GRPUnderground Water Tanks

Maximum burial depth depends on the specific tank's structural rating and the groundwater conditions on site, so this varies model to model. As a general rule, deeper installations need additional backfill specification and sometimes a reinforced neck extension to bring the access cover to finished ground level. Always check the manufacturer's installation guide for the exact tank before excavating.

Yes — an empty or partially empty tank in wet ground or a high water table can float, which is one of the most common causes of installation failure. Anchoring typically involves a concrete base slab or ballast straps sized to the tank's buoyancy, calculated from its volume and the groundwater level on site. Your installation instructions will specify the anchoring method required for that tank size.

Some GRP underground tanks can take light or occasional vehicle loading, but this depends on the tank's design specification, burial depth and whether a load-spreading concrete slab or traffic-rated cover is fitted over it. Standard tanks are generally rated for pedestrian or garden areas only. If you need a tank under a driveway or car park, tell us at the enquiry stage so we can confirm a suitable specification.

For an average household using harvested water for WCs and outdoor taps, a 2,500 to 5,000 litre tank is typically sufficient, though this depends on roof catchment area, local rainfall and how many outlets you're feeding. Larger properties or those wanting a bigger buffer during dry spells often opt for the next size up rather than running short. We're happy to help you work through the sizing based on your roof area and intended use.

Yes, we supply GRP underground tanks manufactured and certified to WRAS standards specifically for wholesome water storage. Make sure you select the potable-rated version of the tank, as the non-potable variants use different fittings and liner specifications that aren't approved for drinking water contact. If in doubt, check the product listing or ask before ordering.

Larger GRP tanks are bulky, so you'll need a clear route from the road to the excavation for a HIAB or crane-offload vehicle, plus enough overhead clearance for lifting. Smaller tanks up to around a few thousand litres can often be manhandled into position by a couple of people. Excavation should be ready before delivery, as these tanks are usually lowered straight into the hole rather than stored on site for long periods.