Groundwater seepage is groundwater that rises under pressure and emerges at ground level, in a ditch or in a crawl space. The cause is always a difference in water pressure: somewhere nearby the water stands higher — a river, a canal, a higher lying polder — and that water finds its way through the permeable sand layer below ground into the lower area. The Dutch term is kwel.
Seepage is therefore not a leak from above but a pressure phenomenon from below. That explains why the problem does not go away by sealing the top surface.
Recognising it
- Water in the crawl space that rises with high river levels or after a long wet spell, and falls again in dry weather
- Rust brown deposit — seepage water is often rich in iron; an orange-brown sludge layer in a ditch or crawl space is a strong indication
- A wet patch at the lowest point of the garden that never fully dries out, even in dry periods
- Boils — small upwellings that carry sand grains with them; these need attention, because moving sand undermines the ground
What you can do about it
Seepage can rarely be stopped altogether, but its effects can be managed:
- Measure first — record the water level in the crawl space over several months and compare it with rainfall and the river or polder level. Without that record you cannot tell seepage from a leak or from rainwater
- Ventilate the crawl space — sufficient, well distributed vermin-proof vents ensure moisture is carried away rather than drawn into the floor
- Ground cover — a vapour-tight membrane or a layer of shells over the ground substantially reduces evaporation into the dwelling. This does not stop the water, but it does stop the damp
- Drainage with an outfall — a drain to a lower point, or a pump with a float switch, keeps the level below the floor manageable
- Boils carrying sand — bring in a geotechnical adviser; this is soil mechanics, not construction damp
Do not raise the level in the crawl space by closing off the ventilation: that simply moves the moisture into the timber and leads to wood rot and mould.
Related terms
- Groundwater
- Crawl space
- Drainage
- Construction moisture
- Wood rot
- Ventilation
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