external render for patio walls

[browfish]

I have built some blockwork retaining walls around our new patio. The are about 3 foot tall. I plan on rendering them and installing a slate capping and slate patio.
I had planned laying the capping first which overhangs the wall and then rendering from the underside of this down to the foundation, then laying teh slabs up to the edge. Would this be ok? i appreciate that you wouldnt normally render to ground level as you would breach DPC's however this is only on a garden wall and not a house wall. Will the absorbtion of water from low level cause the render to weaken over time or make it go green etc?
If this is ok what mix should i use. Looking at previous posts they suggest 5 sharp sand, 1 cement and plasticiser. Should i add fibres or a waterproofer?
thanks

 

[browfish]

Does anyone have a view on this. Plan on doing this tomorrow since the weather is good
thanks

 

[Alastairreid]

1st coat dampen the wall down 4/1 plastering sand with a splash of waterproofer 2nd coat 5/1 with a splash of plasticiser.

I would lay the slabs first and render down to them.

 

[r896neo]

One way to do it is to use a course of engineering brick at ground level and then stop the render short of these with a bell-cast so you leave a gap of a couple of inches at the base. The right engineering brick will match your slabs as well.

 

[Alastairreid]

i'm assuming the walls are already built.

 

[browfish]

Yes the walls are already built so i dont think the engineering brick idea will work although i like it.
concrened that if i lay the slabs first and render down to them that i may not get a neat edge due to the uneveness of the slate, and that the ed would be liable to crumble away.
Perahps i should use the base coat with waterproofer as you suggest and bring this out thicker at the base so that water can not penetrate from behind the wall or from the ground. Then us the top coat finishing flush with the slabs. would this work?

 

[r896neo]

Sorry, i'm a dope didn't read that part.

 

[Alastairreid]

Use a stop bead to finish the render at the slabs.

 

[browfish]

The stop bead above the slabs wont work due to the slate being uneven. there would be large gaps in places. I have now laid the slabs up to the edge and i am going to lay a thin strip around the edge as a skirting. The stop bead then sits on this
thanks for the help

 

[browfish]

What is the benefit of plastering sand over normal builders sand. Is it simply the texture? As the stop beads are around 15mm wide, i guess i have to render in 2 coats? Can i use builders sand for the first coat with waterproofer and then do the thin top coat with plasteres sand? I ask because i have 2 tonne of builders sand left over.
How thick should the coats be? is it like internal plaster where the top coat is around 3mm?
Should i put waterproofer in the top coat too?

 

[Alastairreid]

Aim for your scratch coat to be about 8mm and the top about the same depth.
waterproofer in scratchcoat, plasticiser in finish.

Sharp sand

This coarse sand has large particles and is used in mixing concrete. Sharp sand can also be used to mix a tough, extremely hard mortar.

Builder's and plasterer's sand

Builder's sand (or bricklayer's sand) is finer and softer and mixed with cement and water to produce the mortar for laying bricks and blocks. Plasterer's sand is a finer grade than sharp sand and mixed with cement and water to produce render.

Kiln-dried silver sand

Very fine sand with a pale colour, kiln-dried silver sand is used in grouting the joints between exterior paving or patio slabs.
Other materials used in mixing concrete and mortar
Lime
Although cement already has a lime content, adding more lime to the mix produces mortar that is easier to work with and increases its resistance to cracking after is has set. Although very effective for this purpose, lime has generally been replaced with plasticizer
Plasticizer

Plasticizer is available in powder form but is generally supplied as a liquid. It eliminates the use of lime when mixing mortar, while making the mix easier to work. Mixing instructions will come with the plasticizer (usually on the label), which will include the correct quantity to add.

 

[browfish]

Thanks
You mention that builders sand is finer than sharp sand. You also mention that plasteres sand is finer than sharp sand, but what is ther difference between plasterers sand and builders sand? Can i use builders sand for atleast the scratch coat?

 

[Alastairreid]

Can i use builders sand for atleast the scratch coat?

There are different types of building sand, and what type you have will depend on what part of the country you are in....Different quarries supply different types and grades.

Your local builders merchant should be able to give you good advice based on your location, and availability of sands in that area.

A good quality sand for rendering will be made up of grains of slightly differing sizes....This is what gives it its strength.

 

[browfish]

Any ideas on builders sand in southampton
It was bought from Jewsons and dont know whether you class them as a good builders merchants or just a big builders merchant

 

[Alastairreid]

An expensive builders merchants

They will probably source their sand from a local quarry so you would need to find out from them which one.