7 minute read
Good Governance
Managing subsidence, cracks & fl oor level defects
Is underpinning necessary?
So, you have some cracks emerging in your townhouse villa complex in the plasterboard walls, ceilings and cabinet works and/or your external walls (brickwork or painted render). Is this a sign of normal soil consolidation and sett lement over time, or something more sinister like footing failure and structural damage?
Nature of fl oor level defects
During construction it is almost impossible to achieve a completely level or fl at fl oor slab, so the construction industry via its National Construction Code, Building Act(s) and regulations, and Australian Standards generally indulge “allowable tolerances”. What this means is that it is not unusual for fl oor levels inside townhouses or villas to be slightly out-of-level, but still within allowable tolerances.
It may be that your fi nished fl oor slab was never entirely fl at to begin with, so it is important to determine the nature of fl oor level variations before becoming alarmed or planning remedial works.
So, what contributes to fl oor levels changing?
Building defect: The fl oor was not built level during construction, and not picked up as a defect so the rest of construction was constructed around the as-built fl oor level. In this instance you are unlikely to observe any functionality issues, cracks or other evidence of fl oor level defect.
Sett lement or consolidation: The vast majority of new housing is constructed on imported fi ll, which is compacted to strict standards prior to construction. Over time this fi ll (soil) may consolidate further and ‘sett le’, causing minor deviations in fl oor levels and minor signs of non-structural cracking. Heave: This is where the soil foundation is reactive to water and swells/shrinks depending on how wet the weather is. When reactive soil types swell up with water in a heavy wet and rainy season, the swollen soil heaves up against the slab and may cause deviations in fl oor levels and minor signs of non-structural cracking. Footing failure: This is where the footings and/or slab were not designed suffi ciently to deal with the soil condition. As time goes by the normal sett lement and/or heave impacts of the soil adversely aff ects the structural integrity of the footings and slab because they may not have been designed properly to deal with the soil condition.
Footing maintenance: As a lot owner or occupier you have a duty to maintain the veracity of your footings and foundation to ensure no adverse eff ect is caused on the structural integrity of the building, including: • Ensure there are no signifi cant trees with root interference within the zone of infl uence of your footings. • Ensure your drains are serviced regularly by a plumber to eliminate cracks, leaks or root damage. • Ensure your irrigation apparatus does not concentrate excessive water along the perimeter of the house where it could erode the soil around the footings. • Ensure your annual termite barrier inspections are being undertaken by the body corporate. • Report any cracks to the body corporate that are wider than 10mm.
• Report any doors that foul or won’t open/close, or any glass panels that spontaneously break without any real cause. • Ensure your caulking seal maintenance is up to date around the house.
Note: The CSIRO has put out a helpful document for lot owners and occupiers, which is available for purchase online, Foundation Maintenance & Footing Performance. This is a good guide to understanding and preventing soil-related building movement in housing.
Lynda Kypriadakis,
Diverse FMX
Signs of fl oor level defects
There are multiple signs that your fl oor levels may be outside allowable tolerances and defective, including: • Cracks inside in your plasterboard walls, cornices, cabinets and ceilings. • Dummy fl oor tiles or cracks in wall tiles.
• Cracks or gaps opening up in your external brickwork or rendered/ painted external walls. • Doors fouling and not opening or closing properly. • Broken window and/ or door glass. • Broken pipes underground. • The fl oor is noticeably out
of level, for instance you can roll a marble down the fl oor without propulsion.
When to take action
Not all fl oor level defects, or cracks are cause for immediate concern or remedial action.
It is important that extensive investigations are done prior to deciding on a remedial solution as the causes may be many and varied and all causes will need to be considered in the remedy. You need to fi rst discover what is the cause of the defect, followed by what is the eff ect of the defect and then fi nally, what is the proper remedy.
For example…
Building defect: If your fl oor levels are outside tolerance and were constructed that way, there may be no reason to remedy the fl oor levels as the house is completely functional and serviceable in every other way. Having fl oor levels slightly outside allowable tolerance is not always a reason to take remedial action if the house is perfectly functional in every other way. Sett lement or consolidation: If your fl oor levels are outside tolerance due to ongoing consolidation of the imported fi ll soil, you will need to ascertain if that sett lement has concluded, or if the consolidation is still happening. As a minimum three annual fl oor level surveys are required to determine whether the soil foundation is still consolidating, or if it has fully sett led and now stable. Having fl oor level surveys done across three years will ensure that the building owner knows whether the sett ling has stabilised or still worsening. Heave: Similar to sett lement, three annual fl oor level surveys are required to determine the seasonal impacts of heave on fl oor levels. It may be that during the rainy season the fl oor levels change, and then during the drier months they return to previous levels. In highly reactive soil conditions, the structural engineer may have designed your footings and slab to cope with the seasonal heave thus there would be no repairs to be made.
Footing failure: Once the three annual fl oor level surveys have been completed and the structural engineer has assessed the footing design against the as-built work, the engineer may determine that the footings and/ or slab design was inadequate, and the footings/slab have failed.
Note: Three consecutive years of annual fl oor level surveys should be the fi rst step for determining the cause/eff ect of fl oor level defects before designing a remedial solution.
To design a remedial solution on one single fl oor level survey may cause more damage than good.
When to act urgently
Naturally you would not wait for three annual fl oor level surveys if catastrophic failure is imminent, so how do we know when to act urgently? Here are the signs to look for: Take immediate action to repair if the following occurs: • Cracks appear overnight and are wider than 15-20mm.
• Doors in the house are unable to be opened or closed. • Stairs or cabinets start to separate from walls. • Glass breaks in windows or doors
The general rule of thumb is that if you can still live in your home, open/close all the doors and there are no cracks wide enough to allow vermin or water into habitable areas, then it is likely that you have plenty of time to undertake investigations to determine the cause of the issue(s).
National Construction Code, Australian Standards and fl oor levels
In Queensland the QBCC Standards & Tolerances Guide provides information about fl oor level defects in concrete slabs, which is based on the NCC and relevant Australian Standards.
The QBCC Standards & Tolerances Guide is available for free online and here are some excerpts below.
Go to section 3.8: Levelness of concrete fl oors
Except where documented otherwise, new fl oors are defective if within 12 months from completion of the work, they diff er in level by more than 10mm in any room or area, or more than 12mm in any 3m length and such deviation adversely aff ects the safe use or reasonable amenity of the building. The overall deviation of fl oor level to entire building footprint shall not exceed 20mm within 12 months from date of completion of the work and such deviation adversely aff ects the safe use or reasonable.
Go to section 3.11: Termite barrier
Domestic concrete slabs that form part of a termite management system. Where a domestic slab is designed in accordance with the BCA and is to act as part of a termite management system, cracks through the slab are not to exceed Category 1 width as set out in Table 3.10.
Summary
The bott om line is that it is not unusual for fl oor levels to move over time and movement is not necessarily cause for immediate concern. The best fi rst course of action is to undertake fl oor level surveying across the three annual survey events to properly assess the situation. If external cracks are allowing vermin or water ingress these should be sealed. If internal cracks are unsightly, these can be remedied at the next repaint.
