Flat Roof Overhang

[Barney12]

The super sharp lines of @Trw144 roof line (See here:

has prompted me to make a design detail decision about the flat roof on my dormers.

 

My died in the blood old boy roofer friend says "Got to have 50mm overhang from the face and sides of the dormer. Nothing less". But that's not exactly going to produce a contemporary sharp looking roof is it?

 

My intention is now to use GRP/fibreglass with the minimum depth of edge I can get away with. Which is my first point of annoyance as all the pre-formed edging strips are min 100mm depth.

 

So what's peoples thoughts on the overhang issue? Can it only be less if you have some form of internal gutter to stop run off?

[Construction Channel]

I could be wrong but i cant see why it would need an overhang unless it is trying to drip into a gutter,

 

lap the wall below properly i cant see a problem, 

 

ill try to remember to take a picture of the flat roof on our current job tomorrow, but along the side it is pretty tight to the wall. (its a rubber roof but i cant see that would make a difference.)

[Ferdinand]

I think it depends on the robustness of the vertical face.

 

How good is it?

 

[Ian]

@Barney12

The reason you normally have an overhang is to prevent uneven wetting of the facade of the building. The uneven wetting in turn produces uneven staining.

This staining shows up worst on some facing materials such as timber and render.

[Ian]

@Barney12

Edit: if you don't have an overhang on your flat roof then it's important that the parapet flashings all have a good slope pointing back into the roof if you want to avoid the uneven staining that I mentioned.

Not having an overhang means that you are more reliant on good workmanship and weatherproof detailing to keep the water out of the top of the wall.

[Onoff]

I was looking to INCREASE my dormer overhangs if anything when I redo them. Edges and corners leak like a sieve. 

 

 

[Barney12]

8 hours ago, Ferdinand said:

I think it depends on the robustness of the vertical face.

 

How good is it?

 

 

I'm intending to use thermally treated ash with interlocking profile so it should be pretty weather resistant. 

Ill probably treat it with Osmo Oil which tends to be my weapon of choice for treating bare timber. 

[Barney12]

1 hour ago, Ian said:

@Barney12

The reason you normally have an overhang is to prevent uneven wetting of the facade of the building. The uneven wetting in turn produces uneven staining.

This staining shows up worst on some facing materials such as timber and render.

 

That does make sense, 

[Barney12]

1 hour ago, Ian said:

@Barney12

Edit: if you don't have an overhang on your flat roof then it's important that the parapet flashings all have a good slope pointing back into the roof if you want to avoid the uneven staining that I mentioned.

Not having an overhang means that you are more reliant on good workmanship and weatherproof detailing to keep the water out of the top of the wall.

 

The dormers were installed by MBC and the slope is forward not back towards the roof. Thus water is going to run off to the front of the dormer (unless I remove the timbers from the roof they have used to create the slope and reverse them).

[Ian]

7 minutes ago, Barney12 said:

 

The dormers were installed by MBC and the slope is forward not back towards the roof. Thus water is going to run off to the front of the dormer (unless I remove the timbers from the roof they have used to create the slope and reverse them).

It's difficult to give proper advice without seeing a drawing of the relationship between the dormer and the rest of the building. Do you have a drawing you could post?

12 minutes ago, Barney12 said:

 

That does make sense, 

rainwater carries pollutants and dirt with it and these can get deposited unevenly down the facade unless you have either a roof overhang or a very good drip profile to the capping at the interface of the wall and roof.

[Ferdinand]

27 minutes ago, Barney12 said:

 

I'm intending to use thermally treated ash with interlocking profile so it should be pretty weather resistant. 

Ill probably treat it with Osmo Oil which tends to be my weapon of choice for treating bare timber. 

 

TBH I do not know how good that is.

 

Perhaps I'm flying in the dark here, but I had decades of my dad pointing out water staining on every modern building built with flat faces, His point was that if you do not channel it water will stain where it happens to go according to nature. He advocated details where the water would run (ie vertical something of some sort), or channelling it where you want it.

 

Looking back, @Trw144's roof seems to have a detail all the way round all the edges, and it may even have a small upstand (perhaps only 25mm) between the edge and the flat surface of the roof (?). And I think it may rely on a mini-fall on the roof going to wherever he wants the water to go to avoid any runoff. Perhaps the fall alone will do it. Is there a hidden drainpipe somewhere?

 

 

How are you doing that?

 

But if it is able to runoff anywhere visible, it *will* show stains.

 

Just been out looking at my dormers and my neighbour's dormers with binoculars. Both are trad tiled tops with a normal roof fall, and both take the water to the back, then eject it onto the roof at the back corners. Mine has a slightly Heath-Robinson flashing arrangement; his has valley type tiles.

 

I think for a flat dormer I might do a lip (eg 25x25 tanalised batten fibreglassed over) round the edge, and slope the fall towards the back corners, with perhaps something as simple as a 100mm gap in the lip to let it onto the roof. Then any staining will be along the dormer / roof joint rather than down the vertical sides or front.

 

If it slopes the other way another option might be to have a shallow-u profile channel/tray mounted vertically behind (200-300 x 12mm?) and a gap or an overlap in your cladding just round the side from the front corners. That would let the routine runoff run down behind the cladding but in front of your waterproof liner (guessing there is one). Visually.you would just get an accent line near your vertical corners. Then take it onto the roof at the bottom. I suppose a couple of vertical battens mounted behind the cladding 200mm apart with the channel lined in grp would do it as well.

 

That is me pants-flying, so others may want to help by critiquing.

 

Ferdinand

 

Edited June 21, 2017 by Ferdinand

[Ian]

34 minutes ago, Barney12 said:

 

I'm intending to use thermally treated ash with interlocking profile so it should be pretty weather resistant. 

Ill probably treat it with Osmo Oil which tends to be my weapon of choice for treating bare timber. 

I'm guessing that you've already seen this very good Scottish government guide to timber cladding detailing?:

http://www.gov.scot/Publications/2002/03/15098/8744

[Barney12]

6 minutes ago, Ian said:

I'm guessing that you've already seen this very good Scottish government guide to timber cladding detailing?:

http://www.gov.scot/Publications/2002/03/15098/8744

 

No I hadn't. Only the Trada one. I'll take a read. Ta!!

Edited June 21, 2017 by Barney12
Typo

[Ian]

40 minutes ago, Barney12 said:

 

The dormers were installed by MBC and the slope is forward not back towards the roof. Thus water is going to run off to the front of the dormer (unless I remove the timbers from the roof they have used to create the slope and reverse them).

It sounds like it needs some very careful thought before changing the original design.

 

I've got timber cladding (cedar) on my own building. Expect movement of the timber and design accordingly. You may find after the timber t&g cladding has been on the building for a while that you'll get gaps opening up at the butt joints between the boards. If one of these joints happens to be directly below a part of the roof where there's no gutter or roof overhang you will get water penetration through the wall.

Edited June 21, 2017 by Ian

[Trw144]

7 hours ago, Ferdinand said:

 

TBH I do not know how good that is.

 

Perhaps I'm flying in the dark here, but I had decades of my dad pointing out water staining on every modern building built with flat faces, His point was that if you do not channel it water will stain where it happens to go according to nature. He advocated details where the water would run (ie vertical something of some sort), or channelling it where you want it.

 

Looking back, @Trw144's roof seems to have a detail all the way round all the edges, and it may even have a small upstand (perhaps only 25mm) between the edge and the flat surface of the roof (?). And I think it may rely on a mini-fall on the roof going to wherever he wants the water to go to avoid any runoff. Perhaps the fall alone will do it. Is there a hidden drainpipe somewhere?

 

 

How are you doing that?

 

But if it is able to runoff anywhere visible, it *will* show stains.

 

Just been out looking at my dormers and my neighbour's dormers with binoculars. Both are trad tiled tops with a normal roof fall, and both take the water to the back, then eject it onto the roof at the back corners. Mine has a slightly Heath-Robinson flashing arrangement; his has valley type tiles.

 

I think for a flat dormer I might do a lip (eg 25x25 tanalised batten fibreglassed over) round the edge, and slope the fall towards the back corners, with perhaps something as simple as a 100mm gap in the lip to let it onto the roof. Then any staining will be along the dormer / roof joint rather than down the vertical sides or front.

 

If it slopes the other way another option might be to have a shallow-u profile channel/tray mounted vertically behind (200-300 x 12mm?) and a gap or an overlap in your cladding just round the side from the front corners. That would let the routine runoff run down behind the cladding but in front of your waterproof liner (guessing there is one). Visually.you would just get an accent line near your vertical corners. Then take it onto the roof at the bottom. I suppose a couple of vertical battens mounted behind the cladding 200mm apart with the channel lined in grp would do it as well.

 

That is me pants-flying, so others may want to help by critiquing.

 

Ferdinand

 

 

 

My flat roof has a fall in it, and surrounding it all is a parapet that is probably 600mm high at its highest point. The cladding profile should be installed so that any water runs back in to the flat roof, and out through one of the internal gutter outlets, rather than down the outside. I had one area where it was running down the render slightly and I solved this by putting a thick bead of silicone on the top of the parapet so the water could nt track to the front and drip down, but instead ran back internally.

[Barney12]

43 minutes ago, Trw144 said:

 

 

My flat roof has a fall in it, and surrounding it all is a parapet that is probably 600mm high at its highest point. The cladding profile should be installed so that any water runs back in to the flat roof, and out through one of the internal gutter outlets, rather than down the outside. I had one area where it was running down the render slightly and I solved this by putting a thick bead of silicone on the top of the parapet so the water could nt track to the front and drip down, but instead ran back internally.

 

So are your gutter outlets behind the cladding detail?

[Ferdinand]

24 minutes ago, Barney12 said:

 

So are your gutter outlets behind the cladding detail?

 

CAn we have an aerial photo? 

 

I think someone on here has a drone, and your location may be within the navigation tolerances.

[Trw144]

1 minute ago, Ferdinand said:

 

CAn we have an aerial photo? 

 

I think someone on here has a drone, and your location may be within the navigation tolerances.

 

Yes, my gutters are behind the cladding and run internally down through the walls - I have 4 or 5 from memory. I ll try and find some section drawings - they are readily available as I have changed computer/server/company.