Flashing details for side abutment with very limited upstand?

[lineweight]

Here is a situation that quite often occurs at the party wall on terraced houses where there is a change in roof height, but not parapet rising above the roof surface:

 

 

If the change in height is more than about 300mm then it's just about OK - there is room for, say, a stepped flashing, as per the first image, or some kind of apron flashing as per the second image, with a bit of masonry still left above the flashing chase line.

 

If the change in height is less than that, though, it gets tricky.

 

Is there any 'proper' detail, or roofing product, that can deal with a change in height that's less than about 300mm?

 

 

 

 

[PeterW]

Yep should be done as a hidden valley with the lead lapped under the top verge board and then the lower tiles laid over the top of the valley leaving as small a gap as possible. 

[lineweight]

6 minutes ago, PeterW said:

Yep should be done as a hidden valley with the lead lapped under the top verge board and then the lower tiles laid over the top of the valley leaving as small a gap as possible. 

 

Is that going to work with a bedded verge, as shown in the photos, though?

[Temp]

I think 150mm upstand is normally the minimum recommended. That would be 150mm above the tiles.

 

If a hidden gutter is used (as @PeterW said) it would be 150mm above the bottom of the gutter which would be lower.  Hidden gutters can work with zero height above the roof if the gutter is deep enough. 

 

 

Image from here.

https://1stassociated.co.uk/box-gutter-sketch.asp

 

If this is an existing roof I think the main issue is likely to be a rafter in the way (fixed to the wall).

 

 

 

 

Edited December 15, 2020 by Temp

[PeterW]

1 hour ago, lineweight said:

 

Is that going to work with a bedded verge, as shown in the photos, though?

Depends. I’d want the verge off, strip back and bed the lead min 50mm back onto the brickwork and then bed a new cloaking board over the lead / wall junction and then the tiles. Hidden gutter wants to be non 75x25mm, deeper if possible, and get the last row of tiles within 20mm of the wall. Needs lots of careful detailing and the lead would probably need to be 450mm wide so wants to be in short bays no more than 1200mm long with decent overlaps.
 

You’ll need a decent roofer who knows how to work lead to do that and it’s not a quick job, especially if there is a chimney in the roofline too. 

[ToughButterCup]

15 minutes ago, PeterW said:

...

You’ll need a decent roofer who knows how to work lead to do that and it’s not a quick job, especially if there is a chimney in the roofline too. 

 

Its the short bays and the need for overlaps  that got me . Overlaps of 50mm perhaps (?) amount to more than I had guessed before someone asked me if I'd allowed for overlaps. But then, I'm no decent roofer.

 

Just an oft-bitten self builder.

Edited December 15, 2020 by ToughButterCup

[PeterW]

Depending on the slope I would go with 75-100mm overlaps. More is better but leaves lumpy lead work to deal with ..!

[lineweight]

33 minutes ago, Temp said:

I think 150mm upstand is normally the minimum recommended. That would be 150mm above the tiles.

 

If a hidden gutter is used (as @PeterW said) it would be 150mm above the bottom of the gutter which would be lower.  Hidden gutters can work with zero height above the roof if the gutter is deep enough. 

 

 

Image from here.

https://1stassociated.co.uk/box-gutter-sketch.asp

 

If this is an existing roof I think the main issue is likely to be a rafter in the way (fixed to the wall).

 

 

 

 

 

Yes, 150mm is generally what you're aiming for; the problem is when you're trying to chase into an upstand with very little space above that line.

 

An obvious approach is to take that flashing higher up, right up and over the top of the masonry and under the tiles of the higher-level roof, which is essentially what @PeterW is suggesting - what I'm not sure is whether that's a conventional detail in this scenario, and how you would then secure that flashing.

[lineweight]

30 minutes ago, PeterW said:

Depends. I’d want the verge off, strip back and bed the lead min 50mm back onto the brickwork and then bed a new cloaking board over the lead / wall junction and then the tiles. Hidden gutter wants to be non 75x25mm, deeper if possible, and get the last row of tiles within 20mm of the wall. Needs lots of careful detailing and the lead would probably need to be 450mm wide so wants to be in short bays no more than 1200mm long with decent overlaps.
 

You’ll need a decent roofer who knows how to work lead to do that and it’s not a quick job, especially if there is a chimney in the roofline too. 

 

Ok, so the lead sandwiched between the cloaking board and the masonry, secured by friction/weight of cloaking board & tiles on top?

 

What's the reason you suggest a hidden valley, rather than a conventional abutment detail, just with the lead cover flashing secured in the way described above, rather than chased into the brickwork joints?

Edited December 15, 2020 by lineweight

[PeterW]

1 hour ago, lineweight said:

 

Ok, so the lead sandwiched between the cloaking board and the masonry, secured by friction/weight of cloaking board & tiles on top?

 

What's the reason you suggest a hidden valley, rather than a conventional abutment detail, just with the lead cover flashing secured in the way described above, rather than chased into the brickwork joints?

Space. You don’t have 150mm depth and you will also get a lot of wetting of that cloak the way it’s done. Makes more sense to just overcloak it and ensure you have a decent vertical waterproof detail.  

[lineweight]

2 hours ago, PeterW said:

Space. You don’t have 150mm depth and you will also get a lot of wetting of that cloak the way it’s done. Makes more sense to just overcloak it and ensure you have a decent vertical waterproof detail.  

 

In case we are talking at cross purposes - I mean taking a standard abutment detail, with soakers and then a step flashing covering them, like this:

 

And a standard bedded verge detail, like this:

 

I think your suggestion was to tuck the lead flashing in under the undercloak (shown on this detail as a face-down tile, but could also be a fibre cement undercloak)

 

So if you can forgive the crude mash-up of details, I mean something like this:

 

What I've drawn in blue replaces the step flashing, and instead of being chased into the brickwork like normal, it's tucked under the verge undercloak (which I think is what you were suggesting to do with the hidden valley detail).

 

This would work, I think, as long as you had a minimum of about 150mm between the top surface of the lower roof, and the level of the undercloak for the upper roof?

 

 

[Temp]

 

7 hours ago, lineweight said:

which I think is what you were suggesting to do with the hidden valley detail).

 

@PeterW might correct me but i think he was saying the same as me.

 

Eg there are three cases depending on height of the wall..

 

If its tall enough go up 150mm as normal.

If its marginal go up to the undercloak as per your drawing.

If its too short make a hidden gutter. 

[lineweight]

8 hours ago, Temp said:

 

 

@PeterW might correct me but i think he was saying the same as me.

 

Eg there are three cases depending on height of the wall..

 

If its tall enough go up 150mm as normal.

If its marginal go up to the undercloak as per your drawing.

If its too short make a hidden gutter. 

 

yes, that makes sense I think.

 

I'd comment that a "secret gutter" like this:

https://redland.co.uk/products/components/abutment-systems/side-abutment-grp-secret-gutter

 

Is not quite the same thing as a box gutter as shown in your sketch.

 

I think you've both been meaning a box gutter?

 

I'm now trying to figure out whether using a "secret gutter" can buy you a little bit more space, compared to conventional detail with soakers.

Edited December 16, 2020 by lineweight

[PeterW]

6 minutes ago, lineweight said:

 

I'd comment that a "secret gutter" like this:

https://redland.co.uk/products/components/abutment-systems/side-abutment-grp-secret-gutter

 

Is not quite the same thing as a box gutter as shown in your sketch.

 

I think you've both been meaning a box gutter?

 

No I’ve definitely been talking about a leadwork secret gutter, not a GRP one. 
 

Decent leadwork manual here

 

https://www.calderlead.co.uk/uploads/documentsearch/id4/Calder-GTGLW-210213.pdf

 

10 minutes ago, lineweight said:

I'm now trying to figure out whether using a "secret gutter" can buy you a little bit more space, compared to conventional detail with soakers.

It buys you a lot more as a standard set of soakers will need 120mm vertically above the tiles, which in that picture you don’t have, more if it’s cut and stepped. 
 

This has a secret gutter cut into the brickwork below the roofline as it was quicker and easier than stepped flashing - all done with a single length of lead replacement as it’s only an outbuilding. 
 

 

leadwork round the top is around a pillar but the remainder is below the roofline. Quick and simple and no leaks and allows the wall to remain as is. 

[lineweight]

Do you mean a variation of what the Calder guide calls a "secret gutter" recessed only to the level of the underside of the battens, but with one side turned up and chased into the brickwork?

 

Or do you mean more like what they call a "box gutter"?

 

I assume in the photo above, you've used something more like the box gutter, because it must be recessed well below the batten level in order for there to be no lead showing above the tiles.

 

My question there would be - doesn't this lose you the protection for the "splash zone" where the brickwork gets wet from water splashing off the tiles, and which is one of the purposes of the general principle of 150mm upstands?

[PeterW]

No I mean a secret gutter as per page 15 of the leadwork manual

 

 

Depending on how far down you can get (as per comment from @Temp about joists etc) you can essentially remove the soaker completely and do a single lap onto the verge. 
 

Essentially you want 150mm from the bottom of the gutter up to the bottom of the tiles of the roofline above as a minimum, and this is one way of achieving it. 

[lineweight]

Ah right, got you, sorry, I missed that page.

 

Details like that of course rely on the gutter never getting blocked.

 

I suppose the idea is that because it's covered over by the tiles, it's much less likely to get leaves and suchlike in it.

 

 

Edited December 16, 2020 by lineweight

[Temp]

3 hours ago, lineweight said:

I suppose the idea is that because it's covered over by the tiles, it's much less likely to get leaves and suchlike in it.

 

I'd be tempted to leave it open. I think less likely to block and easier to clean it did.   

[lineweight]

Thinking about this further - if you do it as a single lap, what is the logic that says there needs to be 150mm from gutter bottom to the underside of the tiles on the *higher* roof but not on the *lower* roof?

 

The "single steps with secret gutter" detail from the Calder leaflet shows much less than 150mm from the gutter bottom to the underside of the tiles visible there. That's in common with other "secret gutter" details where the roofs are at the same level on each side - it appears it's considered acceptable for the bottom of that gutter only to be about the depth of a tiling batten below the underside of the tiles on each side.

 

So it seems to me that the 150mm requirement then becomes redundant.

[makie]

3 hours ago, Temp said:

 

I'd be tempted to leave it open. I think less likely to block and easier to clean it did.   

 

That's how we do it in Scotland, looks tidy when done right also

[Temp]

6 hours ago, lineweight said:

Thinking about this further - if you do it as a single lap, what is the logic that says there needs to be 150mm from gutter bottom to the underside of the tiles on the *higher* roof but not on the *lower* roof?

 

Think I mentioned 150mm first, but I think that's normally the figure used when you have a wall at the eaves rather than at a verge. Can probably be less at a verge. 

Edited December 16, 2020 by Temp

[lineweight]

11 hours ago, Temp said:

 

Think I mentioned 150mm first, but I think that's normally the figure used when you have a wall at the eaves rather than at a verge. Can probably be less at a verge. 

 

I think at least 150mm would be needed at an eaves, where you'd probably have a near-flat gutter and where you are collecting water from a large area of roof.

 

Typical details suggest that less is considered necessary at abutments where it's running parallel to the slope of the roof. That I imagine is because the water will run away quickly on the steep slope, and it's not gathered from a large area of roof so won't be large in quantity. The only way for the water to build up depth would be if there was some kind of blockage (which is what would worry me with hidden gutters).

 

But my understanding is that the typical 150mm upstand is not just about preventing water from overtopping it - but to protect the lowest part of a wall from getting saturated from water splashing off a roof surface next to it. That would be an argument for taking any leadwork somewhat above the top of the tiles even if there's a dropped gutter. It becomes irrelevant if the lead runs all the way up a small upstand that is less than 150mm though.

 

So the main question for me is whether tucking lead in under a tiling undercloak is a conventional detail that's recognised as an adequate way of securing the lead.

Edited December 17, 2020 by lineweight