plaster vs render

[Stephen H]

Hi all,

I've bought a do-er upper house.

The ground floor internal walls are those porous bricks that are the
same size as thermalite blocks. Believe they are called breeze blocks?

Upstairs internal walls are stud and partition.

The external walls are plasterboarded & skimmed upstairs and downstairs
whereas the internal walls downstairs are rendered and then skimmed.

well upon stripping the wall paper on the downstairs internal walls, the
plaster skim has come off in places with the wall paper and underneath
is rendering rather than coarse plaster.

A few questions?

why was render used instead of normal coarse plaster before skimming over?

What has caused the skim to blow off the render? house is 25 years old.

Can I reskim plaster on top of the rendering or have I got to remove the
rendering and have two coat plastering on the bare walls instead?

Regards

Stephen.

[The Natural Philosopher]

Stephen H wrote:
> Hi all,
>
> I've bought a do-er upper house.
>
> The ground floor internal walls are those porous bricks that are the
> same size as thermalite blocks. Believe they are called breeze blocks?
>
> Upstairs internal walls are stud and partition.
>
> The external walls are plasterboarded & skimmed upstairs and downstairs
> whereas the internal walls downstairs are rendered and then skimmed.
>
> well upon stripping the wall paper on the downstairs internal walls, the
> plaster skim has come off in places with the wall paper and underneath
> is rendering rather than coarse plaster.
>
> A few questions?
>
> why was render used instead of normal coarse plaster before skimming over?
>

crap brickwork being strengthened, or as a water barrier perhaps -
cinder block not great for rain without a waterproof outside..I HOPE its
cavity..or they had a lot of sand and cement left over?

> What has caused the skim to blow off the render? house is 25 years old.
>

blocks sucked the water out of the render too fast and it dodnt set
properly?

> Can I reskim plaster on top of the rendering or have I got to remove the
> rendering and have two coat plastering on the bare walls instead?
>

either Id say.

> Regards
>
> Stephen.


--
To people who know nothing, anything is possible.
To people who know too much, it is a sad fact
that they know how little is really possible -
and how hard it is to achieve it.

[NT]

no need to remove render before plastering - just knock off any weak
about to drop off bits.


NT

[MuddyMike]

"Stephen H" <ilov...@pleasespamme.com> wrote in message
news:Lrb6r.135111$Uj7....@newsfe05.ams2...

> Hi all,
>
> I've bought a do-er upper house.
>
> The ground floor internal walls are those porous bricks that are the same
> size as thermalite blocks. Believe they are called breeze blocks?
>
> Upstairs internal walls are stud and partition.
>
> The external walls are plasterboarded & skimmed upstairs and downstairs
> whereas the internal walls downstairs are rendered and then skimmed.
>
> well upon stripping the wall paper on the downstairs internal walls, the
> plaster skim has come off in places with the wall paper and underneath is
> rendering rather than coarse plaster.
>
> A few questions?
>
> why was render used instead of normal coarse plaster before skimming over?

I'm no plasterer our last house was completely re plastered, partly onto old
red brick and partly onto new blockwork and the plasterers rendered all the
walls before plastering.

Mike

[Andy Wade]

On 08/03/2012 23:40, Stephen H wrote:

> The ground floor internal walls are those porous bricks that are the
> same size as thermalite blocks. Believe they are called breeze blocks?

Unlikely to be breeze if it's only 25 years old. Coke breeze was a
by-product of the old town gas industry. They're probably just concrete
blocks of a heavier weight than Thermalite or Celcon, etc. Dark grey,
almost black in colour?

> why was render used instead of normal coarse plaster before skimming over?

By 'render' do you mean that it's a sand and cement undercoat? That's
quite normal - a lot of plasterers still prefer it to the ready-mixed
undercoat plasters. (And any plaster undercoat can be referred to as
the 'render coat', whether cement, lime or plaster based.)

> What has caused the skim to blow off the render? house is 25 years old.

Possibly poor workmanship originally - too much cement in the mix,
causing excessive shrinkage, undercoat trowelled up too smooth, giving
insufficient 'key', or the skim applied too soon, before the cement had
set and shrunk.

> Can I reskim plaster on top of the rendering or [...]

Yes, remove all loose topcoat, apply PVA and 2-coat skim as normal.

--
Andy

[Rick]

On 08/03/2012 11:40 PM, Stephen H wrote:

>
> A few questions?
>
> why was render used instead of normal coarse plaster before skimming over?
>
> What has caused the skim to blow off the render? house is 25 years old.
>
> Can I reskim plaster on top of the rendering or have I got to remove the
> rendering and have two coat plastering on the bare walls instead?

Fairly normal practise to rough out a block (or stone wall) with a
sand/cement coat.
This was to get the wall to a true condition ... fill in all the deep
pits and bellies in the work.
It is cheaper than doing it with plaster .. and harder finish.

It was then keyed ready to take a plaster skim on top.

If the blockwork was good to start with then they could use a base coat
of plaster (scratch coat) followed by a finish coat.

here is a section I wrote on plaster for the SelfBuild FAQ:

>>>>>


3.11 Plaster

3.11.1 What is plaster?
Plaster is calcium sulphate hemihydrate. Gypsum is the dihydrate, and is
roasted to extract the water of crystallization and ground. Certain
plasters contain additives to improve texture, adhesion etc. Old plaster
(even a couple of months) is best thrown away, as it either sets very
fast, or not at all.
Confusingly, in some parts of the world "plaster" is a term used to
describe a sand/cement render.

2CaS04.2H2O (heat)»»» 2(CaSO4).H2O + 3H2O

The reaction operates in reverse when the plaster sets.

3.11.2 What should I use and where?

Previously there were 3 main plasters ranges used – CARLITE, THISTLE &
SIRAPHITE

3.11.2.1 CARLITE Plasters
The Carlite range being lightweight retarded hemihydrate premixed gypsum
plasters. Consisting of 4 types of undercoat plasters and a topcoat.

The Carlite range has now been merged by British Gypsum with their
Thistle range of retarded hemihydrate premixed gypsum plasters.

3.11.2.2 THISTLE Plasters
Thistle Plasters are a range of retarded hemihydrate premixed gypsum
plasters.
The Finish grades are used neat, while the undercoat grades are usually
mixed with sand.

Thistle Browning (formerly Carlite Browning) An undercoat plaster for
moderate suction solid backgrounds, which have good mechanical key.

Thistle Slow-setting Browning is as above with longer open time.

Thistle Bonding Coat (formerly Carlite Bonding Coat) An undercoat
plaster for low suction backgrounds (eg. concrete and plasterboard or
surfaces first sealed with a pva wash).

Thistle Plaster Finish (replacing Carlite Finish) is the ideal choice
for over Sand & Cement bases. It can be used on still damp backgrounds.

Thistle Board finish is a one-coat plaster for skim coating of plasterboards

Thistle Multi-Finish is used where both undercoat and skim coat are
needed on
one job. Suitable for all suction backgrounds, and ideal for amateurs.

Thistle Hard-wall an undercoat plaster which provides a much harder more
durable
finish and is also quick drying.

Thistle Universal One Coat A one-coat plaster for a variety of
backgrounds. Suitable for application by hand or mechanical plastering
machine.

Thistle Tough Coat (formerly Carlite Tough Coat) An undercoat plaster
for solid backgrounds of high suction with an adequate mechanical key.

Thistle Dri-Coat A cement-based undercoat plaster for application after
installation of a damp-proof course.

Thistle Renovating An undercoat plaster for replastering.

Thistle Renovating Finish A final coat plaster for use in conjunction
with Thistle Renovating to form a complete replastering system.



3.11.2.3 SIRAPHITE Plasters
This is of historical interest, Siraphite was a plaster that stiffened
quickly, and then a progressive slow set plaster used to give a smooth
finish over undercoat plasters.
It was not suitable for direct application to plasterboards.
A plasterer often mixed this with a Carlite board finish plaster to
increase it’s first set time.
However it is no longer in production by British Gypsum.

3.11.3 What is typically used in practice?
For Ceilings:
For plasterboard ceilings that are being plastered - pretty simple they
are skimmed with Thistle Board finish plaster.

For walls:
If Blockwork the choice is between-
A floating or undercoat of Thistle Browning, followed by a top skim of
Thistle Finish. With the advantage that the skim coat can be applied
while floating coat is still damp. i.e. same day.
Plasterers also like the lightweight of the floating coat - makes the
job quicker & easier. - but expensive.
(it is also possible to use thistle Multi-Finish for both floating &
skim coats, but the finish is not quite as good)


Alternatively rough out with 1:1:6 render mix, followed by a skim coat
of Thistle Plaster finish.
This has advantages that it is cheaper especially over rough walls, but
the floating coat has to be left several days to dry before skimming.
The fact that you then have a high suction dry undercoat, means that you
cannot use the Carlite finish plaster.


Your Plasterer will probably have his own foibles - mine always wants
the top
skim coat to be a mixture of Thistle & Siraphite - Thistle for the good
hard
finish, Siraphite to speed up the initial stiffening.

3.11.4 Wall boards
If you're using plasterboard you don't need to plaster it (see dry-lining
section) this is OK for matt or semi-matt paints, but for silk or gloss
it does
need a skim coat with a 'board finish' plaster.

3.11.5 How do I mix it?
The simplest way in is in a bucket … pour in water to start, then have
someone add the plaster slowly while you mix with a large stick. It is
physically hard going.

I made a plaster mixer - which greatly eases the process, this consists
of a flat round disc 6mm thick, 150mm in diameter welded at it’s center
to the end of a T-bar handle made from 15mm steel tube, about 1.6m long.
The disc has a number of slots cut into it circumferentially about 35mm
in width and 75mm long.

You simply pump it up and down as the mix squirts in between the slots -
makes it a lot less work, faster and a smoother mix.

Many plasterers have scorned this - and then tried to get it off me
after they have used it! (far better than a whisk made of scrap wire)

For Carlite Bonding plaster it is usual to mix this in a galv bath, as
the stuff made with expanded vermiculite and is so light that it tends
to float off everywhere if you try to mix in a bucket.

There are plaster mixing machines - but not really feasible for a single
SelfBuild.
Don’t be tempted to use your cement mixer - it does not mix well enough.

3.11.6 Should I do my own Plastering?
No - this is one of the skills that it is uneconomical to attempt - if
you feel guilty about this, hire a plasterer and act as his labourer.
It is too expensive on materials and too long a learning curve to make
it worthwhile doing it yourself.

3.11.7 Further Information
The British Gypsum White Book is available on line at:
http://www.british-gypsum.com/literature/white_book.aspx
product application guide at:
http://www.artexltd.com

New plaster is highly absorbent. Non-thinned emulsion cannot be brushed
out neatly as it sets straight into the plaster leaving streaks which
would show. You would also use a lot of paint. The first coat, thinned
with water, has enough liquidity to brush/roll on and seals the surface
so further (normal, unthinned) coats may be applied.
Good practise is to use 5:1 water:pva adhesive to seal the surface first.

[js.b1]

On Mar 9, 2:49 pm, Rick <rick_hug...@remove.btconnect.com> wrote:
> 3.11.6  Should I do my own Plastering?
> No - this is one of the skills that it is uneconomical to attempt - if
> you feel guilty about this, hire a plasterer and act as his labourer.
> It is too expensive on materials and too long a learning curve to make
> it worthwhile doing it yourself.

Bollocks.

1 - ANYONE can get the base coat done.

Be that sand-cement render, undercoat plaster or best of all (re long
term heating) stripping the walls and fixing plasterboard backed
celotex or Mapei keraflex bonding marmox to solid brick wall. However,
do not wallpaper direct to plasterboard because removing future
wallpaper makes a right mess of the plasterboard paper covering.
Simple rounded board to shuffle level the render/undercoat level
(parge coat), if you must run then between vertical battens which are
later removed. Scratch surface properly before it dries & the
plasterer can PVA before he skims.

Plasterboard must have scrim tape, although it can just move cracks
elsewhere it does pretty much work.


2 - SOME people can manage to get a basic skim done.

The exception is usually ceilings - unless you have learnt by
plastering the entire house.

First do not use one-coat, it is fine for behind a kitchen cupboard or
a few repairs, lousy for a wall and difficult even for a pro to get
something they will be happy with (and their standards are generally
high and will refuse to use it). Use Thistle finishing plaster or
multi-finish. Mixed in a clean bucket, with clean water, with a
plaster paddle in a drill (preferably a two-speed cheap Ryobi / AEG /
TTI jobbie, but a high-end cordless will do fine).

Learn by skimming the understairs or a large board. Understand
consistency re not too wet, not too dry. Take it all off if necessary
(practice on a board), and repeat. It takes at least 6 goes on at
least 1m^2 to learn the basics. Learn how to polish, when to polish.

There is slim chance of a DIYer getting a good enough finish for
perfect painted walls or ceiling. Usually after finishing an entire
house they take an SDS to the first wall they did and redo it, just
like wiring. Skimming a ceiling is difficult due to the necessity of
wrist action practice, DIY will take a lot of filler and a lot of
sanding (ball jointed unit on a pole).

Skimming a ceiling is plain difficult, and DIY will struggle unless
practiced - it really need stilts or low height tower.

You do not dilute PVA the surface of plaster, it re-activates when
wet. Far better to use dilute emulsion. PVA causes a lot of problems
in kitchens & bathrooms re moisture induced failure. Most tilers will
pull a right face if you PVA. Emulsion can end up a mess with a PVA'd
surface.

Worth saving £2500-3500 on for a house, definately, you could fit
underfloor heating or insulate 50mm celotex everywhere.

All started with Atlee and got worse thereafter...

[Rick]

On 09/03/2012 5:50 PM, js.b1 wrote:

> Bollocks.


Thank you for your valid comment ...



I have built 3 houses ... it makes much more sense for self-builder not
to bother learning skimming ...
Waste material is expensive, speed can often be important .. and it
needs a flawless finish or you can't use just paint.

It's far quicker to get a plasterer in ... and the time you save save
can be spent on the hundreds of other jobs.

I have still save money bay doing the labouring for the plasterer.

I stand by what I said ... although anybody can feel free to ignore it.

I am capable of roughing & skimming, but I'm too slow at it, ends up
with too much going off before I have used it.

[js.b1]

DIY is nothing without learning, and it is not an easy path in many
cases. Mistakes will happen, must be corrected, and there is a cost.
Too many give up, hopefully lack of money might cause some skills re-
acquisition - going by the increasing old age of uk.d-i-y members and
many DIY web forums.

There are plasterers out there who would be better regarded as
creating plaster sculptures, but some doing pretty rough work. The
argument if you can get a tradesman very easily in (previous) good
times, there is a reason the market is putting the best work on
someone else.

The money saved from substantial plastering work can DIY a poor
bathroom into a wet room, DIY a rotten solid walled kitchen into an
insulated warm living space. At least a DIYer can get to a good parge
& undercoat coat, scratched ready for skim. Ceilings are a messy job.


Blown plaster browning / render can sometimes be saved by large-drill-
n-plaster-peg (like failed flooring screeds) with PVA poured into the
holes (screeds etc use SBR). That is assuming the cause has been
rectified, eg, defective guttering which is going to be a boom in the
next years looking around. Looks like the 2010 & 2011 winters pushed
old installs over the edge.

[js.b1]

On Mar 9, 6:10 pm, Rick <rick_hug...@remove.btconnect.com> wrote:
> I am capable of roughing & skimming, but I'm too slow at it, ends up
> with too much going off before I have used it.

It is very hard work.

No worries with standing by what you said, skimming gets harder as you
go from good enough under wallpaper -> good enough for mouldings ->
good enough not to worry about lighting. Seen too many pro's make a
slash job of it, not surprising because it is often a short job and
the real job they were waiting for has suddenly come in... and they
have to finish quick. A case of no trains and then five come along.

[Stephen H]

There seems to have been a bit of confusion regarding where the render
was exactly.

Firstly the house is all brick externally.

It appears that for all of the downstairs rooms, the internal walls are
Celcom blockwork. Rendering has been applied to all of these celcon
internal walls. All the external walls are plasterboarded thermallite
blockwork.

Obviously there is a skim plaster layer on all of the walls in the
internal rooms.

Now in the dining room, the skim plaster came off in various places when
using a wall paper steam stripper. The rendering seems sound so I expect
I can PVA this for the plasterer to skim over. DO I need to remove the
skim plaster off from the whole wall in case more is soon to blow?

Now in the family room,, the skim actually came off the whole wall with
the wall paper. There was a light covering of render powder on the back
of the skim plaster.

The rendering appears to be so powdery that I can rub a finger backwards
and forwards and actually wear a groove in the rendering. I suspect the
rendering is the fault here so I am probably looking at having to take
off the rendering. Now can plaster be applied to celcom blockwork or
have I got to have rendering again?

Stephen.

On 09/03/2012 18:32, js.b1 wrote:

> On Mar 9, 6:10 pm, Rick<rick_hug...@remove.btconnect.com> wrote:

>> I am capable of roughing& skimming, but I'm too slow at it, ends up

[The Natural Philosopher]

Stephen H wrote:
> There seems to have been a bit of confusion regarding where the render
> was exactly.
>
> Firstly the house is all brick externally.
>
> It appears that for all of the downstairs rooms, the internal walls are
> Celcom blockwork. Rendering has been applied to all of these celcon
> internal walls. All the external walls are plasterboarded thermallite
> blockwork.
>
> Obviously there is a skim plaster layer on all of the walls in the
> internal rooms.
>
> Now in the dining room, the skim plaster came off in various places when
> using a wall paper steam stripper. The rendering seems sound so I expect
> I can PVA this for the plasterer to skim over. DO I need to remove the
> skim plaster off from the whole wall in case more is soon to blow?
>
> Now in the family room,, the skim actually came off the whole wall with
> the wall paper. There was a light covering of render powder on the back
> of the skim plaster.
>
> The rendering appears to be so powdery that I can rub a finger backwards
> and forwards and actually wear a groove in the rendering. I suspect the
> rendering is the fault here so I am probably looking at having to take
> off the rendering. Now can plaster be applied to celcom blockwork or
> have I got to have rendering again?
>

celcon sucks like a bitch

not sure on this, but PVA may be a good thing to reduce that a bit

[js.b1]

Older style sand-cement browning can suffer failures.

Bond failure on external walls, often re damp.
- It literally ends up a free standing structure where a spade can be
used to separate several square feet at a time.
- Typically rattles when tapped with your fingers with numerous
surface cracks of the plaster skim.
- Fixed by pouring dilute PVA behind from higher up, or drilling
multiple large holes (25mm) and re-plastering to plug the plaster back
to the wall.

Bond failure to plaster skim.
- Where the surface was either not keyed, too dry, or skim coverage
simply too thin to be mechanically self supporting.
- Comes away with wallpaper, few surface cracks but does not rattle.
- Fixed by heavy application of PVA to create a very hard crust and
stabilise the structure, and kill suction for a re-skim.

It is not necessary to "knock it all back to celcon" (which is highly
porous and sucks water like fish). Doing so leaves you with quite a
sizeable cost if repeated through a house - as well as "pixie-dust"
sand particles everywhere.


One room at a relatives was atrocious. The plaster skim had hundreds
of wide surface cracks from failed plaster skim and browning undercoat
was completely blown. Over 80 large holes were drilled just scraping
the block surface in a pattern over the wall, dilute PVA poured in
(pipette) and the wall took a lot, then one-coat used filled the
holes. All the rattle was gone, the wall was stable. However the skim
was still pretty thin so they were picked open in the worst places,
PVA brushed in to stabilise and create a hard crust, kill suction,
then one-coat skim. Whilst one-coat is not good for skim work, it is
fine on such hole filling with a deep fill then quick skim flush and
quick stanley blade scrape when dry.

It saved a lot of mess on a large wall, and a fair amount of cost
(even to DIY the materials add up), and also mess.

Celcon is difficult to bond to (and solar blocks impossible re
differential expansion), so ripping it all off may not actually be the
best solution. Generally old browning needs a 3-4mm skim coat, often
it got 2mm which is not enough and it will craze crack permitting the
underlying "honeycomb" of sand-cement to destabilise or even crush.
Classic case is when you tighten big screws into rawlplugs and the
shelf/bracket crushes the hard plaster into the softer browning
behind. Make the hole a little bigger after drilling and filling with
plaster around a temporary screw stabilises re compression force from
tightening big screws.

[Stephen H]

but this rendering is so powdery I can rub my finger up and down and
wear a groove into the render....... the skin that came off had render
dust on the back of it which leads me to believe its the render at fault
rather than the skim.

Stephen.

[The Natural Philosopher]

Stephen H wrote:

>
> but this rendering is so powdery I can rub my finger up and down and
> wear a groove into the render....... the skin that came off had render
> dust on the back of it which leads me to believe its the render at fault
> rather than the skim.
>
> Stephen.

Comndier: you have a rtough wall and want a really cheap finish. Sand
with a tiny bit of cement will JUST stick to the blockwork as a crappy
'car body filler' or 'body putty' then skim over. the skim will hold
the render together

anyway slap PVA on it all and reskim