In estimating the size of gutters on internal roofs and behind parapet, the amount of rainfall should be provided for. An exceptional rainfall is about .05 in. per minute, and this gives about '026 gal. for each square foot of catching surface. An average rainfall in London would be about one-third of the above, but for preventing gutters on internal roofs, or behind parapet walls, oversowing inside the house, the maximum should be allowed for.