Plastering / rendering behind kitchen sink?

[FrankE]

Due to a poor tiling and poor seal behind the kitchen sink unit and a waste pipe fracture the plaster behind the kitchen sink is shot. The unit is on an external brick cavity wall with cavity wall insulation and is under the kitchen window.
~The kitchen installers situated the sink further to the left than the sink in the previous installation and didn't [edit:render]

The plaster was mush under the tiles above the countertop behind the sink When I took the tiles off to investigate what substrate the loose tiles was on and took out the base unit backboard. I discovered the plastering / rendering was damaged right down to the floor. This was made worse by a fractured waste pipe. This was all hidden behind the units.

I removed the window frame apron, have chipped the plaster above the countertop line, renewed the waste pipe and allowed the area to dry out.

I haven't finished chipping out all the plaster, I wanted to make sure I have all the materials at hand first. There's only me in the house and I've got kinda used to not having a kitchen sink, so I can live with long drying times

What is the best render / plaster and additives (if any) to use given that it will be prone to regular moisture in the event of sinktop seal or tile seal failing in future?

Mortar? (if so what mix?)
Dri-coat
Hardwall?
Waterproofing admixture? (brand?)

 

[ree]

You would really help your case if you posted pics of above the sink and the wall below it.
Anyhow, the best material, given the sketchy information, would be sand and cement render.

 

[FrankE]

Thanks. Will try and borrow a digital camera and upload image.
Is that soft or sharp sand?

 

[FrankE]

You would really help your case if you posted pics of above the sink and the wall below it.
Anyhow, the best material, given the sketchy information, would be sand and cement render.

The sink bowl is on LHS the drainer on RHS for reference. Spills and spllashes from the sink are hiting the gypsum side while behind the drier draining board side the plaster is cementitious.

I will be replacing the pipework as some is 70s plastic, some in copper.

Below sink where waste goes to cast iron soil stack. On the RHS the plaster is cementitious, On the left it isn't cementitious and there are various bits of what appears to be full thickness one coat, undercoat and skim from over the years. Where I drilled the pipe clips to hold the pipes out of the way of the waste while I work on that (to the left of the tee) the plaster crumbled.

View media item 79401Above sink. To the right some of the old plaster that didn't mush/break away Note I have applied some ready mix mortar (Tarmac Multipurpose, which is cement / builders sand).
View media item 79400Below counter: 1/5 margin at left is plaster and crumbling/mush. Right 4/5s of clip is cementitious and intact (doesn't look like it from pic but had to chip that hard to get a gash screw out ).
View media item 79402
What mix should I use?

 

[NickB_99]

What mix should I use?

There are rendering mixes in the Plastering Wiki
//www.diynot.com/wiki/Plastering-and-Rendering:index

 

[FrankE]

What mix should I use?

There are rendering mixes in the Plastering Wiki
//www.diynot.com/wiki/Plastering-and-Rendering:index

Good FAQ, which covers things in general.

Use a plasticiser/waterproofer in the batch (mix). This will help to aerate the mix, make it easier to use/spread, and the w/proofer will reduce the suction when you come to the top coat. Always “measure” any admix correctly, especially waterproofer!! ....... Lime in a batch is optional, but it allows flexibility
in the finished render coats, and reduces the risk of cracking….. I prefer lime in a mix.

In this instance do i use Plasticiser, Waterproofer or both? I have Feb plasticiser (red container) in the chem store which as far as I know isn't a waterproofer but I didn't get plasticiser this morning because I was already carrying 75kg home on the bus and needed further advice.

Is this stuff OK for waterproofer?
Cementone Freeflo Retarder 365063 5L EAN: 5011734001367 (£7.52)
Retarder, waterproofer and plasticiser - protects against driving rain and damp. Slows setting time - ideal for large areas. Reduces water penetration without acting as a vapour barrier.

or
Cementone Integral Waterproofer 365490 5L EAN: 5011734001268 (£11.54)
Damp-proofer and plasticiser. Does not inhibit vapour transfer. Increases strength of final mix

Neither product is a vapour barrier, which is good as there is cavity wall insulation and moisture that might find its way between brick and render can breathe out and assist in the drying cycle. I'm not bothered about drying times

 

[ree]

Your best practice would be to remove the units and work top and the pipework.
You will then have a free shot at the wall - patching here and there wont fly in the long run.
The above would also give you an opportunity to perhaps re-arrange the sink unit more to your liking? And re-arrange the plumbing.

Your difficulties might come from the splash-over you mention plus condensation below the sink unit.
Or there may be penetrating damp at the lower levels.
Removing a few interior bricks and examining the cavity for blockage and/or sodden cavity insulation might help?

A basic mix of 3:1 sand to cement will do fine for your job - no skimming required, just a rubbed up S&C surface.

 

[FrankE]

Your best practice would be to remove the units and work top and the pipework.
You will then have a free shot at the wall - patching here and there wont fly in the long run.
The above would also give you an opportunity to perhaps re-arrange the sink unit more to your liking? And re-arrange the plumbing.

Your difficulties might come from the splash-over you mention plus condensation below the sink unit.
Or there may be penetrating damp at the lower levels.
Removing a few interior bricks and examining the cavity for blockage and/or sodden cavity insulation might help?

A basic mix of 3:1 sand to cement will do fine for your job - no skimming required, just a rubbed up S&C surface.

Many thanks.

After two trips on the bus to B&Q yesterday I've now got plastering sand, portland, hardwall, waterproof PVA, GP mortar, Cementone Freeflo for this and other jobs around the house, so I've no excuses not to get on with it

There isn't any moisture ingress from outside. There was only egress from the fractured waste pipe I removed. I chiselled out the old waste which was wrapped in the aluminium core of a cable drum, inspected the cavity, fitted a 110mm pipe in the penetration, formed it at the cavity and mortared it in outside. It goes about 3/4 the way through. The cast iron socket the waste caulking joint goes into fits quite snugly into the 110m pipe
There's a couple of slater brothers who also do roughcasting down the road who can finish the outside but I have underground clay drainage to get repaired/replaced first.

That has all been sprayed with anti sulph (vinegar/surfactant/water) and thoroughly dried out. I've got some nice bits of coated aluminium from a cladding job to fold up a splashback terminating inside the bowl to help prevent spillages getting down the back but 'm in the habit of planning for worst case scenarios hence the re-render.

I'd like to but am unfortunately unable to shift things left or right as the kitchen is small and L-shaped such that moving the washing machine which is on the right of the sink under the draining board would require lifting it over the end of the sink unit to situate it in the foot of the L. Chain block and ceiling rail job.

Damn 1920s-built place wasn't designed for mod cons!

I have however marked to position of the units to pull them out and get a run at the wall. I got 5L of waterproof PVA to seal up the exposed faces of the units and the worktop joints, something else the fitters didn't do.
There isn't much space in the rest of the house to move units and worktops to but it's going to have to be done.

The idea was also to replace the plumbing with Armaflexed and Armaclad copper. A succession of plumbers have refused to tee into the horizontal plastic run and have run pipes all over the place to where they can tee into copper. The result is a convoluted flow path and cludge of pipe fittings where a simple Tee below the boiler would have sufficed.

My late dad had the kitchen and bathroom work done when I lived abroad (in a nice Passivhaus flat in DA that never needed heating). so what would have been a small amount of work bring the work up to a good finish (and in the bathroom airtightness) is taking much much longer. Holes one could get one's arm through around the shower tray waste, WC waste and gaps of up to 12mm at the floor perimeter under the shower is taking natural ventilation a bit far.

The plumbing should have been replaced with copper horizontal runs before the kitchen and bathroom (adjacent) were fitted and the pipes made inaccessible by the kitchen units and shower enclosure.

 

[ree]

Well done, i admire your energy and commitment. Getting on a bus with building materials is something else. You must be pretty determined.