Flashing and felt upstand against stonework

[Jackswan]

I am looking for some advice on how to fit a felt upstand and lead flashing against a stone wall.

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The original flashing is shown following the mortar line of the blocks in the middle, then it goes under the larger blocks at the end. I have attached some temporary stick on flashing to seal it at the moment, which can be seen.

The lead flashing presently fitted is only 80mm going down to almost level with the outer blocks above the roof and had no felt upstand underneath, due to it following the original mortar joints.

I am just wondering what is the best way to fit the felt upstand and where to fit new lead flashing (height wise)

Just to get a perspective of measurements.
Height to underside of window ledge from roof is 280mm
Three coarses of stone blocks beneath window, each at a height of approx 90mm.
The larger stone blocks on either side of the window are the height of the three coarses.
The mortar coarse is continuous above and below the larger blocks.

from info that I have gained the felt and flashing should be about 150mm above the roof but this would mean cutting a slot in the larger stones (reluctant to do this, would they later crack?)

Any advice would be much appreciated.

Julian

 

[Nige F]

Tricky one ....but I`d be inclined to put the lead directly under the cill and maybe even render the wall up to cill first to get a smooth surface to work to.......not many like that here in Sussex

 

[^woody^]

I would use any joint nearest to 150mm from the finished roof surface. And if you need to, cut a joint in those double course stones. Stones are relatively soft, so it is easy to do, and no it will not crack. Cut in 30mm by 10mm high

But it looks like the joint under the cill may be the best option - depending on if this looks like 'too much lead' when you look at the elevation from the ground.

What it will look like after is important, and worth a little effort now if needed

I would also render the stonework behind where the lead will go to give the flashing a nice smooth surface, so that it can be dressed wrinkle-free.

 

[Jackswan]

Thanks everyone for your comments.

I am going for the mortar line below the window ledge as I would rather not cut into the stone.

Next dilemma is the size of flashing, 150mm or 240mm. The window ledge is 240mm above the roof and not 280mm as I originally stated.
If I use the 150mm that will leave 115mm of the capsheet upstand showing taking into account 25mm tucked into the mortar joint.
If I use 240mm that will almost cover the upstand and will end up half way down the 75 x 75mm triangular fillet tucked in at the abutment to the wall.

I am just thinking about where the felt leaves the triangular fillet and attaches to the wall. Would this be a weak point in a few years time and possibly crack the felt? Would it be better to cover this joint or leave the flashing above it? Hope that you understand where I am going.

Julian

 

[^woody^]

The lead should overlap the felt upstand by at least 100mm, the felt upstand should be 150mm. Its function is just to cover the joint into the wall (where felt may crack and displace) and not to protect the felt upstand or fillet.

The lead should finish just above the fillet, so you will need to cut the wider lead to suit.

And don't forget to use the lead in 1.5m strips and overlap by 100 mm minimum

 

[Jackswan]

Thanks

If I understand correctly what you are saying is that no part of the lead should overlap the uppermost part of the 75 x 75mm fillet where the felt leaves the wood and comes into contact with the stone.
I have done a quick sketch

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I have put the positions where the lead will come to if using either 150mm or 240mm.
What you are saying is that the lead flashing if I used 240mm should end above where I have marked "Point A". I would have thought that "Point A" would have been the weakest point, if the deck expanded or contracted as the wooden fillet is screwed to the deck.

Looking to fix the flashing in tomorrow if the rain stops

 

[masona]

Do you really need to go that high? Could you not cut into the mortar line which I can see approximately 100mm from the flat roof?

 

[Jackswan]

Surely there would be capillary action between the lead and the capsheet if the upstand was so low? Water would surely rise into the joint?

 

[masona]

I wouldn't have thought if you put the lead flashing cut into the mortar joint and then use sealant?

I suppose you could use a small square/angle timber on the angle fillet and shape the lead over it to act as a drip bar?