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 yesterday at 19:27 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 4 hours ago 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?