Castle Drogo: water pressure on the granite

Castle Drogo near Okehampton is an amazing modern creation by the architect Lutyens. It is less than a century old but has had problems with water penetration almost since it was built. I went up the scaffolding observation tower to see what they are doing. I was told that they have found that even granite can be penetrated by water if it under enough pressure. They have recorded 90mph winds and say water penetration is between 10 and 20mm. 90mph is 144km/h or 40m/s. Take a rain drop of diameter 2mm. That is a volume of 4 x 10^-9 cubic metres and a mass of 4 x 10^-6kg. Change in momentum on impact assuming it is stopped would be 1.6 x 10^-4kgm/s. Now suppose it takes 0.01 seconds to stop the drop. Rate of change of momentum is 0.016N. Area = 3 x 10^-6 square metres so pressure = F/A = 5000 Pa which is 0.05 bar. Mains water pressure is given as 1 bar. If the raindrop is stopped in a much smaller time, then the pressure will be greater.