Whilst I dry off...
Semi, with a shared down pipe all new within the last few years, both front and rear. I cleaned out our gutters earlier this week. This afternoon, we were hit by the thunderstorms and the gutter at the rear filled up and we had an almost wall of water down the rear of both halves of the semi. The front was fine. I checked at the bottom where it discharged into the drain, during the worst of it and there was some flow, but not as much as I would have expected at the rear. The weather was hitting the rear slightly more than the front.
Neighbour's gutter slope to the down pipe is rather excessive, put up by 'professionals'persistent, ours was DIY and set accurately to recommendations. I checked the pipe and it was clear, when I did the gutter cleaning, but I will be checking it again...
Might the poor flow via the down pipe, have been due to the two heavy flows of water clashing in the middle at the down pipe, rather than due to any blockage?
The over- spilling gutter, completely flooded the back garden.
Just thinking aloud! Its been the heaviest and most persistant downfall I've seen in the last 30 years.