In 1936, as part of a £35,000 refurbishment, an 80-yard-long roof was added to the United Road stand (now the Sir Alex Ferguson Stand) for the first time, while roofs were added to the south corners in 1938. Upon the outbreak of the World War II, Old Trafford was requisitioned by the military to be used as a depot. Football continued to be played at the stadium, but a German bombing raid on Trafford Park on 22 December 1940 damaged the stadium to the extent that a Christmas day fixture against Stockport County F.C. had to be switched to Stockports ground. Football resumed at Old Trafford on 8 March 1941, but another German raid on 11 March 1941 destroyed much of the stadium, notably the main stand (now the South Stand), forcing the clubs operations to move to Cornbrook Cold Storage, owned by United chairman James W. Gibson. After pressure from Gibson, the War Damage Commission granted Manchester United £4,800 to remove the debris and £17,478 to rebuild the stands. During the reconstruction of the stadium, Manchester United played their "home" games at Maine Road, the home of their cross-town rivals, Manchester City F.C., at a cost of £5,000 a year plus a percentage of the gate receipts. The club was now £15,000 in debt, not helped by the rental of Maine Road, and the Labour MP for Stoke, Ellis Smith, petitioned the Government to increase the clubs compensation package, but it was in vain. Though Old Trafford was reopened, albeit without cover, in 1949, it meant that a league game had not been played at the stadium for nearly 10 years. Uniteds first game back at Old Trafford was played on 24 August 1949, as 41,748 spectators witnessed a 3-0 victory over Bolton Wanderers F.C..
Question: How many years after the roof was added to the United Road stand was the stadium damaged by German bombs?
Answer: 4