How to Sand and Cement Render?

[wetcoat2]

Can anyone clear up some queries I have about sand and cement rendering as I hear contrasting opinions. I would like to Render a large block wall, hopefully whilst the weather is nice ;

1 Should I use sharp sand or building sand and to what mix per cement?

2 Do I apply PVA direct to the wall or add it to the mix?

3 To what thickness is the scratch coat applied?

4 To what thickness is the render in total?

5 What does it mean to float off using a wooden float?

6 Do I use mortar plasticiser or Lime to make the mix more pliable?

I have a good idea of the general principles but would like to clear up these points before I start slapping stuff on the walls.

 

[roughcaster]

Use plastering sand to cement render. 4 sand-1 of cement (4-1) for scratch coat with a combined waterproofer/plasticiser added. If the blockwork has a good enough key, you wont need to use PVA on the wall or in the mix. Just give the blockwork a wet down as you go, and apply the first coat to the wall say 6 or 7 mm thick, and then scratch it, ( an old 6" paint brush handle with "small" nails hammered in along the top works fine), scratch just deep enough to provide a key, but not right through to the blocks. There is no set thickness for the overall render, but the two coats combined would be around 15mm give or take the odd ml. The top coat, applied a day or two after the scratch coat has set and cured, would be a mix of 5 plastering sand, 1 of cement and 1 of lime (5-1-1). A waterproofer/plasticiser would be added to the water prior to mixing. Again, wet the wall before you top coat it.To "float off" using a wooden float, is to close up the render surface after being screeded/straightened off with the straight edge or darby. Floating is a process that is ongoing, and involves going over the same area of wall with the float a few times as the render goes through the stages of going off. When the render is at the final stage of "floating off", it is then sponged over (sponge finish)to give it more closer finish, and an even, slightly sanded texture.. There is a lot of work involved in cement rendering and if you're without experience, you have a daunting task ahead of you, but good luck to you anyway. A good dry day with not too much sun is perfect, you don' want it to dry out too quickly.

Roughcaster.

 

[wetcoat2]

Thanks mate, I'll give it a go on that solid bit of advice

 

[MisterSandman]

Just a quick note - a Sharp Sand refers to the angularity of the grain. Make sure that any sand you use for plastering or rendering is suitable for the purpose.

Typical names include 0/2 Washed Medium Plastering Sand and 0/4 Washed Fine Plastering Sand, also Type A plastering sand and Type B plastering sand are sometimes used. Or just ask the quarry or builders merchants for a plastering/rendering sand.

Regarding sand/cement or sand/cement/lime mixes, basically you're aiming for each successive coat to be a little weaker than the last, to avoid cracking.

Hope this helps and good luck with the project.

 

[major]

hi roughcaster,

as i have never rendered, just interested in why the lime is used?

 

[roughcaster]

Sand and cement render with lime mixed in it, gives the finished render coat greater flexibility, against any small structural movements when it's on the wall, and will greatly reduce the risk of the render cracking..... Sand and cement (render) on it's own, "having little or no give", is more liable to develop small hairline/movement cracks, although not always....... Lime in each batch also gives a good smooth texture to the mix itself, giving it great workability, making it far easier to spread etc...... A lot of stonework is also built using mortar with lime, again allowing flexibility to any movement in the structure,..... There's quite a few benefits in using lime........ I like the odd drop in a pint of lager too.

Roughcaster.

 

[major]

Oh ok, brilliant I never knew that, thanks for the info.

I'm more of a beer on it's own man so if I should ever need to render I will just keep the lime in that mix

 

[Jonnyboy40]

Hi

I have an area of penetrating damp halfway up a wall. I've been told to use a sand/cement render with a salt retardant additive. Seems to fit with the advise above.

When I remove the existing plaster should I be removing it right back to the brickwork?

Cheers

J

 

[roughcaster]

Absolutely,, remove it right back to the brickwork, then give it a good brush/wash down. Make sure it's got a good key as well. When you put on the scratch coat, make sure you don't scratch it too deeply, meaning, don't scratch right through to the brick themselves,, and use a good quality waterproofer/plasticiser.

 

[Richard C]

Hi

I have an area of penetrating damp halfway up a wall. I've been told to use a sand/cement render with a salt retardant additive. Seems to fit with the advise above.

When I remove the existing plaster should I be removing it right back to the brickwork?

Cheers

J

Jonnyboy40 please look at the last post dates; this thread is 18 months old, if you have a query, please post your own thread. Resurrecting old threads, no matter how relevant they may appear, causes confusion.

 

[gday2uk]

This is a very informative thread. Should be 'pinned' near the top or whatever they call it....

 

[thechief13]

i ve been doing sand and cement with skim finish for a few months and find that when the job as been done a couple of weeks later the walls start showing little cracks, is there any way to stop this thank you for your repiles

 

[Richard C]

i ve been doing sand and cement with skim finish for a few months and find that when the job as been done a couple of weeks later the walls start showing little cracks, is there any way to stop this thank you for your repiles

Poor preparation; please post in your own name; do not hi-jack someone else's post & particularly not one that's 18 months old.

 

[thechief13]

sorry new to this did not know

 

[Micilin]

Sand and cement render with lime mixed in it, gives the finished render coat greater flexibility, against any small structural movements when it's on the wall, and will greatly reduce the risk of the render cracking..... Sand and cement (render) on it's own, "having little or no give", is more liable to develop small hairline/movement cracks, although not always....... Lime in each batch also gives a good smooth texture to the mix itself, giving it great workability, making it far easier to spread etc...... A lot of stonework is also built using mortar with lime, again allowing flexibility to any movement in the structure,..... There's quite a few benefits in using lime........ I like the odd drop in a pint of lager too.

Roughcaster.

it also gives the render a bit of 'fat' - if you ever see a napped finish, you could not do this without the benefits of lime, and it makes it nicer to sponge up after floating.