Drainage Problems can Cause Other Building Defects and Attract Termites

Why does that downstairs room always smell damp?  –  Site Drainage Problems can be the Cause of a Range of Moisture Issues such as Damp Penetration

Drainage problems can be hard to spot for the untrained eye, especially if the prevailing weather conditions are dry. We look at the site more generally for possible drainage concerns, as well as things like roof plumbing, provisions for surface drainage and methods of construction.

The site topography (slope of the site) can direct stormwater toward the building and into the subfloor areas. Once in the subfloor this moisture has no way of escaping and will cause a range of problems and damp issues such as rising damp blistering, buckling timber floor boards, stuck windows and doors, or even structural settlement and attracting termites.

Basement and lower storey rooms often do not have enough separation between the building elements,  and the damp subfloor soil. This moisture seeps into the walls and floors slowly over a period of time, causing damage to internal finishes. This is on top of no subfloor drainage or ventilation.

Whilst good external surface drainage (strip drains) are essential to collect stormwater and direct away from the building into the stormwater system, sometimes these aren’t enough. Subsurface water is common in areas with rock foundation material or on steeply sloping sites.

This house on the North Shore in the image below had a steep backyard sloping toward the back of the house. The home was built with no provision for site drainage. Large pools of water were found in the subfloor up to 100mm deep. A series of damp related problems were found inside the house and the chipboard timber floor required complete replacement – costing in excess of $30K – $40K.


inspection of drainage problems

These problems are very expensive to rectify once a building has been constructed and landscaped. We recommend a building inspection be carried out on any prospective house purchase.

Subfloor drainage around the room is often required to collect water that finds its way into the subfloor and direct it out of the building.

External ground levels must be lower than internal floor levels by at least 75mm and external paths must slope away from the building.

Soil in the subfloor must not be heaped up against internal walls and must be lower than the lowest floor level in the basement room or other habitable area.

Sometimes more complex remedies include subsoil drainage pits dug deep around the building to collect ground water before it enters the subfloor.

Good damp proof building practice is essential in the rooms / building to prevent the absorption of water. Brickwork must have a damp proof course (DPC). Timbers must be separated from damp masonry and concrete with DPC and concrete slabs must have adequate plastic vapour barrier installed at the time of construction. Double brick and brick veneer construction must have clear cavities to prevent moisture bridging.

To aid the drying out process improved subfloor ventilation is required. Often an active fan system with with negative and positive air flow (one fan pushing air in and one fan pulling air out) can be highly effective.