Garden Room - Flat Roof Design

[efunc]

Hello all,

I’m in the initial build stages of a small Garden Room. It’s about 2m x 3m with an open storage area on the right side:

The roof will be supported by a wall along the long edge at the back (5.4m) and on the right (2.9m). It will be basically flat, although I will try to introduce a fall to the front by using firrings. It will be done in GPR. The wall on the right is a shared garden wall with a neighbour and the wall to the rear is also a garden wall with another neighbour's house into which the roof flashing will go.

Obviously I would like to keep water off the neighbour’s wall on the right. My builder wants to build a raised upstand or ridge all the way around the roof to prevent runoff and just have a break on the left side of the roof to a drain. However this worries me because it could allow water to pond on top, and snow too in the winter. That would be a lot of weight and possible leaks. Is this advisable? What would be the best way to design the GPR roof for this build? A simple gutter on the front, and one on the left side to keep the wall dry?

Also the builder has suggested he use lead flashing on the back wall. Is this appropriate for a flat GPR roof?

Any advice would be very welcome. Thanks!

 

[noseall]

You just need a raised kerb the same as any flat roof edge. This is done with an angle fillet etc. Lead cover flashing will be fine on the abutment.

 

[efunc]

I see, thanks for that. So just a drip trim on the non-drainage edges of the roof like this would be sufficient?

Would a slight fall to the front with a gutter be advisable?

thanks

 

[Notch7]

Would a slight fall to the front with a gutter be advisable

The term flat roof is a misnomer -flat roofs are not flat! Typically felt roofs should have a 1 in 40 fall -in fact the min fall for sufficient run off is reckoned to be 1 in 80, but felt is done at a steeper fall to allow for overlaps and uneven rafters etc.

I generally aim for around 1 in 60 for grp -however roofers often seem to double up the matting near the edge with grp and there tends to be a little bit of ponding.

I would suggest the simplest option is to have a fall back to front with the roof discharging into a gutter at the front.

I wouldnt do as the builder suggsst by making an upstand all round and discharging with a shoot into a gutter -its more complicated.

 

[^woody^]

I will try to introduce a fall

Try hard!

 

[efunc]

Great! That's a plan then. Firrings rear to front, guttering along the front, raised edge trim along the two sides, fillet and flashing along the back wall. Thanks for the pointers.

 

[efunc]

I'm returning to the thread while I tweak my design prior to work starting on the stud work started tomorrow. Here's what I currently have - firrings from the back to the front at about 1:40. The roof depth is about 2.5m, so I worked them out as about 60mm high at the back wall tapering to 1mm at the front overhang:

What I'm having last minute doubts about is whether or not there is an alternative to running a gutter along the front edge of the roof? It's the longest side at 6.1m and since it's facing the house and garden would be relatively big and prominent. Short of having no gutter and instead allowing the rain to pool on the roof with some sort of hidden outlet to one side, is there any other alternative? Do I just have to accept the inevitable for the good of the structure. I'm just wondering if there's another solution I can explore before we get too far with the build. Thank you all!

 

[^woody^]

Do I just have to accept the inevitable for the good of the structure

No. You could design the getter to be concealed by soffit and fascia.

 

[efunc]

Thanks for that. It's a great suggestion. After about an hour of googling around though I can't really see a simple off-the-shelf system I can use and everything else seems complicated and over engineered. For a properly designed build with skilled specialists it could be a great solution but I think for my humble garden room it might be beyond my ability to design something from scratch that wasn't needlessly expensive. And then my poor builder would have to muddle through it too.

Could I possibly notch a channel out of the joists above the roof overhang, clad it in OSB along with the rest of the flat roof, then line it in GPR? Is this too simple? Or can anyone point me in the direction of an actual pre-existing system that might be easier to design in?

I think I understand what you mean now. I would just end the roof joists as they are in the current design, keep the guttering too, but just extend the fascia further out so it covers it, and extend the soffit below too?

 

[Leofric]

A box gutter shouldn't be that difficult for a builder.

 

[Notch7]

What I'm having last minute doubts about is whether or not there is an alternative to running a gutter along the front edge of the roof? It's the longest side at 6.1m and since it's facing the house and garden would be relatively big and prominent

Guttering is likely to be the least prominent solution -I think it woukd be more attractive than having a hidden box gutter which would mean some form of fascia upstand to hide the roof falls and is more complex.

I thiink guttering would look fine, if it was me Id run it around 2 sides -which would require different firrings -that way the roof would look the same without the stop end on the gutter at the corner.

A gutter will give you more roof overhang so helping to protect the sides

 

[efunc]

Thanks all. I'll school up on box gutters, but I think Notch7 is right, traditional guttering is simple, cheap and effective. It's not the most offensive thing in the world to look at and the alternatives are slightly over complicated for me and may pose problems down the line if I execute it badly.

One of the things that threw me is that if you look on google images at the hundreds of similar builds out there, literally none of them have any visible guttering, so it looks like mine will be unique!

https://www.google.com/search?q=gar...2pTjAhWIfMAKHVAqCOMQ_AUIECgB&biw=1920&bih=966

But I guess that's mostly because mine is a lean-to structure and all the traditional designs have pent roofs draining to the rear...

 

[Notch7]

draining to the rear..

Yours could drain to the rear, but it would need falls in 2 directions and the highest point would be the rear corner where the 2 walls are -which would mean the highest point would be higher than current design.

 

[Leofric]

You could have no gutter or outlet through the perimeter of the roof if you fall to an outlet through the flat roof to a rainwater pipe down into the open storage area.

 

[efunc]

You could have no gutter or outlet through the perimeter of the roof if you fall to an outlet through the flat roof to a rainwater pipe down into the open storage area.

Yes, I'm aware these options exist, but I understood that they're more challenging to implement, maintain and also repair. For example, should leaves block the outlet and rainwater end up ponding around it it's more likely to seep into the internal structure than if a gutter was breached. Also I think there's more need to insulate internal drains because otherwise condensation can build up causing its own damage. Also creating effective falls are a bit more complicated.

I mentioned the possibility of doing a concealed gutter to my builder today and he also suggested something like my plan (which I thought was probably crude and ill informed!) ie, creating a trough in the roof over the front overhang by notching out a channel about 10-12cm deep across the joists, lining it with 12mm ply and then covering it with the GRP like the rest of the roof. This would fall to the left of the roof at about 1:80 over 6m. This at least could be done relatively cheaply compared to box gutters and the like in aluminium costing several hundred ££ including matching fascias. But at the moment I'm still playing with ideas and yet to settle on a final plan.