Solving problems with Concrete Floors and Replacing a Timber Floor with Concrete After Dry Rot

This project is sponsored by Property Repair Systems

Summary: How to solve problems with dry rot by laying a concrete floor. The best guide to prepare your foundations for a concrete floor, and ensure that dry rot is thoroughly treated before you pour your concrete slab. Solving problems with existing concrete floors including damp patches, holes and cracks.

If your ground floor timber floors have become infested with insect damage or dry rot you might choose to replace them with concrete floors. However laying a concrete floor to replace your timber floors may not be the end of your troubles if you don’t take care to treat the problem before you lay a concrete slab for your new floor.

Dry rot can spread through and over most building materials once it is established so it is really important to treat it thoroughly before you replace your floor.

If you would like any additional expert advice on treating timber decay and damage call Property Repair Systems on 01626 336180. They will be able to assist you and answer any further questions you may have.

These points all need consideration before you lay a new concrete floor in your home:

  • 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 and extensive 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 filling the void under the old timber floor to raise the level with rubble hardcore. Rake any soil thoroughly for timber chippings. The soil must be sterilised, you can do this using a boron treatment which is available from Property Repair Systems.
Dry rot present on timber joists

Dry rot present on timber joists

  • Hardcore rubble and brick, brought in to fill up the old floor void, may also be contaminated with Dry rot, so be sure to also treat this thoroughly with the same boron fungicide.
  • The original wall damp proof course (DPC) was designed to protect the timbers, skirting boards and plaster from rising damp. The damp proof course is usually made from black bitumen, slate or plastic, and should be should be visible below the timber joists. By laying a concrete slab you may alter the original level of the floor and bury the DPC. You will in effect cause a bridge between the ground and the wall by doing this and in fact we use the term ‘bridged’ when describing this effect. If your new concrete floor bridges your existing DPC you may need to inject a new damp proof course at the correct level, and we explain how to do that in Damp Proof Injection Project Page.
  • Unless you live in a detached property your new floor may also ‘bridge’ your neighbour’s damp proof course in the Party Wall, causing their 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.You can also ring Property Repair Systems for more advice on the Party Wall Act on 0843 636 8193 . Check the position of the DPC and the relative ground levels of your two properties. If there is a problem with the level of your new floor in relation to your neighbours DPC you may need to consider tanking the wall to prevent lateral penetration of damp between the two properties. To find out what to use when tanking walls you can visit Property Repair Systems for more information. In our experience neighbours are frequently involved in expensive legal actions when this Law is ignored, so please seek expert advice before you start work.
  • Most new concrete floors are laid well, BUT, even the best builders can forget to seal the floor to wall joint properly. This leaves a small gap between the wall face and the plastic DPC sheet that rises up the wall from under the new concrete. This becomes a high pressure area for water vapour. Even if a damp proof course has been correctly fitted, if the gap is not prepared and sealed properly, damp can be drawn up the wall and the vapour alone can cause rot in the skirting boards and ‘salting’ in the plaster above. This can be avoided by sealing the gap with and epoxy grout, which you will find on Property Repair Systems website.
Epoxy pouring resin used to fill gaps

Epoxy pouring resin used to fill gaps

  • Pipes and other services may pass through the original void – gas, water, electricity, sewage etch. You should consult a professional before burying any service pipes or wires, most require ducts to be laid and specific safety regulations apply. You will also need to seal around the pipes or ducting to prevent moisture build up.
  • If you are laying a concrete floor following extensive insect damage you should ensure that all remaining timbers and skirting are thoroughly treated before you pour your concrete floor, to eliminate the chance of further infestation in other parts of the house.

Concrete Floors DIY

If you have decided to lay a concrete floor yourself there are some points you need to know. The floor construction of a house, or dwelling, must fulfil several criteria, and the following design functions must be taken into consideration when laying your concrete slab.

Your floor must comply with the following:

  • You must provide a uniform, level surface
  • It must be sufficiently strong and stable
  • You must exclude damp from coming inside the building
  • Thermal insulation to current regulations
  • Resistance to fire

We have provided a sketch below to show the most common form of concrete floor slab construction. However it will not be applicable in every case. Speak to the Building Control officer of your local Council for instruction and to obtain the necessary Building Regulation approval. You may also need to apply for Planning Permission, if you are constructing a new building, and we go into this in more detail in our Planning Permission project.

Foundation cross section for concrete floor

Foundation cross section for concrete floor

Where we indicate insulation on the drawing, we should point out that this must be to the specification shown by the architect, if it is required.

All hardcore should be compacted to refusal, that is to the point where is will not compact any further – this should be done using a mechanical compacter. Then place a layer of sand blinding on top to avoid puncturing the Damp Proof Course. As previously mentioned any pipework or cables placed in the screed must be accessible through proprietary ducts with removable covers.

When it comes to the actual thickness and construction of the foundation you may find it helpful to visit our Foundations Project for a fuller understanding. Basically the foundation sizes and depths for local authority approval is the entire thickness of all the layers: hardcore, concrete, insulation and screed, together with the gauge of the damp proof membrane.

If you have Cavity walls then bear in mind that a new concrete floor could bridge the cavity at doorways where the floor screed meets up with a door frame. You can find out more in DIY Doctor’s Cavity Walls Project.

Cavity wall cross section

Cavity wall cross section

Repairing a Concrete Floor

Concrete floors are very durable and are relatively maintenance free during their long life. However if there were any problems with the original laying of the base it is possible that, if you have an existing concrete floor, it may need some repair or maintenance after it has been down for a period of time.

Concrete floor problems

Concrete floors suffer from four main defects:

  • Open joints between the edge of the slab and the walls. This can cause floor-to-wall damp, resulting in damp patches, wet and dry rot in skirting boards, and ‘salting’ in plaster.
  • Cracks. There can occur due to subsidence, a poor or unstable foundation, or they may be a symptom of rapid drying when the concrete was poured.
  • Holes and voids, this can be caused if the concrete is not poured correctly or again by a poor or uneven foundation. It can result in localised damp patches, hollow spots and in severe cases collapse.
  • Punctured Plastic or Liquid Membranes, usually caused by a poor foundation where rubble has not been properly compacted and levelled and then a blinding sand laid to protect the DPC form puncturing. This can cause damp patches, and lifting of floor tiles and other coverings.
  • Lack of DPC membrane. If the foundations were not covered with a damp proof membrane before the concrete slab was poured then this can cause moisture to leach up into the floor from the ground. This can cause damp patches, overall dampness, which in turn will damage floor coverings by lifting floor tiles and laminates, and causing fungal and mould growth under floor coverings such as vinyl, lino and carpet.

Damp Patches on Your Concrete Floor

If you see damp patches forming on your concrete floor you must make sure you check out the possibility that you have a water leak, above or below the floor. The possibility of a leak MUST be eliminated before you attempt to carry out a repair. Otherwise you could cause a problem in an adjacent area (See our Checklist). If the damp is caused by a water leak eg from a leaking radiator pipe, it will be much more easily fixed then a punctured DPC.

Cracked and broken concrete floor

Cracked and broken concrete floor

If cracks, holes or voids have developed you should consult a Structural Engineer to establish the cause. If they recommend that you make repairs then the remedy depends on the problem with the floor, here is a guide to the best way to deal with damaged concrete floors:

  1. 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.  Clean the slot thoroughly to remove all dust and debris and then filled with a special Structural Epoxy Pouring Grout, which can be bought form Property Repair Systems.
  2. 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 should be filled with a special Structural Epoxy Pouring Grout, see link above.
  3. Holes and voids – excavate to the full depth of the defect carefully. If you reach as far down as the waterproof membrane layer then extend the hole laterally to expose enough sound membrane to be able to effect a repair. This should be at least 150mm (6 inches) from the edge of the hole. Remove all dust and debris from around the area. The hole can then be repaired, either using Structural Epoxy Pouring Grout (as mentioned above), or for holes larger than 8 litres in capacity, a combination of the Epoxy Grout as a base layer and concrete as a filler. The final surface can then be covered with levelling floor screed, however the repair edges must be primed and the screed mixed with Ultra BondAcryl, to avoid shrinkage and cracking.
  4. Punctured Membranes – remove all floor coverings to expose bare concrete, and then run a Dehumidifier until the damp recedes to defined areas, and does not reduce any further. This drying time can takes a number of weeks depending on weather conditions, temperature in the room and the amount of moisture the concrete has absorbed. When you have a defined damp patches, mark the areas with chalk and excavate them carefully to expose the defective membrane punctures.  The hole can then be repaired in a similar way to Holes above, either by using Structural Epoxy Pouring Grout, or as above if the holes are larger than 8 litres in capacity, use a combination of the Grout as a base layer, and concrete as a filler. As previously mentioned the final surface can be screeded, but prime the repair edges first and mix screed with Ultra BondAcryl, to avoid shrinking and cracking of the repaired areas.
  5. Lack of Membrane – To be sure there is no membrane you should first remove all floor coverings to expose bare concrete, and then run a Dehumidifier until the damp completely disappears. If damp patches remain see (4) above. If not, you can clean the floor to remove all contamination (hot wash with detergent, and then thoroughly rinse), then 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 an Epoxy Sealer (you can also buy this from Property Repair Systems). If you intend to lay flooring that requires an adhesive, then check with the manufacturer for bonding details – you may need to sand ‘blind’ the second coat of Epoxy Seal to provide a mechanical grip.

Causes of Dry Rot and Insect Attack

Many buildings with suspended timber floor suffer from rot and insect attack in the ground floor timbers due to poor sub floor ventilation. We look at this more thoroughly in our air bricks and air vents project, but quite simply if air bricks become blocked with debris or ground levels are raised this cuts off the air flow and water leaks may also occur. Timber damaging insects and wet and rot all thrive in these moist humid surroundings.

So in many cases the floor timbers and joists become so badly damaged you may feel the only answer is to replace the damaged timber floor with a concrete floor. Again, this is not a small undertaking, so if you would like any further advice to rule out any other possibilities of repair Property Repair Systems will happily provide this. Give them a call on 01626 336180.

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