt know what a bond bridge was. We used to use a really wet mix, which was ok but could get a little messy.

Stick with the sharp sand IMO, it will be stronger than soft. You have taken enough precaution by using a bond bridge

Steve

[lutonlagerlout]

i find if you use sharps sand and cement quite wet and strong on top of mot or such the wetness soon gets sucked into the subbase,but the flags are stuck
never had any problems like this but you do need to be careful
LLL

[Tony McC]

Class IV mortar is typically used for non-structural brickwork and masonry and so a building sand tends to be used, but it is not mandatory. A coarser sand can be used if preferred.

When we discuss mortars for bedding paving, we often tend to talk in terms of mix ratios, 4:1, 6:1, etc., but the mortar classification system is still applicable even though we prefer to use a coarser sand. It gets even more complicated when some contractors use a mix of building/soft/plastering sand with the coarse/grit/concrete sand in the mistaken belief that this somehow makes for a better mortar.

Anyway, I'll have a look at the wording you;ve pointed out and see if I can clarify it somehow.

[mrrodge]

lutonlagerlout,

Someone at work said I should wet the sub base first to help stop this happening. Would this be a good idea?

Tony McC,

Thanks! I was looking at 4:1, grit sand and cement as I have loads of grit sand left, semi-dry.

[lutonlagerlout]

before i ever found this site i found by trial and error that natural stone goes down better on a wettish mix
when you use grit sand it doesnt hold the water like soft sand so within an hour or so the flags are stuck
never have any rockers or problems with hollow sounding flags as the wettness as you tap it down makes sure the bed is full
always furrow the bedding away from adjacent flags before placing the next flag
cheers LLL
PS half a bag of cement,half a bucket of water and 12 shovels of grit sand is our preferred mix
works good for me