Why sharp sand for rendering?

Anyone know why they recommend sharp sand for mortar rendering onto brickwork? I would have thought that the finer sand would stick better...

TIA

JK

Reply to
J Kemph
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I'm not a bricklaying expert so if anyone knows better then they should shout, but ...

Its not sharp versus fine, its sharp versus rounded. Spheres tend to slide over each other whereas angular bits fit together in a less slippery way. Well graded sand is recommended too, which means that the grains come in a wide range of sizes, helping the sand to pack together well.

For plastering and also I expect for bricklaying, a pure sharp sand is not very plastic and easy to use, so a mix of sharp sand and building sand is preferred 1:1 to 3:1 depending on the sands in question.

Anna

~~ Anna Kettle, Suffolk, England |""""| ~ Lime plaster repairs / ^^ \ // Freehand modelling in lime: overmantels, pargeting etc |____|

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Reply to
Anna Kettle

I **believe** it has to do with allowing moisture to escape

If you have a chemical damp-course injected, the guarantee will always specify the use of loam free, washed sharp sand to BS 882 grade M (often called concreting sand) for re-rendering. The grade governs the particulate size (M = Medium). On the other hand, soft sands are generally finer with much smaller average particulates sizes than the rather course grade M.

The reason for this is that large particles packed together result in larger microscopic spaces between the grains, and this helps moisture escape from previously damp walls. Using fine grained sand will fill the interstices and slow the rate at which a damp wall can dry out.

I imagine what is good practice for known damp walls is also good practice for walls generally. Basically it allows them to "breathe" better.

Reply to
Coherers

You can get something called plastering sand which is between sharp and building sand. It gives a finer finish but I'd say the sharp sticks best.

Reply to
Mike

Its also more expensive, and is generally only used for the final coat which may need to be smooth for painting etc so that it can be finished with a float.

Reply to
Mark

But is this effect negated as soon as the walls are painted with emilsion? Thanks to everyone for the explanations. -JK

Reply to
J Kemph

I think you are okay with emulsion as it still allows a certain amount of moisture through. In fact now you mention it, I seem to recall the same damp-course specs recommend the use of thin emulsion paints for just this reason.

However, I imagine that thick oil-based paints would be a different matter.

Another reason I have heard why they use sharp sand relates to what Anna said. The roughness of the grains gives a better key for the cement "crystals" which form between grains, and therefore results in stronger bonds. However, I don't think strength is the main reason because, if so, presumably it would also be specified more widely for mortars etc.

Reply to
Coherers

Yes and that is one reason to eschew (good word huh?!) emulsion paint in favour of limewash, which contrary to bad press does not rub off in clothing if it is put on properly

And the other reason is that any business that can afford adverts on primetime TV is making too much money. Limewash is cheap

Anna

~~ Anna Kettle, Suffolk, England |""""| ~ Lime plaster repairs / ^^ \ // Freehand modelling in lime: overmantels, pargeting etc |____|

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01359 230642

Reply to
Anna Kettle

Great word. I use it as often as I can, never eschew the opportunity.

But it makes you have babies :-(

Mary

Reply to
Mary Fisher

Limewash, the noted aphrodisiac

Oh - and a third reason to use it. It is very fashionable according to some clients of mine who are in marketing and up on such things

Anna

~~ Anna Kettle, Suffolk, England |""""| ~ Lime plaster repairs / ^^ \ // Freehand modelling in lime: overmantels, pargeting etc |____|

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01359 230642

Reply to
Anna Kettle

LOL! My mother's generation always felt the need to whitewash the outside privvy or cellar just before they went into labour. I always thought it was an old wives' tale but guess what I had an urge to do before I was carted off to the labour ward?

Oh well, I'll do it if only for that and risk the babies ... :-)

Mary

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Reply to
Mary Fisher

Welll I've heard many good things assigned to the use of lime, many true, but never that one.

Reply to
Mike

Mortars aren't supposed to be strong. They are supposed to break in preference to the bricks breaking, in the even of any movement.

Reply to
Andrew Gabriel

That is what my builder said when I caught him using 10:1 ;-)

Reply to
Coherers

That reminds me of a question which has never had a 100% satisfactory answer:

What is mortar for?

a) to hold bricks together?

b) to hold bricks apart?

Mary

Reply to
Mary Fisher

Yes.

Reply to
Rob Morley

I forgot the 'or' between a and b. Silly me :-)

Mary

Reply to
Mary Fisher

No, that's what gravity is for.

Not really. The purpose is to prevent (or at least to limit) the realative movement between adjacent bricks. In theory, you could build the same building with no mortar, but it would be fragile (think of a dry stone wall or a pile of childrens building blocks).

Reply to
Andrew Gabriel

You think a dry stone wall is fragile???

But it's a good answer - in relation to bricks. Thanks.

Mary

Reply to
Mary Fisher

Although true in a brick house, in a random stone house the stones lain on top of each other are quite stable - 24 inches of thickness sees to that - and I'd say the mortar, which is only on the inner and outer 4 inches or so is to stop the weather getting into the wall (outer) and to provide a sensible surface (inner).

Reply to
Mike

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