I have a 1930s house too with a slate roof. The house was built by a reputable builder in 1939 to be exact. Occasionally, due to high winds, a slate or two will need replaced. Fortunately there is a local roofer who does the repairs and his bill is always reasonable. One of his other customer, my nextdoor neighbours, had their slate roof replace during summer this year. I asked him if I should consider replacing mine. He thought it would be a waste of money in my life time because the durability of a slate roof should be about 100 years. I'm already a golden oldie therefore I feel sure that I will be pushing up daisies long before the slate roof here needs replace!