Raising part of flat roof - drainage issues?

[Gooman]

We recently moved into a detached house with an attached double-length garage. The other side of the garage shares a party wall with the neighbour's identical garage. See front and back pics attached, together with the view from our first floor over the garage roof.

 

We're planning to take out the wall that separates the kitchen from the rear half of the garage and install a wall in the garage to create an open plan kitchen diner. You can see that wall from the garage in the attached pics.

 

Of course, since the garage floor is about 110mm below the kitchen floor (and is uninsulated) we'll be putting in insulation and a floating chipboard floor. To enable that, we need to raise the part of the garage roof that's above the newly converted area, by about 300mm (including insulation and service void). The neighbour has agreed to our Party Wall Act notice.

 

So far, so good. But here's the potential problem ...

 

The garage roof has a slight pitch for drainage from front to back (you can see on the pics that the front has a raised lip and the rear has the (shoddy) guttering. So if we raise our part of the rear half  of the garage, rain water will run from the front towards the new raised part. The least worst of the problem is that this will run across the front of that raised part onto the neighbour's garage roof, and they then have our run-off going into their gutter. My major worry is that the water won't all just run horizontally across to their part of the garage, since there's no noticeable pitch running across, and that it might pool on the roof next to the wall.

 

Am I being paranoid? Our builder seems to think so.

 

If not, what are my (reasonably priced) options here? In an ideal world I'd rather not have our run-off going across the neighbour's roof - although they've agreed to the Party Wall notice, we don't want to be unreasonable and would like to avoid any future issues.

 

For example, could we have a gully at the base of the new raised roof section, with in internal downpipe inside the remaining part of the garage? You can see in the internal garage pic that there's a soil pipe that runs from the toilet above. That soilpipe will be moved to run diagonally from the external wall above and then down through the roof of the remaining part of the garage. Could a gully connect with that soilpipe?

[Tennentslager]

I’d go with the builder as the new roof isn’t that big. If I understand correctly the new layout means rain from one garage roof will run onto the neighbours garage roof then into his gutter?

you could offer to replace the gutter at the back for both garages...looks like next door doesn’t even have a gutter 

[ProDave]

Is this a wise conversion?

 

Doing so, if the neighbour then did the same, would turn both houses into "semi detached" and devalue both of them.

 

A rear extension might make more sense?

[Temp]

+1 to what @ProDave said.

 

looking at the rear elevation.. Is there enough clearance below the eaves of the house to raise the garage 300mm? I cant tell from the photo due to the angle but if it's the same height as at the front it looks tight.

 

Edit: Looking at the view out of the window towards the neighbours the gap looks about 3 brick courses or 250mm ish . So If you raise the roof 300mm this will cut into the facia. This area would need careful design or you will have a leak in a difficult area to fix. Think I'd want to see a sketch of the proposed solution 

Edited December 21, 2019 by Temp

[Gooman]

24 minutes ago, ProDave said:

Is this a wise conversion?

 

Doing so, if the neighbour then did the same, would turn both houses into "semi detached" and devalue both of them.

 

A rear extension might make more sense?

Not enough space in the rear garden for a rear extension (and still leaving a reasonable garden. This is our "forever" house, so we're less concerned about devaluation - plus, any devaluation from it being classified semi rather than detached would be more than offset by the amenity value of the conversion. Lastly ... since we're going first, that's more of a judgement for the neighbour anyway.

[Gooman]

10 minutes ago, Temp said:

+1 to what @ProDave said.

 

looking at the rear elevation.. Is there enough clearance below the eaves of the house to raise the garage 300mm? I cant tell from the photo due to the angle but if it's the same height as at the front it looks tight.

It's close, but there's enough.

[Gooman]

3 hours ago, Tennentslager said:

I’d go with the builder as the new roof isn’t that big. If I understand correctly the new layout means rain from one garage roof will run onto the neighbours garage roof then into his gutter?

you could offer to replace the gutter at the back for both garages...looks like next door doesn’t even have a gutter 

That's right,

 

My main concern is to ensure that rain from the front of our garage does go somewhere, and doesn't just pool against the new raise section due to the lack of any pitch across the roof from left to right.

[Temp]

3 minutes ago, Gooman said:

It's close, but there's enough.

 

Ideally you need 300mm plus two brick courses for the flashing. On my house that's about 6 brick courses. Only looks like 3 on the neighbours house.  

 

 

 

 

[Temp]

19 minutes ago, Gooman said:

My main concern is to ensure that rain from the front of our garage does go somewhere, and doesn't just pool against the new raise section due to the lack of any pitch across the roof from left to right.

 

How old is the garage flat roof? If it's not got much life left have you considered raising the whole thing?  Might be possible to make the new internal dividing wall all blockwork as none of it would then project above  the roof line.  You might also be able to run a gutter on the neighbours side to catch any run off if they are happy with that. Obviously you would have to raise the front wall and a bit more of the party wall as well.

Edited December 21, 2019 by Temp

[AliG]

Looking at that last picture, is there any flashing between the flat roof and the side of either house? It doesn't look like it which would make me more concerned about where the water might go. It could start to pool and run into the house wall. 

 

The roof also looks in pretty poor condition, I would be considering whether to replace the whole thing whilst work is being done anyway. I guess though the builder might have mentioned this if he thought it needed done.

 

This seems a bit of a thing in the south east, people seem happy to put lots of mismatched extension onto houses, but really it won't look great with a little piece of flat roofed area jutting up on just one side. I think I'd actually be more concerned with this that next door converting the back of their garage.

 

Raising the entire roof on both garages would probably be best, but looking at your neighbour's house versus yours and the lack of gutter, it wouldn't seem like they will be in a hurry to contribute to anything.

 

Depending on the current set up of your drain BC may not want you to connect a rainwater drain to a wastewater drain.

 

 

 

[Gooman]

Just to close this out, I've reviewed the situation with our builder. The roof is in good enough condition to keep, and doesn't need to be replaced. The roof needs to be raised by 250mm and that is exactly the gap between the existing roof and the bottom of the fascia - the fascia can be trimmed back and flashed into the new roof height.

 

As for the drainage, the builder agrees with me that it needs somewhere to run other than onto the neighbour's roof (which doe have guttering - hard to see on the pics as it's black). He will be running a downpipe through the roof out to a soakaway at the rear.

 

[Temp]

9 hours ago, Gooman said:

the fascia can be trimmed back and flashed into the new roof height.

 

Running flashing up the fascia sounds like a bit of a bodge. Can't really see an obvious alternative.

 

9 hours ago, Gooman said:

He will be running a downpipe through the roof out to a soakaway at the rear.

 

It is possible to put an internal drain in a flat roof but that's also something to be avoided if possible. Its one of those things that works if done right but is a pain if not. Again can't see you have much choice.