Hi

Ive tried looking in the archives and the FAQ but cant find a definitive
answer.

I was taught to use builders sand and cement (and water obviously) to mix a
render suitable for then skimming with plaster.

In this context is there really any or much difference between builders sand
and sharp sand ? or are both suitable.

Thanks

Tim

"Tim Smith" wrote in message
...
Hi

Ive tried looking in the archives and the FAQ but cant find a definitive
answer.

I was taught to use builders sand and cement (and water obviously) to mix
a render suitable for then skimming with plaster.

In this context is there really any or much difference between builders
sand and sharp sand ? or are both suitable.

Thanks

Tim

I recently put a base layer of render on couple of walls in my cellar with a
bit of waterproofer added, prior to plastering and used cement to sharp sand
in a mix of 1:4. Seemed to go on pretty well.

Alistair

Tim Smith wrote:

Hi

Ive tried looking in the archives and the FAQ but cant find a definitive
answer.

I was taught to use builders sand and cement (and water obviously) to mix a
render suitable for then skimming with plaster.

In this context is there really any or much difference between builders sand
and sharp sand ? or are both suitable.

Thanks

Tim


Sharp sand is grittier and tends not to slide about so much. I don;t
think its any more than that.

Ive used both, and buiolder sand makes a smoher slip[perier mortar, but
once dry there is leiitle difference,

However a screed of sharp sand with low cement content seems to be more
porous and permeable than builders sand.

Tim Smith wrote in message
...
Hi

Ive tried looking in the archives and the FAQ but cant find a definitive
answer.

I was taught to use builders sand and cement (and water obviously) to mix
a
render suitable for then skimming with plaster.

In this context is there really any or much difference between builders
sand
and sharp sand ? or are both suitable.

Thanks

Tim

You may find the odd larger stones in sharp sand annoying with a coat of
under 10mm thickness.
Plastering sand is better and easier, but more expansive and is best to use
for a finish render.

--
Mark§
This is an automatic signature of unknown origin

"The Natural Philosopher" wrote in message
. ..
Tim Smith wrote:

Hi

Ive tried looking in the archives and the FAQ but cant find a definitive
answer.

I was taught to use builders sand and cement (and water obviously) to
mix a
render suitable for then skimming with plaster.

In this context is there really any or much difference between builders
sand
and sharp sand ? or are both suitable.

Thanks

Tim


Sharp sand is grittier and tends not to slide about so much. I don;t
think its any more than that.

Ive used both, and buiolder sand makes a smoher slip[perier mortar, but
once dry there is leiitle difference,

However a screed of sharp sand with low cement content seems to be more
porous and permeable than builders sand.


Sand varies all over the country. Some building sand is very fatty and some
dry when mixed with cement, the latter may require additives to make it
usable. Most building sand is quarried straight from the ground. Sharp sand
or sometimes known as washed sand comes from a water course, either a river
or gravel pit course. Its the very small partials of the graded gravel.
Sharp sand or washed sand is usually used for path's, floor screeds and the
finer sharp/ washed sand for the backing coat for plastering.
It can also be mixed with building sand, Hydraulic lime and cement for
bricklaying. This mortar mix is very sticky and pliable.

"keith_765" wrote in message news:...

"The Natural Philosopher" wrote in message
. ..
Tim Smith wrote:

Hi

Ive tried looking in the archives and the FAQ but cant find a
definitive
answer.

I was taught to use builders sand and cement (and water obviously) to
mix a
render suitable for then skimming with plaster.

In this context is there really any or much difference between
builders
sand
and sharp sand ? or are both suitable.

Thanks

Tim


Sharp sand is grittier and tends not to slide about so much. I don;t
think its any more than that.

Ive used both, and buiolder sand makes a smoher slip[perier mortar, but
once dry there is leiitle difference,

However a screed of sharp sand with low cement content seems to be more
porous and permeable than builders sand.


Sand varies all over the country. Some building sand is very fatty and
some
dry when mixed with cement, the latter may require additives to make it
usable. Most building sand is quarried straight from the ground. Sharp
sand
or sometimes known as washed sand comes from a water course, either a
river
or gravel pit course. Its the very small partials of the graded gravel.
Sharp sand or washed sand is usually used for path's, floor screeds and
the
finer sharp/ washed sand for the backing coat for plastering.
It can also be mixed with building sand, Hydraulic lime and cement for
bricklaying. This mortar mix is very sticky and pliable.

Oh dear, oh dear. When will people stop rendering with cement and
plastering with gypsum. Its too impermeable and too brittle and will
end in tears. Use lime putty and sand 1:3 mix with a little horse or
goat hair for the base coat. Then lime putty and a fine sand at 1:2
for the top coat. 1:1 if you want a really high class finish. Your
wall will then be breathable (so long as you don't then ruin it with
an impermeable paint). Sharp sand has a variety of grain size and is
stronger when grains are angluar. Good for bricklaying. The spaces
between the large grains are partly filled with small grains so you
need slightly less lime than when the grains are all about the same
size. Angular grains interlock and don't slip past each other as
easily as the rounded grains typical of 'soft' sand so develops a
higher load strength. For plastering soft sand is better but it is
not very critical. Just make sure that the sand is as fine as
possible for the top coat of lime plaster.

On 8 Apr 2005 11:46:07 -0700, (Biff)
wrote:

Oh dear, oh dear. When will people stop rendering with cement and
plastering with gypsum. Its too impermeable and too brittle and will
end in tears. Use lime putty and sand 1:3 mix with a little horse or
goat hair for the base coat. Then lime putty and a fine sand at 1:2
for the top coat. 1:1 if you want a really high class finish. Your
wall will then be breathable (so long as you don't then ruin it with
an impermeable paint). Sharp sand has a variety of grain size and is
stronger when grains are angluar. Good for bricklaying. The spaces
between the large grains are partly filled with small grains so you
need slightly less lime than when the grains are all about the same
size. Angular grains interlock and don't slip past each other as
easily as the rounded grains typical of 'soft' sand so develops a
higher load strength. For plastering soft sand is better but it is
not very critical. Just make sure that the sand is as fine as
possible for the top coat of lime plaster.

Is there any evidence (preferably not anacdotal) that gypsum is less
permiable to water vapour than lime? I mean it passes through 10mm
gypsum plasterboard like its not there. Surely the addition of 9"
brick would make any difference between 10mm of lime or gypsum
irreleevent?

In article ,
basil writes:

Is there any evidence (preferably not anacdotal) that gypsum is less
permiable to water vapour than lime? I mean it passes through 10mm
gypsum plasterboard like its not there. Surely the addition of 9"
brick would make any difference between 10mm of lime or gypsum
irreleevent?

Gypsum is very permiable, but passes water through in liquid form,
which then evaporates from the surface. This is why you will see
salts deposited on the surface. It actually sucks water up like a
sponge. Prolonged exposure will wreck the gypsum though.

Raw sand and cement is much less susceptable to this, and it can
be reduced to pretty insignificant levels with the addition of a
waterproofer into the mix. If the mixture is weak as normally
used for renedring and mortar, then it will allow some breathing
(gas permiable so water vapour can pass through), though not as
much as lime.

Neither gypsum nor sand and cement will move, so if you use them
in a lime mortar house, at best you'll get hairline cracks forming
where the house is routinely moving (at worse, bits will fall off).
This will not happen with lime and horse hair plaster, which will
move in sympathy without any such problem. The lime plaster is not
damaged by moisture directly. However, damage can occur on either
side; many modern decorations are not sympathetic to breathability,
and over the years, any salts which are deposited on the underlying
brickwork surface due to continued moisture traversal can cause the
lime plaster to lose key.

--
Andrew Gabriel

"basil" wrote in message
...

Is there any evidence (preferably not anacdotal) that gypsum is less
permiable to water vapour than lime?

Gypsum is very permeable to water. Problem is when it dries out it is
weakened whereas lime dries out at its original strength.

On Sun, 10 Apr 2005 09:49:40 +0000 (UTC),
(Andrew Gabriel) wrote:



Raw sand and cement is much less susceptable to this, and it can
be reduced to pretty insignificant levels with the addition of a
waterproofer into the mix. If the mixture is weak as normally
used for renedring and mortar, then it will allow some breathing
(gas permiable so water vapour can pass through), though not as
much as lime.


Good post!

On the subject of waterproofer for sand and cement would wickes's
waterproof PVA be the correct thing? What do you use? I'm missing a
DPC on one small area of internal wall.

In article ,
basil writes:
On Sun, 10 Apr 2005 09:49:40 +0000 (UTC),
(Andrew Gabriel) wrote:



Raw sand and cement is much less susceptable to this, and it can
be reduced to pretty insignificant levels with the addition of a
waterproofer into the mix. If the mixture is weak as normally
used for renedring and mortar, then it will allow some breathing
(gas permiable so water vapour can pass through), though not as
much as lime.


Good post!

On the subject of waterproofer for sand and cement would wickes's
waterproof PVA be the correct thing? What do you use? I'm missing a
DPC on one small area of internal wall.

No, I don't think that's what the 'waterproof' in waterproof PVA
means. (I think it just means the PVA won't degrade inside damp
sand/cement when the PVA is used as a bonding additive.)

You can buy a waterproof additive for mortar mix. The one I use
is also combined with plasticiser, but you can get it separately.
I think it came from a large B&Q, but I can't remember the make
(may be their own brand).

If you have a measurably damp brick wall, a sand and cement scratch
(base) coat with waterproofer (and I use lime too -- makes the mix
very nice to handle) will protect a gypsum finish coat. I've done
this a number of times, and whilst the underlying wall is still
damp, the gypsum remains bone dry. However it probably isn't a
substitute for tanking, where you have water being forced through
the wall due to being below the water table for example (at least
I've never tried, and I wouldn't expect it to work).

The general sequence for doing this is...
PVA the wall just as you would before any plastering -- if it's
very absorbent, a 1:5 dilution with water soaked in to dry to seal
the surface. Then apply a 1:3 dilution and allow it to get nearly
dry before applying the sand/cement. I use the standard 1:1:6 mix
of cement:lime:sand plus waterproofer (plus plasticiser as that's
mixed with my waterproofer, but the lime acts as a plasticiser
anyway so that's not necessary). You want to apply the gypsum
finish coat after the sand/cement has gone off but before it dries;
this means waiting at least 24 hours, but probably not more than
48 hours. If you leave it longer and the sand/cement does dry out
before skimming, then you'll probably want to PVA it as as though
you were doing a reskim.

--
Andrew Gabriel

On Mon, 11 Apr 2005 08:00:26 GMT, basil wrote:



On the subject of waterproofer for sand and cement would wickes's
waterproof PVA be the correct thing? What do you use? I'm missing a
DPC on one small area of internal wall.

I've just got some!

On Mon, 11 Apr 2005 10:26:23 +0000 (UTC),
(Andrew Gabriel) wrote:



You can buy a waterproof additive for mortar mix. The one I use
is also combined with plasticiser, but you can get it separately.
I think it came from a large B&Q, but I can't remember the make
(may be their own brand).

SOunds like the one I got from Wickes £2.99 / 5ltr

If you have a measurably damp brick wall, a sand and cement scratch
(base) coat with waterproofer (and I use lime too -- makes the mix
very nice to handle) will protect a gypsum finish coat. I've done
this a number of times, and whilst the underlying wall is still
damp, the gypsum remains bone dry. However it probably isn't a
substitute for tanking, where you have water being forced through
the wall due to being below the water table for example (at least
I've never tried, and I wouldn't expect it to work).

I'll try this now. I dont think its serious enough to warant tanking,
there are some stains and the dust at the base of the wall looks damp
and I can smell the BG "plaster finish" which I applied several months
ago as if its stll damp.

The general sequence for doing this is...
PVA the wall just as you would before any plastering -- if it's
very absorbent, a 1:5 dilution with water soaked in to dry to seal
the surface. Then apply a 1:3 dilution and allow it to get nearly
dry before applying the sand/cement. I use the standard 1:1:6 mix
of cement:lime:sand plus waterproofer (plus plasticiser as that's
mixed with my waterproofer, but the lime acts as a plasticiser
anyway so that's not necessary). You want to apply the gypsum
finish coat after the sand/cement has gone off but before it dries;
this means waiting at least 24 hours, but probably not more than
48 hours. If you leave it longer and the sand/cement does dry out
before skimming, then you'll probably want to PVA it as as though
you were doing a reskim.
Thanks!