Building? Why your floor needs to be at least two brick courses above ground level
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.
This is the part that is constructed on top of the concrete footings to the floor level of the house. Some people refer to it as the foundation. The building of the plinth is very important and you will have to spend some time making sure that it is correct. It will save you a lot of heartache later.
Albert says: “If you don’t do anything else, do this!”
Study the detail in the sketch about the plinth very closely, and insist that your house is built that way. Note that the plaster is stopped about 50mm above the concrete footings.
Do not extend the plaster to the top of the concrete footing. Water will penetrate the soil and surround the concrete and the brickwork of the foundation, from where it will rise with capillary action in the plaster and the bricks. When the plaster is stopped above the concrete, this action is delayed.
The DPC (damp proof course) must be visible after the plastering is done.
It is advisable to build the outside walls of a double-storey house 330mm wide and the inside walls 220mm wide in the plinth. Also do this when the height of the plinth is more than 1m.
Your presence during the setting out process is important in order to give final instructions on the different levels. Alternatively ask the contractor to mark it out and wait for your approval before he proceeds. Ask for advice when you are not sure because it will be costly to alter it later.
The first thing is to determine the floor height of the house above the natural ground level. There will be many different levels when the building is on a slope, so make sure that it is done the way it was designed and how you want it to be.
The floor level must be a minimum of 2 brick courses above natural ground level, but higher is better. This also applies to the floor of the garage. This should be lower than that of the house when they are connected. The ground must slope away from the house. This slope must be 50mm per horizontal metre for a distance of at least 2m. There must be a step from a sliding door to the patio and another step from the patio to the garden. These steps must be in multiples of a brick course (85mm).
Check that the building is square and level.
All the walls must rest in the middle of the concrete footings. Mistakes can be rectified by making small changes to the sizes of the rooms but only with your permission.
Pay special attention to the height of all the outside and garage doors so that water will not get in.
- It must not contain any wood, plants, plastic or paper because they disintegrate and pockets will form in the filling, which will lead to subsidence of the slab.
- Take a handful of damp soil and roll it into a ball. If the ball dries in a hard lump, then it is too clayey and not suitable to be used.
- The filling material must be damp when it is being compacted, as it is impossible to compact dry materials.
The filling in the plinth must be done in compacted layers of about 200mm each. It is recommended that the top layer of about 20mm should be river sand. It will provide an even surface before the water proof membrane is put down.
- The waterproof membrane on top of the filling must be 375Micron thick. Thinner material can easily get damaged by the filling and concrete.
- The joints between the sheets should overlap and must be sealed with tape.
- The membrane should be taken up the inner face of the outside wall.
- The thickness of the surface bed concrete will be shown on the plan, either 85mm or 90mm. Some contractors make the filling higher in the middle of the rooms in order to save on the quantity of the concrete.
The surface bed must NOT be taken over the internal walls, i.e. one slab across the entire floor of the house. The brickwork of the inside walls must divide the concrete slab into blocks, especially in large houses in order to accommodate movement and to prevent cracks. Expansion joints in the surface bed must be transferred to the screed and the flooring.
- It is recommended that 15MPa or 20MPa concrete be used for the surface bed.
- Curing of the concrete, by keeping it wet, must start within 2 hours after casting, and be done for as long as practically possible.
If the height of the plinth is more than about 1m, then reinforced mesh must be installed above the waterproof membrane. This is done in order to strengthen the surface bed concrete and limit the development of cracks of the tiled floors. The mesh must not rest on the membrane but be raised in order for concrete to be underneath it.
- It is recommended that mesh always be installed in the surface bed.
- Insulation material like polystyrene must be installed under the surface bed in areas where under tile or under carpet heating is installed. This is an energy saving requirement.
- The surface bed is supported by the inside 110mm skin of the outside wall. Ensure that it is clean before the concrete is poured.
- The poison spray must be done on top of the filling and around the outside of the house before the concrete is placed.
Vertical damp course
Vertical damp course (375Micron thick plastic sheet) must be installed where there are steps in the floor levels. The plastic must be installed on the higher side of the wall to prevent damp to surface the lower side.
The most efficient way to do this, is to have those walls water proofed before the filling is done with a torch-on product, like Derbigum, which will guarantee that no damp will ever surface on the walls.
Make sure that the vertical damp course is not damaged by the filling as this mistake will only be visible after a few years when the paint and plaster will blister and peel.
Next time: Why the bricklayer MUST lay the floor in a single session