plaster vs render

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.

Reply to
Stephen H
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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?

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

Reply to
The Natural Philosopher

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

NT

Reply to
NT

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

Reply to
MuddyMike

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?

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.)

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.

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

Reply to
Andy Wade

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:

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application guide at:
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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.

Reply to
Rick

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 =A32500-3500 on for a house, definately, you could fit underfloor heating or insulate 50mm celotex everywhere.

All started with Atlee and got worse thereafter...

Reply to
js.b1

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.

Reply to
Rick

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.

Reply to
js.b1

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.

Reply to
js.b1

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.

Reply to
Stephen H

celcon sucks like a bitch

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

Reply to
The Natural Philosopher

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