Marine ply overflow - how to protect it?

[Visti]

I could use a 2nd opinion on how to protect my overflow from the elements!

 

Long story short, I've had to retrofit a box duct (9mm marine ply, 35x90mm internal cross-section) onto each end of my hidden box gutters. 

It is lined with a single ply membrane (PVC) but not much else at the moment.

 

Dark blue in the detail below:

 

 

I want to cap off the exposed part with some Alu trim as shown below:

 

 

 

Any idea whether this will help, or whether it'll just make things worse with drip marks and the like?

[LA3222]

In my non professional opinion I would imagine that it will tidy things up - I can't see anything wrong with your plan.

Edited October 4, 2019 by LA3222

[LA3222]

On second thoughts, would it be worth angling the drip bead to encourage the drips to occur away from the face of your cladding?

[Visti]

That is not a bad idea at all @LA3222! I think I'll take that on board!

[gc100]

Can I ask my you need to add the overflows? I have hidden gutters in my design but there is no spec for overflow

[PeterW]

1 minute ago, gc100 said:

Can I ask my you need to add the overflows? I have hidden gutters in my design but there is no spec for overflow

It is very advisable. Water weighs a lot, we did a commercial job with hidden gutters and on a 30m building, we worked out the hidden gutters held 5.7 tonnes of water between them before they overflowed ... 

[Visti]

Aye, the primary factor was the eventual weight of the water as @PeterW explained. If we didn't have an outlet, we'd accumulate 1.5tonnes per gutter across 10m. It might have held, but was an unacceptable risk to our Building control officer, so he applied balcony regs to get an overflow installed since we couldn't go over the edge.

 

[gc100]

This is probably a dumb question but why would they fill up? In case the downpipe was blocked basically? Or are you saying during heavy rain the gutter can't empty itself quick enough?

[PeterW]

4 minutes ago, gc100 said:

This is probably a dumb question but why would they fill up? In case the downpipe was blocked basically? Or are you saying during heavy rain the gutter can't empty itself quick enough?

Both.... you have to plan for both of those, and tbh your issue is when the water reaches the top of the gutter it will find the shortest route to the ground and that could be via your cavity wall. 

[Visti]

Yup. The regs specify the amount of rainfall you should scale your gutters too per roof based on area, but that isn't 100% foolproof. There could be 1/100yr rainstorms or other peak events you have to cater to as well, hence the need for some overflow in one way or another. I've oversized my downpipes too just in case so that I've at least double capacity at all points in case one end fails entirely.