Many villas have uneven floors and need to be repiled or levelled.
When working on old villas it is not unusual to find that the floor is not particularly level. Reasons for this can include:
- rotten/deteriorated timber piles
- missing piles or bearers – they may have been removed during previous alterations to provide storage space under the floor
- soft ground conditions that have allowed the pile to sink
- the villa was not levelled during previous repiling
- the span for floor joists is too great, causing them to sag (see structure)
- rot in wall framing supporting an upper floor (see borer, rot and mould).
Before carrying out a renovation, as part of a general inspection all floors should be checked with a level to make sure that no movement has occurred. In most cases, where there has been a failure of the foundations the signs will be obvious.
Failure due to deterioration
Generally, unevenness in the floor of older buildings is caused by deterioration of foundation piles and/or subfloor framing caused by rot or wood-boring insects.
In most cases, the perimeter piles are the first to deteriorate. Usually, chimneys based on concrete or brick foundations are still sound so floors tend to ‘hump’ and slope away from chimneys.
Failure due to bad ground
The earliest New Zealand houses were generally built on flat land with good bearing capacity. As cities grew, the more suitable land was used up and gradually the suburbs crept into the surrounding hills, onto reclaimed land, or onto drained swampy areas where the bearing capacity of the ground was lower.
Uneven floors may be found in houses built on:
- deep organic topsoil
- soft or expansive clay
- uncompacted back fill.
In these cases, it is likely that the heavier elements, such as chimneys, masonry and brick walls will have sunk, cracked and be out of plumb. In cases of subsidence, get advice from a qualified structural engineer.
The most effective way to correct a floor that is out of level is to employ a firm experienced in this work to jack the sunken parts of the building back up to as near level as possible. The piles and/or damaged subfloor can then be replaced to hold the building in the level position.
This process may result in damage to wall linings. In some cases, doors and windows will have been eased as the building sank and will require remedial work after jacking. An assessment must also be made of the effect of jacking on additions, alterations, fittings and services added to the building after the floor sank.
Alternatives to jacking
If jacking is not practical, the floor may be:
- left as it is, if acceptable to the owner
- overlaid by a new timber, plywood or particleboard floor packed up level.
In some circumstances, where there is a ‘hump’ in the middle of a room, the centre piles may be removed, allowing the centre of the floor to settle level with the perimeter walls. Sometimes, weighting is necessary to force the floor down.
Rotted timber piles should be replaced with H5 treated timber or precast concrete piles.
New treated timber or concrete piles can be installed:
- from under the floor where there is sufficient working space (minimum 450 mm clear)
- from above the floor by cutting holes (approximately 450 mm square).
When repiling, it may be necessary to also install earthquake bracing. See inadequate foundation bracing.
Some villas have insufficient clearance between the floor and ground beneath. When repiling, consider whether to raise the floor level to provide increased clearance.
When repiling in soft ground, it is sometimes necessary to install deeper piles in order to reach firmer ground. These can be small diameter steel pipe piles that are coated to resist corrosion. They are driven or augured down to the required level and then a concrete pile is cast around the top. In extreme cases where the house is built on deep unsuitable ground, it may be necessary to remove the roof so that long piles (such as railway line) can be driven down to firmer ground.
Soft ground conditions are usually as a result of wet soils so the renovation project needs to address the cause of the dampness. Once the soil dries, bearing should return but, in some cases, it may be necessary to install an additional bearer to remove the load from the offending pile.
Gap between pile and bearer
If there is a gap between the pile and the bearer, or both are basically sound but there are signs of moisture on the bearer, the bearer can be jacked up off the pile (after cutting any fixings), a DPC inserted into the space, and the bearer lowered and refixed.
Corroded connecting wires on concrete piles
Repiling carried out since the 1920s and before the introduction of H5 treated timber is likely to be concrete piles with the bearers tied with galvanised wire to the concrete pile. Any original connecting wires are likely to be corroded and of limited structural value so replacement will be required.