WELCOME TO OUR GROUND FLOOR SLAB PROJECT
This project is presented by Property Repair Systems
Summary: Construction and Repair - concrete ground Floor Slab.
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If you need help with any type of Concrete Slab Repair, Floor to Wall Joint Dampness, or other damp problem Property Repair Systems will be pleased to give you completely free, no obligation advice on 01626 331351. The information you need may be in the Project below, but if not, call them for expert, unbiased assistance with building products.
Many buildings suffer from rot and insect attack in their ground floor timbers and this is mainly due to poor sub floor ventilation (See our Project – Air Bricks and Air Vents). Air bricks become blocked with debris, ground levels are raised cutting off the air flow and water leaks occur. In many of these cases a new, concrete slab floor is laid, to avoid future timber decay and repair cost.
CHECK LIST – timber floor replacement in concrete
However, simple replacement with concrete can cause further building problems, so beware of the following;
- Dry rot in the original timber floor must be properly treated or it may re-emerge at the edges of the new concrete, causing expensive damage to skirting boards and other joinery. Dry rot can spread out into dry areas, so great care must be taken to remove ALL debris before infilling with rubble hardcore (rake the soil thoroughly for timber chippings) and the soil must be sterilised (See ‘How to Kill Dry Rot’) .
- Hardcore rubble and brick, brought in to fill up the old floor void, may also be contaminated with Dry rot, so be sure to treat it thoroughly with a suitable fungicide (See ‘How to Kill Dry Rot’).
- The original wall damp proof course (DPC), which should be visible below the timber joists (e.g. a layer of slate or black bitumen), was designed to protect the timbers, skirting boards and plaster from rising damp. This will now be buried and therefore ‘bridged’ by the new concrete floor, so you may need to inject a new damp proof course at the correct level (See our Project – Damp Proof Injection).
- Your new floor may also ‘bridge’ your neighbour’s damp proof course in the Party Wall, causing the wall to suffer from rising damp. Under the Party Wall Act you MUST ask your neighbour for permission before doing any work that might affect the Party Wall. Check the position of the DPC, the relative ground levels of your two properties and check with us in case Tanking is required to prevent lateral penetration of damp between the two properties (See ‘Seal Damp Walls’). Neighbours are frequently involved in expensive legal actions when this Law is ignored, so please seek expert advice (Contact Property Repair Systems for Party Wall Act advice on 01626 331351).
- Most new concrete floors are laid well, BUT even the best builders tend to forget to seal the floor to wall joint properly. The small gap between wall face and the plastic sheet that rises up the wall from under the new concrete is a high pressure area for water vapour. If this gap is not prepared and sealed properly damp is drawn up the wall and, even if a damp proof course has been correctly fitted, the vapour alone can cause rot in the skirting boards and ‘salting’ in the plaster above (See ‘Epoxy Grouts’).
- Services may pass through the original void – gas, water, electricity, sewage – consult your suppliers before burying any service pipes or wires – most require ducts to be laid and specific safety regulations apply.
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The floor construction of a dwelling must fulfil several criteria and the following design functions must be taken into consideration.
- The provision of a uniform, level surface
- Sufficient strength and stability
- Exclusion of dampness from inside of building
- Thermal insulation (max.0.45 W/square metre K
- Resistance to fire
The sketch below details the most common form of floor slab construction. It will not be applicable in every case. Instruction & building regulation approval must be sought from the Building Control officer of your local Council when you are in doubt. You are reminded that, in a great many cases Planning permission must be given for the construction of new buildings. See our
planning permission project for details.
The cavity can be bridged (closed) at doorways see Cavities Project to carry the floor screed above it, to a doorframe. The insulation shown must be to the specification shown by the architect if it is required. All hardcore should be compacted to refusal and a layer of sand blinding placed on top to avoid puncturing the dpc. Any pipework placed in screed must be accessible through proprietary ducts with removable covers. Foundation sizes(See Foundations Project ) and depths are for local authority approval as are thickness of the layers of hardcore, concrete, insulation and screed together with the gauge of the damp proof membrane.
Repairing a Concrete Floor
Concrete floors suffer from four main defects;
- Open joints between the edge of the slab and the walls – causing floor to wall joint dampness, rot in skirting boards, ‘salting’ in plaster
- Cracks – a symptom of rapid drying when laid or subsidence
- Holes and voids – causing localised dampness, hollowness and collapse
- Punctured Plastic or Liquid Membranes – causing dampness in patches, lifting tiles
- Lack of membranes – causing overall dampness, fungal and mould growth under floor coverings
In all cases the possibility that you have a water leak, above or below the floor, MUST be eliminated before a repair is carried out, or you may cause a problem in an adjacent area (See our Checklist). If cracks, holes or voids have developed you should consult a Structural Engineer to establish the cause.
- Open floor to wall joints – these should be opened up to a slot at least 1 inch x 1 inch (25mm x 25mm) by cutting a slot at the floor edge, in the slab surface, without damaging the plastic membrane. This slot is then cleaned thoroughly to remove all dust and debris and then filled with a special Structural Epoxy Pouring Grout (See ‘Epoxy Grouts’).
- Cracks - these should be opened up to slots at least 1 inch x 1 inch (25mm x 25mm), using a disc cutter or grinder, but without penetrating deeply enough to reach the membrane. These slots are then cleaned thoroughly to remove all dust and debris and then filled with a special Structural Epoxy Pouring Grout (See ‘Epoxy Grouts’).
- Holes and voids – excavate to the full depth of the defect. If the waterproof membrane layer is reached extend the hole laterally to expose sound membrane at least 150mm from the edge of the hole. Remove all dust and debris. The hole can then be repaired, either using Structural Epoxy Pouring Grout, or for holes larger than 8 litres in capacity, a combination of the Grout as a base layer and concrete as a filler. The final surface can be screeded, but the repair edges must be primed and the screed mixed with Ultra BondAcryl, to avoid shrinkage and cracking.
- Punctured Membranes – remove all floor coverings to expose bare concrete, and then run a Dehumidifier until the damp recedes to defined areas. Mark the areas with chalk and excavate them carefully to expose the defective membrane punctures. The hole can then be repaired, either using Structural Epoxy Pouring Grout, or for holes larger than 8 litres in capacity, a combination of the Grout as a base layer and concrete as a filler. The final surface can be screeded, but the repair edges must be primed and the screed mixed with Ultra BondAcryl, to avoid shrinkage and cracking.
- Lack of Membrane - remove all floor coverings to expose bare concrete, and then run a Dehumidifier until the damp completely disappears. If patches remain see (4) above. If not, you can clean the floor to remove all contamination (hot wash with detergent, thoroughly rinse) and repair any defects with Ultra BondAcryl Primer and Mortar. Seal the floor to wall joint as in (1) above and then apply two coats of Ultra Epoxy Seal Coat (See www.epoxy-info.co.uk) If you intend to lay flooring that requires an adhesive (See ‘Epoxy Grouts’). Check with the manufacturer for bonding details – you may need to sand ‘blind’ the second coat of Epoxy Seal to provide a mechanical grip.
If you need help with any type of Concrete Slab Repair, Floor to Wall Joint Dampness, or other damp problem Property Repair Systems will be pleased to give you completely free, no obligation advice on 01626 331351.
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