Building? How to know if your plasterer is using high-quality sand
There is no user manual for all aspects of homeownership, from moving, to taking occupation and to maintaining and understanding common and uncommon defects. Albert van Wyk has more than 38 years’ worth of building experience and has put all he has learned into a concise, easy-to-use reference book entitled, The Proud Home Owner. He has granted HomeTimes exclusive access to republish portions of his book to help homeowners make better decisions around buying and selling, as well as maintaining their properties.
The anchors should be evenly spaced where the trusses or beams will rest on the walls.
Double strand of 4mm galvanised steel wire embedded at least 500mm below the wall plate.
30mm x 1.6mm galvanised steel strap should be embedded at least 600mm below the wall plate.
A sliding door consists of a fixed panel and a sliding panel. Make sure that it is correctly installed, with the sliding panel on the outside. Viewed from the outside the slider must be in the front or water will leak into the room.
There should be slots on the bottom part of the frame to drain water away that falls on the outside panels.
Treat the wooden frames before they are installed, making sure that they are also treated on the outside where they are against the bricks.
Apply one coat Pink wood primer if they are to be painted, or use a good oil-based product if they are to have a natural look.
It is not good practice to paint outside wooden door and window frames. Natural timber is “living” and it expands and shrinks due to moisture in the air and the sun will dry it out. Water can get trapped behind the paint which will cause excessive swelling of the timber and it eventually will become rotten.
The door frames must be protected from damage by the workers and their tools during construction, because it is difficult to restore the corners of the frames later.
Albert says: “Nail pieces of wood on the inside of wooden door frames to protect it from wheelbarrows causing damage”
The frames can either be anodised or powder coated. The average anodised thickness is 15Micron. This thickness is suitable for rural areas which are free from industrial pollution. Grade AA25 with 25Micron thickness should be used in the coastal areas and near heavy industrial areas or chemical factories.
First-floor and roof slabs
The design, erection and pouring of the concrete is the responsibility of an engineer who, after completion, will issue a certificate to you. Make sure that the engineer was actually on site to do the inspection and did not sign it off “sitting behind his desk”.
Block and lintel system
It consists of pre-stressed ribs (like a concrete lintel) and hollow-core blocks with concrete on top.
You should inspect the block and lintel system yourself on the top before the concrete is placed.
If it is at all possible, try to be present while the engineer is doing the inspection before the concrete is poured. Demonstrate your knowledge and look at the following:
- There should not be any loose objects or dirt between the blocks or between the steel reinforcing where the concrete will be poured.
- Ensure that the small space of about 50mm between the ends of the lintels and the outside brick course is clean. There should be 15mm thick polystyrene next to the brick course.
- The whole area must be wet before the concrete is poured.
- The concrete slab must be kept wet for 7 days by covering it with either sand, plastic or hessian material.
- Check below that there is a double layer of DPC between the lintels and the walls.
I have promised you that this is not a technical handbook, but a working manual. However, I would like to explain more about a few products which are extensively used during the building of your house.
Portland cement is a common type of cement for general use. It is also referred to as OPC (Ordinary Portland Cement). There are 5 different types of Portland cement. The type as well as other information is printed on every bag.
The other type of cement is “Masonry Cement”, a blend of Portland cement, ground limestone or hydrated lime.
A plaster mix with masonry cement will have lower strength then those made with Portland cement at the same ratio.
This is a mixture of cement, river sand and stone. The mixed proportions will determine the strength of the concrete. Ensure that not too much water is added because that will reduce the strength.
To make 1m³ of medium-strength concrete you have to mix 7,7 bags cement + 0,62m³ river sand and 0,62m³ stone. For high concrete strength you need 9,2 bags cement + 0,6m³ river sand and 0,6m³ stone.
This is a mixture of cement and building sand, which is mainly used to construct the brick work. It is important that the mix is correct and consistent, because this is what keeps everything together.
The commonly used mixture is 6:1 for ordinary brickwork. This means one container of cement and 6 containers of building sand.
However, these days the mix is made by using wheel barrows.
Please note that a mixture of 6 wheel barrows of sand and 1 bag of cement is not correct. 2 Bags of cement is equivalent to 1 wheelbarrow.
The correct mix is, therefore, 6 wheel barrows of sand and 2 bags of cement.
Albert says: “Visit the Cement & Concrete website www.cnci.org.za for more details on the different mixes and defects of plaster”
To lay 1,000 bricks you need 3 bags of cement and 0,6m³ building sand. You can use clean soil from your garden if necessary.
A general guide for the DIY homeowner is that about 15,5 wheel barrows of sand is equal to 1m³ .
This is a mixture of cement and plaster sand. Plaster sand is very fine and some quarries supply washed sand. Remember that the plaster is the final finish that you will see and it should therefore be the best.
Plastering is more sensitive and difficult than it seems. It is more than just mixing cement and sand which is then applied to the walls. You should appreciate good plaster work, because it is not applied with a machine, but by a skilled tradesman. I also like to refer to it as “handmade”.
The type of cement is important, as well as the quality of the sand. A washed sand with a low clay content should be used.
It is recommended that ordinary Portland cement CEM-1 or CEM II-A is used for plastering. You will find this information, as well as the mix proportions on the cement bag. The NHBRC does not allow the use of MC 22, 5 X or MC 12, 5 X for plastering.
CEM II B-V or W is recommended to plaster walls which are exposed to damp conditions, such as the plinth below damp proof course.
If you are sensitive about the quality of the finished plaster product, then you should make sure about the quality of the sand. In order to determine the quality of the sand, you and the contractor can do a simple test: Weigh 2,5kg of Portland cement, and mix it with 12,5kg air dry sand.
The mixture should not require more than 2,75 litres of water to reach a suitable product. If a lot more water is needed, then the sand is not suitable. River sand can be added to the plaster sand to achieve a suitable product. Do another test and add a measured quantity of river sand to the plaster sand.
Checks and balances
Make sure that all the electrical conduits and plumbing pipes are placed deep enough (50mm) in the walls and are well secured.
It is recommended that an expanded wire mesh or chicken wire is nailed over all the pipes in the walls and covered with a mixture of river sand and cement before the plastering is done. This is to prevent plaster cracks over the electrical and plumbing pipes.
The dust must be washed from the walls before plastering can start. As a rule of thumb cement bricks must be dry when they are plastered. However, clay bricks should be wet because they absorb water faster than the cement brick. Cracks will develop in the plaster when they dry too fast.
While the plasterer is working some plaster mix will fall on the floor. It is not good practice to collect this and reuse it after water was added. The strength is now not the same as the original batch. Material which is not applied to walls within 2 hours of mixing should be discarded. Encourage the contractor to rather mix small batches than one large one early in the morning and then keep on adding dry cement and water during the day.
Next time: Plaster finishes for every taste