What did the famous London bookstore, Foyles, use for roof protection during the WWII blitz of London?

In 1903, teenage brothers William and Gilbert Foyle failed their civil service exams and took out an advertisement in a periodical to re-sell their textbooks. The venture met with early success, expanded rapidly and moved several times before settling in their flagship store in Charing Cross Road, London, in 1929. William Foyle stated that it was "the world's first purpose-built bookshop", (although this is disputed), and he had the “Guinness Book of Records” ratify Foyles as the world's largest bookshop in terms of shelf length, at 30 miles (48 km), and for the number of titles on display.

When Hitler started burning books in the 1930s, William Foyle had immediately telegrammed the Führer in 1932 to request that he be able to purchase the books instead and would offer a good price; the response quickly came back that Germany had no books to sell and the burning would continue.

In late 1940, at the start of the Blitz (night-time bombing raids against London and other British cities by Nazi Germany during World War II), Foyles filled sandbags with old books to protect the shop from damage. William Foyle announced that he was covering the roof with copies of “Mein Kampf” (in English “My Struggle”, a 1925 autobiographical manifesto by Nazi Party leader Adolf Hitler) to ward off bombers. The bookstore was not directly hit but there were near misses.