Royal Ordnance Factories (ROFs) was the collective name of the UK government's munitions factories in and after World War II. Until privatisation in 1987 they were the responsibility of the Ministry of Supply and later the Ministry of Defence.

Following the end of WW1, a review of the armaments industry resulted in the closure of nearly all of the ammunition provision facilities, retaining just the Royal Arsenal, Woolwich, the Royal Gunpowder Factory, Waltham Abbey and the Royal Small Arms Factory, Enfield which would satisfy post-war needs. In addition the National Filling Factory at Hereford was retained on a care and maintenance basis as a reserve, should it be needed. This site was reactivated in WW2 to become Filling Factory No.4-Hereford.

The Royal Arsenal at Woolwich was unique, being an explosives factory, a filling factory, an engineering factory, a gun foundry, a proofing range and storage magazine all on the same huge site, which together with the two other London sites constituted a major bombing target in the event of war.

The ROF programme, initially a War Office commitment, eventually (via the Board of Works) ended up under the Ministry of Supply when formed. Wholly government-owned, the Royal Ordnance Factories were constructed and operated by various private civil engineers and agencies. 

The majority of the ROFs were built in the re-armament period just before and just after the start of WW2  in areas regarded as 'relatively safe', which until 1940, meant from Bristol in the south and then west of a line that ran from (roughly) Weston-super-Mare in Somerset northwards to Haltwhistle, Northumberland, and then north-westwards to Linlithgow in Scotland. The South, South East and East of England were regarded as 'dangerous' and the Midlands area, including Birmingham, as 'unsafe'; however this constraint was overcome by the necessity to site factories to where workers were available to staff them.

Woolwich was, however, poorly sited and was especially vulnerable to an air attack. With the rise of the Nazi party in Germany culminating in Adolf Hitler being confirmed as Führer (leader) of Germany on 19 August 1934 another war with Germany seemed inevitable. On 3 December 1934, a decision was taken to close the explosives production and filling facility at Woolwich and build a new factory at a more secure location. The site chosen was Chorley in Lancashire, and construction began there in 1937 with some small-scale production underway in 1938. Before construction had even started, it was clear that it would be impossible to determine the precise demand for ammunition before war had even been declared so construction of a second large factory at Bridgend was also felt necessary to supplement Chorley.

Construction started here in March 1938 but production did not start until March 1940, six months after the declaration of war with Germany. With the proposed demise of Woolwich the Navy made it clear that they would also require a dedicated filling factory. A site at Glascoed, near Pontypool in South Wales, was selected with the intention that it would work in conjunction with Bridgend. Construction started here in February 1938 with production also starting in March 1940.

These are the only sites where construction started before the outbreak of war although in 1936 there had been discussion about an additional site to satisfy the needs of the RAF.  The three factories already under construction were considered sufficient to supply the needs of all three services with Woolwich being retained as back up, followed by the reactivated No.4 Filling Factory at Hereford. All the multiple functions at Woolwich with the exception of explosives production and limited filling were retained throughout WW2 and beyond..

The main administration block. This was sited away from the main production buildings for safety in the event of an accident. It was also close to Thorp Arch station which would have been used by visitors.

Some of the Group 5 buildings looking south. By the time this photo was taken in the mid-1950s the 'cleanways' between the buildings were no longer maintained to the spotless condition that was required during WW2. Where they crossed the lagged pipes carrying steam around the factory, and also the pipes of the compressed air ring, these were carried above the ‘cleanways’. An air reservoir tank is seen on the right.

In 1938, while construction of the three factories was underway, the government reviewed the situation as war was rapidly approaching. It was clear that the projected production capacity would be inadequate as the army increased to ten divisions, and the needs of the RAF were also reassessed. In May 1939 government approval was given for at least two additional factories; one of these was to be at Swynnerton near Stafford. Before the site of the sixth factory could be decided war was declared on 3 September 1939. Five days later the sites for two more factories were chosen at Risley and Kirkby, both in Lancashire.  Construction of both started within a few weeks.  The eight factories now under construction were expected to be sufficient to supply an army of up to 20 divisions.

Five months into the war the government once again reassessed the need for ammunition to support an army that was expected to increase in size to 32 divisions.  It was clear that additional factories would soon be required.  Three sites in Yorkshire had been considered at a meeting on 24 January 1940 with the most suitable being Thorp Arch between Leeds and York. The main criteria for its selection were good rail connections and the availability of labour, which could be brought in by train from Leeds, Bradford, Wakefield and Normanton. At the end of February 1940 the war cabinet approved the construction of No.8 Filling Factory at Thorp Arch and No.9 at Aycliffe in Durham. Construction was underway on 18 May 1940. This was not to be the end of the programme; by the close of war 44 factories had been built, many for specific types of ammunition or purposes.  The largest filling factories covered an area of up to 1,200 acres.

Group 5 looking north towards the larger buildings adjacent to Avenue C (West).

Aerial view of ROF Thorp Arch in 1946. The exchange sidings and carriage sidings are seen on the left. Thorp Arch station is seen north-west from the sidings. Click to enlarge.

Thorp Arch was built on 642 acres of good quality, flat agricultural land adjacent to the River Wharfe, from which an ample supply of water could be extracted. It was also close to the LNER’s lines from Church Fenton to Harrogate and Leeds, which would be used to bring in both materials and labour. The only drawback was the proximity of the villages of Thorp Arch and Boston Spa but this was far outweighed by the advantages of the site.  The projected cost of building the facility was £4,450,000, but the final bill was £5,950,000. Surveying of the site started in February 1940

With a shortage of petrol for road transport, nearly all the building materials were brought in by rail and unloaded at the goods yard at Thorp Arch station. It was soon clear that the yard was too small to cope with the huge influx of building material. To overcome this, sidings were built close to the factory and a connection was laid by the LNER on 24 June 1940.

Initially construction workers also arrived at Thorp Arch station but the small station was unable to cope with several hundred workers arriving each morning.  Although this put a strain on the few station staff it appeared not to trouble the workers who were content to jump from long trains directly onto the track. In June 1941 the platforms were lengthened to accommodate the long workmen's trains, but it was soon apparent that they were still inadequate and that more suitable facilities would soon be required, especially when the factory went into production with a much larger workforce.

King George VI and Queen Elizabeth talk to women munitions workers during the official opening of ROF Thorp Arch in 1941. Although the factory was not completed until 1942 production had already started in those areas that were ready.

Two hostels, each capable of taking 1,000 workers were built at Wetherby, but with an expected workforce of 18,000 these would do little to relieve the problem as the bulk of the workforce, mainly women, would be arriving and departing by train each day. The LNER provided the solution by proposing a circular railway running round the perimeter of the site. The single-track line would be provided with automatic signalling which would allow trains to follow each other in quick succession. Carriage sidings would also be built to allow the trains to be stabled on site after dropping off the workers. They could wait in the sidings until they were needed to collect workers after the off-going shift.

Four workers’ platforms were proposed which would allow them do disembark close to their actual place of work. Construction started immediately with two of the factory platforms being brought into use on 20 November 1941 and the other two platforms used on completion of the circular railway on 19 April 1942.

Royal Ordnance Factories elsewhere in Britain were also provided with new passenger railway facilities and stations for the workers. For example at Swynnerton (Staffordshire) a branch line to Cold Meece served ROF workers, and at Aycliffe (County Durham) extensive facilities were provided at Heighington and Demons Bridge.

Group 10 included the engineering workshops for the factory. The machine shops shown here are the largest buildings in the factory. These workshops were served by Walton Platform on
the circular railway.

The structural shop handled mainly repairs to the structures within the factory.

The factory was divided into 'clean' and 'dirty' areas. Workshops where the actual filling and most of the other procedures involved in producing the final shell took place were ‘clean’ areas. These could only be reached by passing through a 'shifting house' where all personnel changed into protective clothing and shoes. ‘Dirty’ areas were generally stores or magazines served by railway sidings; these handled the incoming materials and components or finished munitions ready for dispatching.

The factory was also divided into a number of different areas or groups of buildings performing specific functions: 

Group 1: Buildings in this group handled and manufactured the most delicate explosives. One of the primary functions of this group was to fill caps and detonators.

Group 2: Buildings here handled powdered TNT and Tetryl, a sensitive explosive compound used to make detonators and explosive booster charges. The function of this group was the production of fuse magazine pellets, exploder pellets and exploder bags.

The cap pressing shop in Group 1 after refurbishment in the 1950s. Belt-drive reduced the risk of explosions with electrically driven machines.

Group 3: Fuses were assembled and filled in this group using the detonators that were produced in Group 1 and the magazine pellets from Group 2. In military munitions, a fuse is the part of the device that initiates ignition.

Group 4: Specific blends of gunpowder for use in the delay mechanisms of time fuses were produced here. 

Group 5: Buildings here were involved with the filling of cartridges which could then be sent to the magazines awaiting dispatch. The processes included filling cloth bags with cordite, for breech loading ammunition; filling cordite into brass cartridge cases for individually-loaded ammunition; filling cordite into brass cartridge cases; and assembly of those cases to filled shells, for issue as a complete round, was also carried out here.

Group 6: Buildings here made smoke-producing compounds for use in smoke shells. Tracer ammunition for marking targets was also produced here.

Group 7: Filled small arms ammunition.

Group 8: All the heavy work was carried out in the buildings of this group including the mixing of high explosives for filling large shells and bombs.

Group 9: This group contained all the large storage magazines for completed shells awaiting dispatch. Many of these magazines were covered with soil and surrounded by an earth bund, and each was served by a rail siding from the perimeter railway.

Group 10: Test ranges and proof yards where a percentage of each batch would be tested.

Most groups had their own canteens, some more than one; this is the Group 3 canteen.

Construction of the factory progressed quickly, and production started before the factory was completed in October 1942. It was opened by King George VI in 1941.

The Thorp Arch ladies with their presents for Adolf.

One of the Group 8 magazines on the north side of Road 7 in April 2013. The magazine is of brick construction with a thin roof surrounded by a substantial earth bund. In the event of the magazine exploding all the debris should go upwards and not damage adjacent buildings. The magazine is connected by a short siding to the main railway network. Although the site was sold in 1959 for redevelopment as an industrial estate. not all of the buildings have been reused or demolished. This is one of the few sections of the internal rail network to survive.
Photo by Nick Catford

Within the factory there was a huge network of internal roads and railway sidings. There were also 'cleanways’ which were special roads for taking components between buildings. They were made of gritless asphalt coated with a shellac-like material, and were kept spotless at all times by constant sweeping and washing.


In December 1997 many of the buildings were still derelict. During the 1950s the Group 5 building was used for filling rockets.
Photo by Bob Jenner

The first 20mm ammunition was produced at Thorp Arch on 8 February 1952, and production increased rapidly with 13,263,000 rounds being turned out in the first twelve months.  Once the factory was in full production it was able to produce 500,000 rounds a week in a single shift.  Gradually other areas of the factory, including proof yards and ranges, bulk magazines, laundry, components shops, and clothing distribution, were also provided to meet the factory production requirements.

Unlike during WW2 the workforce was mainly male, although when the requirement for exploders (detonators) increased it was necessary to start a night shift which was initially worked by women, although the factory eventually adopted all-male night working.

By the middle of 1953 the demand for 20mm ammunition was in decline and production switched to the production of high explosive fuses.  Other areas of the factory were renovated for the production of rockets, which started in June 1953, and 1,000 lb bombs which started in October 1953.  As other forms of ammunition were required more areas of the former factory were renovated and brought back on line.

The Group 9 magazines on either side of Street 4 in the centre of the site all had substantial earth cover. These magazines now form the UK's first out-of-town retail park. It first opened in 1961 and remains a popular destination with a wide range of furniture outlets as well as electrical and fashion stores and the ever-popular garden centre and café.
Photo by Nick Catford

By 1955 the workforce had been built up to 2,000 but within two years production at Thorp Arch was in decline. In 1957 the government published a Defence White Paper, in the aftermath of the Suez campaign of the previous year. It was clear that the reduction in the country’s armed forces which was envisaged would result in a reduction in the munitions requirement, and in
1958 a number of munitions factories were closed. Thorp Arch ceased production in April 1958. The last passenger trains to visit the circular railway were two railtours. The first was a DMU running the R.C.T.S. (Lancashire Branch) 'The Roses Rail Tour' from Manchester Victoria on 8 June 1958. The tour called at Walton and Ranges platforms. The second was an unknown steam-hauled railtour on 22 July 1958. The line was due for closure soon after this tour.

The site was put up for sale in March 1959 and was bought by a local consortium for development as an industrial estate.  The circular railway around the perimeter was lifted and sold for scrap. Part of the site comprising 385 acres is now in use as the Thorp Arch Trading Estate and retail park; this first opened in 1961 and remains a popular destination with a wide range of furniture outlets as well as electrical and fashion stores and the ever -opular garden centre and café. Other parts are used to house the Northern Reading Room, Northern Listening Service and Document Supply Centre of the British Library; and another part is a prison, originally HMP Thorp Arch, now HMP Wealstun.

Many of the original buildings still survive and have been put to new uses. while some remain in a derelict state awaiting renovation and reuse or demolition.


  • ROF Thorp Arch by Mike Christensen - two articles in Archive (magazine) Nos. 22 and 23 June/September 1999.
  • Cocroft, Wayne D (2000) Dangerous Energy: The archaeology of gunpowder and military explosives manufacture. Swindon:English Heritage ISBN 1-85074
  • Hay, Ian (1949) R.O.F. The story of Royal Ordnance Factories: 1399-48 London: His Majesty's Stationery Office
  • Hornby, William (1958) Factories and Plant. (History of the Second World War: United Kingdom Civil Series) London: HMSO and Longmans, Green & Co.
  • Kohan, CM (1952) Works and Buildings. (History of the Second World War: United Kingdom Civil Series) London: HMSO and Longmans, Green & Co
See also Thorp Arch station and platforms on the Thorp Arch Railway: River Platform, Ranges Platform, Roman Road Platform & Walton Platform




[Source: Nick Catford & Bob Jenner]

Last updated: Thursday, 18-May-2017 17:22:56 CEST
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