How do I get the right roof overhang to drain into the gutter?

[Alan Ambrose]

I'm monkeying with this geometry (below). How do I judge how much the metal roof should overhang the fascia so rainwater slooshes down and goes into the gutter rather than shoots over the top of the gutter (i.e. something like the dotted blue line)?

 

[Owain1602]

Have you chosen the supplier for the metal roof yet?

There will typically be an eaves trim that gets fixed to the edge of the top deck, with a downturn that sits in the gutter. The metal roof sheets are then swaged to this trim to lock it all together.

[Mr Blobby]

Our metal roof supplier told the joiners what was needed for this.  He told our joiners to leave about 20 mm wall plate overhang for the gutter brackets and these were installed prior to the metal roof.

 

 

 

 

... which is great, but, the roof supplier didn't plan the metal detail for the gable end / gutter correctly. 

 

There should be an allowance at the gable end so that the gutter can continue through.  Look at the photo below and you can see what the issue is. 

The metal on the gable end stops the gutter running through to an end cap.  We had to butcher it to make it right.   Its worth checking with your metal roof installer that they will cater for this properly.  Unlike our roof 🙄

 

 

 

 

 

 

 

 

Edited May 10 by Mr Blobby

[Russell griffiths]

I would add that I would look at wider gutter, I used 125mm rectangle profile. 
water comes offf these metal roofs far quicker than a tiled roof. 

[Owain1602]

FYI, this is the gutter that Tata supply with their Catnic roof.
 

 

I can’t make out from your drawing what is cladding, cavity etc. But my understanding is that they want to fix the gutter to the outside face of the cladding (or equivalent timber). 
I was told by our steel roof installers to bring the top deck of OSB3 out to the same projection as the external face of the cladding. They then make the overhang and drip edge with their metal eaves trim, and the gutter is fixed through the outside face of the cladding to the timber. 

[Mr Blobby]

2 minutes ago, Russell griffiths said:

I would add that I would look at wider gutter, I used 125mm rectangle profile. 
water comes offf these metal roofs far quicker than a tiled roof. 

 

Good point, and I;m glad you reminded me about this.  If I knew back then what I know now I would have insisted on 150 guttering instead of our builder ordering standard stock items from his regular merchant.

 

We have a large roof but there should have been only one downpipe on the front of the house.  It turns out that one downpipe needs 150 gutter to accomodate the flow.  There's a set of regulations somewhere of gutter sizes required for roof sizes.  It also specifies a fall towards the downpipe. Nobody on our site read it except me, and then too late.  Fall, what fall?

[Russell griffiths]

4 minutes ago, Mr Blobby said:

 

Good point, and I;m glad you reminded me about this.  If I knew back then what I know now I would have insisted on 150 guttering instead of our builder ordering standard stock items from his regular merchant.

 

We have a large roof but there should have been only one downpipe on the front of the house.  It turns out that one downpipe needs 150 gutter to accomodate the flow.  There's a set of regulations somewhere of gutter sizes required for roof sizes.  It also specifies a fall towards the downpipe. Nobody on our site read it except me, and then too late.  Fall, what fall?

I believe the gutter regs are inadequate for our changing climate. 
I based my design on living in oz where you have to have an outlet every 6m of gutter. 

[Alan Ambrose]

OK I've modified the geometry with your feedback above. I think that'll work, thanks. A useless fact I learnt is that the water falls in a parabola, presumably dependent on the velocity it flows off the roof.

 

 

[Mr Blobby]

1 hour ago, Alan Ambrose said:

A useless fact I learnt is that the water falls in a parabola, presumably dependent on the velocity it flows off the roof.

 

It doesn't look like it in the first picture I posted above because the bracket is this side of the roof, but if I put a straight edge on the roof it runs straight to the inside of the gutter.  It doesnt lie over the top of the gutter, so even if the rain ran off at sepped in a straight line, it would still land in the gutter ok.

 

I think the important thing to do is ask your metal roof supplier how they want it.

 

Edited May 11 by Mr Blobby

[saveasteading]

Re downpipes. Once  the water is in the pipe it is well within capacity. But the constraint is in the inlet where a weir effect forms. So a single pipe with a big outlet can carry lots of water where a standard one cannot.

The upmarket makes cater for this.

 

Do you know what brand is being used? They should have info in their brochure or online.

 

It sounds a bit as if this hasn't been designed and is just a sequence of events.

 

Bigger gutters, big outlets, more than one pipe (and/or an overflow)  and a fall, if possible, all help.

 

Also, a downpipe near an end works less well than if centrally positioned. 

 

Bottom line is what happens if it overflows? Does it come clear of the house or run back to the wall?

 

[Alan Ambrose]

OK I'm back on this now.

>>> It sounds a bit as if this hasn't been designed and is just a sequence of events.

Harsh . I'm not aware of any design methodology for guttering? It's normally suck-it-and-see no?

 

You would need a SUDS-like estimate of worst storms + detail of roof geometry and material + hydrodynamic gutter & downpipe info -> therefore max run-off flow s.b. greater than gutter capacity.

 

I can't imagine though that anybody does that calculation or even that the roof and gutter data is available. Seems to me everyone uses rule-of-thumb / best guess.

 

There's an overhang so it should clear the wall, but we've all seen water flow in a big storm - it tends to go everywhere.

[saveasteading]

9 minutes ago, Alan Ambrose said:

normally suck-it-and-see no?

Normally perhaps but it shouldn't be. 

It is hydraulics.

 

 

9 minutes ago, Alan Ambrose said:

can't imagine though that anybody does that calculation

 

The manufacturers publish flow rates. The number, position and quality of outlets makes a huge difference.

As does shoddy fitting that isn't level.

 

The rainfall data is in the building regs but it is wise to allow for the recent increase in continental type downpours.

 

I ask myself what if... 

There is an exceptional storm

The gutter is dirty

There is a blockage

 

Where does the water go harmkessly?

 

 

Ask away.

 

Edited 9 hours ago by saveasteading

[Dunc]

I belive gutter capacities are calculated, rather than intuited, including specifications for rainfall amounts to BS EN 12056-3 e.g. 

https://www.floplast.co.uk/you/diyinstaller

https://alugutter.co.uk/pages/gutter-drainage-performance-eaves-gutter-design-bs-en12056-3?srsltid=AfmBOopwXxj1pTu7ASOsX0Hc1K2cJz8FK6ftk4oQ8J6iE5rW0462CgPn

 

[saveasteading]

On 10/05/2026 at 20:53, Mr Blobby said:

Fall, what fall?

 It of course helps flow if it slopes gown to the outlet but it is tricky to do znc can look untidy.

 

In drizzle the water barely dribbles off a small roof and may even stream back towards the wall, so a clear projection is needed.

In a storm, the parabola could be quite a long and flat one.

 

In your drawing above  I'd like to see a deeper gutter that is higher so that the water always lands inside.

 

The cheap outlets are a round hole in the bottom. The water flows in like a wier, ie only a few mm deep. The classy ones have a curved outlet, almost a hopper, and flow is very much faster.

The price reflects the science and performance, but the dear ones are also sturdier.

Btw for a high gutter, I don't allow leaf catchers in the outlet because they need clearing.

I catch the leaves at the bottom.