How should de-icing grit be stored?

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At a national summit on how to cope better in icy weather, grit reserves topped the agenda. With many local authorities criticised for laying in too little grit too late, how best to store it?

A dome made of wood is the belt and braces approach to storing grit, which is rock salt. A super-size shed may be more common. But if permanent shelter is hard to come by, tarpaulins will do the job.

Rock salt doesn't go off from one winter to the next, but it does need to be protected from the elements. It can become less effective if exposed to rain, and the resulting run-off of salty rainwater can cause environmental problems for flora, fauna and nearby water courses.

THE ANSWER Well-drained, covered areaMay be a salt dome, a big shed open on one side, or tarpaulinsDisused barns or aircraft hangers can be temporary solutions

"In the old days it was stored out in the open," says Matthew Lugg, of Leicestershire Council, who sits on the UK Roads Board, a government advisory body. "But you do get erosion from the rain. And wet salt sticks in the gritting machines. To maximise its effectiveness, you need dry salt."

In the past 10 to 20 years, local authorities and the Highways Agency are among those to have built salt domes, specialist buildings with domed roofs.

A front door keeps the rain out, but is wide enough and high enough for a lorry to drive in and out. The extra height offered by a curved roof means the lorries can upend to offload their cargo once inside.

A salt dome's narrow entrance offers extra protection from the elements...

"Because salt is corrosive, there's a lot of wood. Using steel isn't clever in a building used to store salt, and concrete has to be well protected," says Mr Lugg.

So how big is a typical salt storage depot?

Alan Sheen, of salt dome makers Dome UK, says sizes vary but there is a move towards larger facilities that hold about 2,000 to 3,000 tonnes.

"That's a couple of swimming pools' worth, with a dome of 15 to 16m high. It's similar to the dome of St Paul's Cathedral - although I don't think Sir Christopher Wren had grit storage in mind."

Slip-sliding away

Grit stocks have been discussed at Tuesday's severe weather summit, hosted by Northamptonshire County Council, and attended by transport experts, weather forecasters and rock salt suppliers to work out how to improve their handling of icy conditions.

...and its curved roof has space for lorries to upend to offload

With weather conditions flip-flopping between thaw and freeze, the UK's salt mines are still working around the clock to keep up with demand for rock salt to de-ice roads and pavements.

As soon as it comes out of the ground, it's shipped off to local authorities - many of which have been caught out with insufficient supplies during the coldest winter in about 30 years. With public spending under scrutiny, and after a run of mild winters, few councils wished to splash out on stockpiling rock salt that might not be used within the financial year.

Those buying extra supplies have, in some cases, been caught short as to where to store it. Mr Sheen says about 400 to 450 of the grit stocks in England, Wales and Scotland are currently uncovered. "That's about half the salt piles in the country, but local authorities are increasingly interested in covering these up."

In a review of its icy weather preparedness, Rochdale Borough Council in Lancashire found its depots had insufficient shelter to protect grit from the elements. The grit stored outside proved "notably less effective" at melting snow and ice than fresh salt that arrived later.

This is because rock salt is a mix of sodium chloride - which is what melts snow and ice - and up to 10% of other materials, such as gravel or clay, which are mined along with it. So if a rock salt pile is rained on, some of the sodium chloride is dissolved and washed away, rendering it less effective.

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"Salt doesn't oxidise or rot, so used stocks are good from one year to the next," says Alastair Kight, managing director of Gritit. His winter maintenance company has about 90 salt stores up and down the UK, to serve customers such as hospitals, retail parks, industrial parks and universities.

"The only thing that will degrade salt is moisture. It doesn't need to be stored in a moisture-controlled environment, but it does need to be protected from the rain. So you need a well-drained area and no puddles - that would defeat the object."