Can a gutter downpipe exit onto a paved driveway

ThisIsWeird

grumbler said:

Whatever roof it is, I cannot imagine how adding a porch can increase the total amount of water.
Yes, the area gets smaller, but the amount of water doesn't get bigger.

That is such a good point! That rain would have landed on their driveway regardless  

Albeit more focused now.

Section62

grumbler said:

Section62 said:

grumbler said:

FreeBear said:

Flugelhorn said: Suppose the actual answer is to ask the building regs people

Buidling Regulations Part H - ... this water should be collected and directed to a soakaway, water course, or combined sewer. 

Makes no sense to me. I can understand this for a big roof, but if you add a small porch, the total amount of water getting to the paved area doesn't increase.
That said, it's not the first time when regulations make no sense to me.

If it is a pitched roof then the total amount of runoff may be greater than the equivalent from the footprint of the structure... rain doesn't always fall vertically.

Whatever roof it is, I cannot imagine how adding a porch can increase the total amount of water.
Yes, the area gets smaller, but the amount of water doesn't get bigger.

The total amount of water running off the porch increases (in some cases) because the pitched roof represents a greater surface area to 'catch' the rainfall than the plan area is.

That's why gutter drainage calculations use 'effective area' rather than 'plan area' to calculate the maximum runoff.

Because rainfall usually has a horizontal component, the maximum runoff occurs when the rain is perpendicular to the slope of the roof.  If there is a mismatch in angles then some of the 'plan area' rainfall will miss the roof entirely and could, for example, land on the house walls (or in the neighbouring garden).

Section62

ThisIsWeird said:

grumbler said:

Whatever roof it is, I cannot imagine how adding a porch can increase the total amount of water.
Yes, the area gets smaller, but the amount of water doesn't get bigger.

That is such a good point! That rain would have landed on their driveway regardless  

Albeit more focused now.

Google 'effective roof area' for further insights into why it isn't a good point.

grumbler

Even if the porch roof has bigger effective area than the porch footprint and  it can catch more rainwater than a flat roof,  the surrounding paving will catch less rain. The total amount remains the same.

Yes, if there is, say, lawn next to the porch, the distribution between the paved area and the lawn can change a little depending on the wind direction and speed, but this cannot be a serious reason for demanding the water to be collected and directed somewhere.

ThisIsWeird

Section62 said:

ThisIsWeird said:

grumbler said:

Whatever roof it is, I cannot imagine how adding a porch can increase the total amount of water.
Yes, the area gets smaller, but the amount of water doesn't get bigger.

That is such a good point! That rain would have landed on their driveway regardless  

Albeit more focused now.

Google 'effective roof area' for further insights into why it isn't a good point.

The difference on this porch will be?
And with the rain in some directions, then, it will be less.
The literal issue will be that the collected water is sent out at a single point.

DavidMunro38

Guttermaster have a free rainwater calculator you can use which is fairly useful.