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Swimming pool queries

Making a swimming pool smaller 

I have a gunite swimming pool with a marble-plaster finish. The pool has a volume of approximately 50 000 litres.

Due to the water shortage in the Western Cape, I have decided to reduce the size of my pool. I intend building a wall

across the width of the pool using cement blocks. The wall will have to be 4.5m long by 1.6m high.

My questions are:

1. How do I anchor the wall to the bottom and sides of the gunite pool?

2. As I progress with building the wall, do I fill the block cavities with mortar or concrete?

3. What waterproofing product should I mix with the mortar for plastering the finished wall?

4. Is there paint on the market that I can paint the completed pool with? The marble finish is very rough as the pool is 40 years old. Some of the marble finish has also worn away and there are some faint cracks. The pool doesn’t leak and topping up is only required during summer due to evaporation.

Terry Brown, by email


Sharl Bennie, independent building expert, replies:  Drilling steel rebar pins into the existing gunite wall is a good way to anchor the brickwork. I find it better to alternate the angle of the bars slightly in different directions for additional strength (8mm rebar is good). The wall should be completed in a single session and a brick force should be placed at least every second line to reinforce the brickwork (these can be bought at building supply shops). Give the wall enough time to cure and then backfill the unused cavity left on one side. This is done by laying plenty of compaction in layers not exceeding 150mm at a time. Do not use large pieces of building waste in the backfilling – it does not compact very well..


Cindy Engels, sales and marketing director at A.Shak, replies: When the new wall has been completed as per the given advice, paint the blockwork with a slurry made up of two parts river sand and one part cement mixed with TileLoc, which will waterproof and provide a good bonding key to the plaster. You can always paint another coat of two parts cement mixed with one part TileLoc over the plaster to give an additional waterproof membrane; a double barrier is always better.

George Killian
The bottom of my pool is stained and discoloured, caused by black algae. Do I need to drain the pool to clean it or can you
recommend an easier option?

Ed replies:
Most pool companies agree that once black algae is established, it is extremely difficult to control. It is said that the algae penetrates the plaster and concrete and it may develop a mucus layer which needs to be penetrated before you can remove it.

If the staining and discolouration is severe, it may be a problem. First try the bleaching method before opting to drain the pool by adding hydrogen peroxide, which is effective but not guaranteed. The treatment is lengthy and it is best to consult your local pool shop for the correct procedure and aftercare. When using hydrogen peroxide in your pool water, chlorine may not be added for at least five days and no swimming should be allowed during this period.

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