Greenhalgh & Co.
Chartered Building Surveyors & Valuers
21 Towncroft Lane
Bolton BL1 5EN
Tel: +44 (0)1204 845382
Mob: +44 (0)7813 439196
Explanation of typical movement in a solid concrete ground floor
In properties with solid ground floors, the usual modern form of construction is:
- Fill material.
- Damp-proof membrane (in more recent times).
- Asphalt topping.
In older properties solid ground floor slabs can be affected by movement. The two main causes of movement are compaction or expansion of the sub-floor fill material. For more modern properties built in the last fifty years, compaction tends to be more common, and for properties built over fifty years ago, expansion is usually more likely.
The methodology behind this type of movement is detailed below:
Movement caused by compaction
When the contractor lays the fill material inside the foundations, he is supposed to compact it with a vibrating plate in layers. Sometimes this compaction of the sub-floor fill material is not fully undertaken. The result is that the fill material is gradually affected by natural compaction after the house is built.
This causes the floors to become out of level and is often characterised by gaps between the asphalt topping and the skirting boards (where fixed to walls built off foundations). It should be noted that if pipes or drains run under the floor (this mainly relates to drains) they could be cracked or damaged by the movement in the floor.
Movement caused by expansion
Ground floor slabs constructed from concrete laid on hardcore are vulnerable to attack by water-soluble sulphates in the hardcore. The effects of swelling due to other chemical changes are similar but not as common.
The most common cause is the attack on concrete by water soluble sulphates. The likely source for the sulphates is from the hardcore, but occasionally it may be from the groundwater.
Whatever the source, the sulphates will react with the cement on the underside of an unprotected concrete slab in damp conditions.
This causes a gradual breakdown of the concrete, the visual end product being expansion, cracking and heave of the floor slab. Colliery shale is a significant culprit when it was used as the base of a concrete slab. More recently the problem will likely emanate from hardcore contaminated with gypsum plaster or other sulphate bearing waste products.
Unstable aggregate can cause concrete to expand. Some clinkers contain un-burnt coal of a type that expands on oxidation. They may also contain sulphates which will react with the cement. This is a much less common cause and the effects are usually not as severe.
The hardcore may expand due to chemical and volume changes in the material. All rely on the presence of water to support the reaction.
The principal materials that have been identified as causing problems are:
a) unhydrated lime or magnesia in steel slags
b) broken concrete or old, partially vitrified slags from old slag dumps
c) colliery spoil with a high clay content
d) oxidised pyrites which form soluble sulphates in air and moisture.
Suspended timber floors
Common faults with suspended timber floors
The joist ends have no damp-proof course underneath them where they bed on walls. This is usually causes the joist ends to rot and possible dry rot to further affect the floor.
Stress creep deflexion of the floor joists over a long period of time. In older properties the quality of timber used in floor joists was usually much higher than that used in modern buildings. However by the same token the action of stress creep will have had longer to act. This type of movement rarely results in a failure occurring. It is most noticeable in houses built between 1955 and 1980 and in older properties where solid walls are built off the floor (usually this is on the first floor).
Chipboard decking being affected by dampness, particularly in the kitchen and bathroom.
Uneven floors where many boards have been lifted to install new wiring and plumbing.
Dampness in floors due to ground water standing on the sub-floor. This is usually related to when the house is built on stiff clay and water gains access via the foundation walls under the floor. The water cannot drain away because the material of the sub-floor is stiff clay and impermeable.
Poor ventilation in ground floors. Suspended timber ground floors need to be adequately cross-ventilated by providing air-bricks to give at least 3000 mm2 of open area per metre run of external wall.
Building owners and, in particular, keen gardeners, should be careful not to obstruct the air-bricks in any way such as, for example, by the construction of an external patio.
Dampness rising from an unprotected earth sub-floor. Good practice requires the provision of over-site concrete or other damp-resisting material to restrict the passage of water vapour from the ground into the space below the floorboards.
Woodworm attack. It is normal for buildings over 100 years old to have been affected by at least some woodworm infestation. There are now many modern methods of treatment for such infestation.