Sandbags

Filled or empty

Ready filled bags arrive ready to place, which is the only thing that matters when water is rising. They are heavy to transport and they take storage space.

Empty bags are cheap, stack flat and can be held in quantity indefinitely, but they need sand, a shovel, somewhere to fill and people to do it, and filling takes far longer than anyone expects.

The sensible arrangement for anywhere with a known flood risk is both: a modest quantity filled and ready for the first hour, and empty bags in reserve for the rest. Deciding to fill bags once the warning is issued is deciding too late.

Filling them properly

Fill to somewhere between a half and two thirds. That is the single most useful thing on this page.

An overfilled bag is rigid. It holds its shape, so it will not settle against its neighbours, and a wall built from full bags is full of gaps that water runs straight through. A part filled bag slumps, moulds around what is next to it and seals.

Fold the open end under rather than tying it, so the weight of the bag holds the flap shut, and lay it with the fold underneath. Tied bags are only needed where they are being handled repeatedly.

Building the wall

Stack bags like brickwork, overlapping the joints so no vertical line runs through the wall, and lay each bag flat rather than end on.

Point the folded ends away from the water flow. Tread each bag down as it goes so it beds into the one below, which is what closes the gaps between them.

A sandbag wall reduces flow, it does not seal. For a doorway, a plastic sheet laid against the opening with bags placed on top of it performs far better than bags alone, because the sheet does the sealing and the bags hold it there.

Hessian or polypropylene

Woven polypropylene is stronger and does not rot, so it is the general choice for wet work. Sunlight is its weakness: leave a wall up through a bright summer and the bags start splitting where the light reached them.

Hessian beds down against its neighbours better and is biodegradable, which helps where bags cannot be recovered and hurts where they need to last.

Afterwards, and beyond flooding

Bags that have been in flood water are contaminated, potentially with sewage. Handle them with gloves, do not use the sand on a garden or a sandpit, and dispose of them as the local authority directs rather than emptying them out.

Sandbags also do everyday work away from floods: ballasting signs, frames and temporary fencing, holding sheeting down, and weighting anything that would otherwise blow over. The flood and traffic sign kits combine the bags with the signage for a road closure or a flooded route, so one item covers the whole response.

For bulk bags and rubble sacks, see bulk bags and rubble sacks.