Flat Roof Drainage

[siwhitt]

Come Spring I'm replacing this section of flat roof (used as a roof terrace) with a new GRP roof. It's around 16-17 square metres in size.

Can anyone advise how the correct drainage should be achieved?

The roof falls towards a small channel running the entire length of the side wall, although you can see there is some standing water. Water in the channel should then run towards the outlet in the wall, although we also end up with standing water in the channel.

Untitled

• siwhitt
• 21 Jan 2015
• 1

Untitled

• siwhitt
• 21 Jan 2015
• 1

Firrings will give me the fall in the roof towards the channel, but I'm not clear as to how I then achieve the fall in the channel itself and what I lay in the channel?

 

[oldbutnotdead]

The pics aren't that clear- the channel looks very shallow and the outlet pipe seems to be above the level of the roof, which is odd.

Presumably you're ripping the felt etc off and laying OSB? And the plan is to have a gentle uniform slope on the roof down towards that channel so water drains to the channel and then into the outlet?.

So the channel really needs to slope from both ends towards the middle- easily done with packers or whatever under your OSB. Tricky bit is that GRP won't bond to most plastics- you might need to form a rectangular opening through that wall (which you can line with OSB and fibreglass upstands, mat, topcoat etc so it is an integral part of the roof)- if that pipe goes to a hopper or something then easy life.

Back to the channel. Down to about 10mm below roof you can just mat it- you'll get an air bubble in the corner but as long as it doesn't get heavy traffic it'll be fine. Deeper than that and you start needing trims (to form the corners properly) or you need to put a wedge piece in the roof side of the channel (so instead of a 90 degree drop which won't mat well you have a 45 degree slope which will mat very nicely). Trims are pricey so wedges on the roof side might be favourite (and if that section is zero traffic then you might get away with using plastic padding or even mortar to form the wedge). The channel should be lined with OSB like the rest of the roof.


The wall side of the channel (like the rest of the roof) will need GRP upstands and cover flashing (GRP or lead on the cover, up to you).

On that size you might want to think about expansion joints as well