Flood damage to floors

Identifying problem areas of flood-damaged floors.

Decay of joist

Floor beams and joists will absorb significant moisture during a flood. If the sub-floor is adequately ventilated, the beams and joists will slowly dry out and permanent damage should be minimal.

However, if sub-floor ventilation is inadequate then fungal decay may occur, particularly at the junction of beams/joists and joists/floorboards. The surest way to reduce this possibility is by adequate sub-floor ventilation.

It is also necessary to clean off any retained mud from timber junctions. Prior to the flood or if timbers are replaced, a number of actions can be taken in terms of timber selection. Firstly, rather than using radiata pine or brush box, more durable timbers such as spotted gum, black butt or mountain ash (class 1 and 2 timbers) should be used. The ends of the beams should also be sealed by a water-proof paint or varnish.

Lifting floor lino or tiles

Quality floor covering such as high quality tiles, water-proof adhesive and quality moisture resistant backing should resist a flood of moderate duration (less than 24 hours). Floods of longer periods are likely to result in deterioration of the backing. However, tile lifting could occur in short floods if any of the components are of marginal quality. If only isolated tiles are lifting it may be possible to replace these tiles.

However, it is important that sufficient time is allowed to ensure that the backing is dry before replacement. If a significant fraction of tiles lift then it is best to replace the whole system (backing/glues and tiles). If quality products lift after flooding and continue to lift in the post-flood period then there is a possible sub-floor moisture problem. Lifting of sub-quality materials is to be expected and does not imply any systematic building problem.

Nevertheless, a check of sub-floor moisture should be made prior to repairing the floor covering. Lino is likely to lift under in short floods and need to be replaced.

Poor glues, tiles or backing

The quality of a tiled system will determine the performance in floods of moderate duration. Good quality clay tiles with moisture resistance less than three per cent should be used with moisture resistant adhesives. If an underlay is required then cement sheeting or a synthetic underlay is best. Tiles with high moisture expansion (greater than three per cent), moisture susceptible glues and hardboard backing should be avoided in flood prone areas.

Springy floor board

Springy floor boards indicate a lack of support by the sub-floor members which may be associated with failure of beams/joists or stumping.

Stumping failures are the most common cause of springy floor boards. Stumping failures are often reported after floods. However, frequently such stumping was in poor condition prior to the flood and the flood simply 'finished it off'.

The best way to limit such problems is to ensure that:

  • foundations are in a good condition prior to the flood and
  • sub-floors drain and
  • water does not remain trapped underneath the house.

If stump failure has occurred, re-stumping should be carried out before repairs to the floor.

Failure of beams/joists or of metal connectors is a second cause of floor problems. This may be associated with flood impacts but it may also be associated with fungal growth in the sub-floor after flood. However, fungal growth takes a considerable time to eat out a timber member and thus failures commonly occur in timber which had significant rot prior to the flood or will occur a significant time after the flood (as long as ten years).

Sub-floor moisture

Moist conditions under the sub-floor are expected after a flood, particularly one of long duration. However, a well drained and vented sub-floor should slowly but surely dry out. The time to dry out is very variable, depending on ventilation rate, soil type and design factors. If the sub-floor has not dried out within two to 12 weeks then there may be additional problems restricting drying. Such problems may include poor sub-floor ventilation or sub-floor moisture traps.

The persistence of high sub-floor moisture can result in significant deterioration of floor and its supports including fungal and mould growth on timber, corrosion of connectors, corrosion of steel work as well as a persistence of warping of floorboards.

Poor sub-floor ventilation 

The building code of Australia recommends that for timber clad houses there should be a minimum of 350 mm between the ground and floor joists. This should provide all the ventilation required as long as air movement across the sub-floor is not restricted.

After a flood remove barge boards and other obstructions to air flow. Soil may have built up against the barge boards, which should be removed. Ensure that the sub-floor can drain away from the house. Also ensure that there are no obstructions to air flow under the house. Flood debris or materials stored under the house may obstruct air flow.

More seriously structural members such as concrete or brick walls may block air flow. Strategic vents should be placed in these members (at least 15600 mm2 per metre). If the sub-floor does not dry out and the separation of ground floor joist is significantly less than 350 mm it may be necessary to lower the ground level. Alternatively, forced venting or as a last resort venting into living spaces may be required.

Sub-floor moisture traps

Moisture may be trapped at various points in the sub-floor. Earth may build up around the perimeter of the house (particularly in old houses) so the sub-floor is effectively dammed.

Alternatively, hollows may be associated with stumps/piers or chimneys. These hollows may form during the flood itself or may be left from the building process. All hollows need to be filled and built up earth removed and temporary channels provided, so the flood water beneath the sub-floor drain away from the houses.

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