Porch flashing against stonework

[Jackswan]

I am presently renewing a flat roof on top of a porch. This is against a random stone wall.
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The original flashing is only 50mm above the roof and the felt had no upstand below it. It was only 50mm above for 3/4 of the way across, then the flashing took a 90 degree turn down to follow the mortar joint below a large stone (the one with the red mark on it)

The original flashing can be seen in this picture with the larger stone on the left. temporary flashing tape has been applied to cure any short term leaks. the original flashing enters the mortar joint where the crease in the flashing tape is. The larger stone block can be seen with the red mark on it.

I am just wondering where to put new flashing in by. The new felt is recommended to have an upstand of 150mm and the only continuous mortar joint above that is the one below the window ledge which is 280mm above the roof.

The only other option would be to cut a slot (25mm depth)through that stone with the red mark and continue the mortar joint across level with the top of the red mark. But I am reluctant to do that as I don't know if it would cause a problem in the future with that stone(cause it to crack?)

I think that it would be better to fit the flashing in under the window ledge, but would this be ok. It would just be the sheer size of it then as I would probably need to use 240mm width flashing.

I would be grateful for any suggestions.

 

[Jackswan]

No suggestions yet?

 

[middi]

I would not install the lead any higher than absolutely necessary, the higher you fit it the more unsightly it will look.

I would settle for the lowest position possible but not lower than 100mm from the roof level ( 150mm or higher is normally recommended), I believe if the new lead is fixed correctly in a groove at least 25mm deep and well pointed you should have no problem on this compact roof.

Too much lead showing will look silly.

If you have to cut through the odd stone because it does not follow an existing mortar line, so be it. An angle grinder should make easy work of it with no danger of the stone suffering any further damage.

 

[Jackswan]

Thanks for the reply.

If I fixed the flashing at 150mm above the roof, that would run along the mortar line just to the right of the top of that red mark. This would mean cutting a slot into a stone block at either side.

I am just worried whether this would cause a weakness in those stones in the future, even though this slot will be filled with the lead flashing and a weak mortar mix.

I agree that if the flashing was fixed into the mortar line immediately below the window ledge would look unsightly as this would mean an upstand of something like 240mm high.

One other thing, there will be a 75 x 75mm fillet fixed on to the roof to allow the felt to take a nice rise up to the stone. Just wondering due to expansion and contraction of the roof would this affect the gap where the felt crosses from the fillet on to the stonework. I am using a 2 layer felt, Marley Rapidseal

 

[Jackswan]

One other question, how far should the flashing extend past either side of the roof?

thanks
Julian

 

[Lime]

I would not cut the stone.
Once done it can never be undone.
The porch may change in the future.
I think, if this is a balance between damaging the structure of the house or looking at more lead than I want to, I'd look at more lead.

As an alternative I'd build the roof up so that it is no longer flat but sloping downwards from the wall a bit higher up.
then you could use the line of pointing that runs under the window.
The roof with a bit of fall on it will be a better roof anyway.
Do it in lead all over and with a few expansion scrolls on the roof's change of direction it would look good in it's own right.
It would complement the porch quite well.

 

[middi]

Lime has made a valid point, but my theory is that if in future generations someone hates the porch and the lead and decides to remove it ( if they will be allowed), it wouldn't be too difficult to replace or restore the grooves in the two stones. It may even leave evidence of previous structures erected to the elevation of the building. A good thing.

Meantime you do not have to stare at loads of lead flashing.

I would also keep the protruding lead across the roof not bigger than 100mm, less if you can get away with it and still end up with a good seal.

The expansion factor is important, even on that small roof , nevertheless, if you allow for a flxible seal against the wall with the lead flushing only covering the drop of the main roof covering, it should be fine.

If you are too concerned about cutting into the stione, you can certainly get a professional to make a bespoke feature of the leadwork itself.

It is a matter of choise and expense.

 

[Lime]

If you are too concerned about cutting into the stione, you can certainly get a professional to make a bespoke feature of the leadwork itself.

It is the fixing and finish that makes all the difference to good lead work.

 

[chuckey]

I have been initiated into leadwork, so I am looking at your job with the eye of a novice. I think that the biggest problem to make it look neat, is the internal corner where the lead, overlapping the width of the roof on the wall, meets the lead that is running either along the roof or down the slope of your fillet. I would make a template or mould and fold and dress the lead to shape first, put it on a plank (to stop it drooping and going out of shape) and then just wedge it in your chosen groove. It is imposible to dress the lead neatly against those ragged stones, because half the time you will want to get in behind it to sharpen up the bends.
The best tool I found for spreading the lead was a tiny* little rubber and plastic faced mallet bought of a market stall for £2, unscrew the rubber and plastic faces and use the side of the head. The conventional leadworkers wooden "thing" can be replicated with 12" of 4" X 2", for flattening sheets.
Frank
* face to face of head = 2 1/2"?

 

[geoff]

I also have random stone walls with a porch, but the porch roof is pointy - c 30 degree apex. I cut slots on each side of the porch roof using an angle grinder with diamond stone cutting blade and used a length of lead for each side, shaping and overlapping them at the top. The lead comes in various widths from roofing merchants - I used about 12" wide, about 6in on the roof and wall. I now think this is wider than necessary, a bit less width would probably look neater on a small porch.

I inserted the lengths of lead in the slots and used small folded lead wedges to keep the flashing in place, shaping and overlapping the strips at the apex to give a neat join. I used a rubber mallet and bits of batten to dress the lead into place - I sanded one of the battens I used to a more rounded edge shape to avoid cutting into the lead or stressing it too much in the finishing (you can buy proper tools but for one job I didn't bother). I wrapped the lead around the slates at the bottom of the roof. You obviously have a flat roof, but I would still dress it right round the edges.

I used a tube of 'lead mate' in a gun instead of mortar or other mastic to seal the flashing in the slot and to seal the overlap between the lengths at the apex. I sealed the edge of the lead on the slates with it too. I used some patination oil on the lead after completion to get a nice finish.

It can be quite fiddly to get the lead dressed into place without it popping out of the cut slots, but persevere and you should manage. Lots of cursing will help. Lots of cursing will be inevitable....

From your photo, your main problem may be that the stones are quite small - my wall is quite large rubble stone, which makes slotting an easier and less noticeable approach. If the stones are small, they may fracture more easily and the flashing may seem more prominent, if you see what I mean.

I did this about 5 years ago, and it is all still in place and neither lead nor lead mate sealant has moved or split - it is still as good as new.

 

[Gervase]

You can get somevery handy stainless steel dog clips that you use to keep the lead in the slot in the masonry. You just knock them in with a tack hammer and they hold the flashing very snugly. Once you've used them you'll never want to faff around with anything else again.
They're called Hall Clips, come in packs of 50 and if you can't source them from your local builders' merchant, can be had from PO Box 492, Enfield, EN3 7ZW.