Render, mortar differences?

[reb78]

I am modifying the hearth in out kitchen to accommodate the rayburn. At the base, the stone work is now exposed and needs covering and blending in with the current render. The walls are currently covered with a render I think- its cement based with a rough finish harder than plaster - its not plaster.

In the past I have had little luck trying to acheive an adhered finish with mortar mixes (generally 5:1 sand to cement that I happened to be using for block work etc).

What is the difference with renders applied to walls? I read that some have lime added but then toolstation seem to sell one that appears to just be sand and cement?!

What should I be doing to get a good finish that adheres to the stone wall and blends well rather than stands out like a sore thumb?

 

[julianf]

Is the wall stone?

If so, use NHL 3.5, in a 3:1 ratio with sharp sand. (3 sand to one lime)

Choose the sand with care to match the texture of your joining area.

 

[Jack Ryan]

I am not a builder but I have laid a few bricks and rendered fairly large sections of a few walls - usually as a base for plaster but not always.

I don't think there is a significant difference between render and mortar, Both need to be "sticky" and so use "brickies sand", a sand that contains a fair amount of clay.

I've never added lime unless I'm trying to make lime mortar - but that's different stuff.

(The wall needs to be damp or it will suck the moisture out of the render and it won't stick).

Jack

 

[julianf]

Builders sand is a firm "no" for cement render.

First coat will have a plasticiser in it, second coat lime. Some will add lime to first coat also. But there should never be any builders sand involved.

 

[pedrobedro]

The last I did I was recommended yellow builder's sand and it was a good call, it goes on like butter, really fine texture.

 

[julianf]

(note - the lime that you add to cement render is a different type of lime to NHL)

 

[reb78]

Is the wall stone?

If so, use NHL 3.5, in a 3:1 ratio with sharp sand. (3 sand to one lime)

Choose the sand with care to match the texture of your joining area.

Yep. Behind what they have done previously to make the current hearth (a lot of cement) it is roughly/unpointed stone (lots of gaps, generally flat stones laid on one another).

I was going to chip back the minimum amount of the concrete they used to make the old hearth height so that it is behind the level of the current render so this finish may be going on to stone in places and old rough cement mix in others. I am worried if I chip back too far, the depth of render needed will be 4+ inches in places and dont imagine this is good!

Interesting read here - https://limeliteheritage.co.uk/lime/nhl-3-5-natural-hydraulic-lime/ it appears sand is a science too that I had never tried to understand!

This is a bit of a cop out but I only have a small area to do - are there any good pre-mixes anyone can recommend to avoid buying the different components in small quantities?

 

[julianf]

If you go with nhl3.5, a bag will cost you about £14 or so at any builders yard, and then just buy whatever sharp sand you need in 25kg sacks. They were about £2 each, but probably more like £50 now...

Anyway, what im saying is that £15 on a bag of nhl, and then £2 or £4 or whatever bags of sand you need thereafter isnt too much "collecting", and probably cheaper than buying some premix.


You can apply the first coat of lime by hand, with some decent gloves. Just "smear" it onto the wall, between all the cracks etc. Dont tool it at all.
Then do your 2nd coat with a trowel. 3 coat is normal, but if you can float up the second well enough then leave it at that.

Sharp sand varies as to how well its graded. You may want plastering sand for the final finish if your sharp is too stoney so as not to drag when floated. But sharp the whole way is fine if you dont want to buy multiple sacks, just so long as you can get the finish how you want it.

 

[Gareth J]

If it's just blending in a bit on an otherwise cement rendered wall, not sure there is a significant enough gain for the relatively high cost of NHL products. If redoing a big section, or the whole wall, definitely.