Neighbour is doing an extension against my wall and installing lead flashing on the roof. Think the builders are just planning to use ordinary mortar not lime mortar (which the rest of the house has).

Would this cause issues? Or is it ok as it is not the whole house and 99%+ of the bricks would still have the breathable lime mortar.

If its on your property and there should no doubt be a party wall agreement in place then you get to decide what they use.
Personally – Lime or nothing.

Lime to avoid the bricks spalling over winter (which they may do with standard OPC mortar).

Lead mate mastic. Done properly with correct lead plugs/clips is a much better solution.

I’d insist on lime, if only because you want the wall restored to its original condition. Also a couple of hundred quid on a party wall surveyor may be money well spent too.

Is lead mate mastic breathable?

My old stone house has lead flashing joining a lower slate roof to the taller main wall. House is 1870s (including the lower section) so originally all lime mortar but seems likely the lower section was re-roofed in recent decades, and cement mortar was used where the lead was tucked into the main wall. When we moved in there were leaks everywhere in the lower roof section, eventually traced it to this line of mortar cracking and falling out. All replaced with lime (by me) and it’s been fine since. Obviously only a year or two so far but to cut my long story short the point is the cement mortar was rubbish in that situation. Lime has a bit more healing/flexibility properties so should do a better job, time will tell on that.

Lots of bits round the house have been patched with cement and I’m fixing it up slowly as we encounter leaks and holes!

Neither. You want something that can cope with the high thermal movement of the lead. There are various propitiatory sealants that would do the job better and last longer.

Jeff

Wrightyson +1
Much easier to repair/replace later too

When’s the work being done? If it’s now, it might be a bit cold for lime (depending on where you live) and it could potentially fail. Might this be why they’re not using it?

Just request leadmate or similar as recommended by myself and the others above who know what they are on about. One pointed joint down a flashing will make **** all difference to breathability in fact the actual lead up against the brickwork isn’t breathable is it? Time and a place for lime mortar and this ain’t it.

Todays piece of new knowledge, thanks Wrightyson. I’ll tuck that away for future use.

The lead itself isn’t breathable but it is only fixed along the top the bricks should be able to breathe out of the sides, especially on a slope where it drops a course every few bricks.

Think the problem isn’t the breathability of the whole house which wouldn’t be affected too much by a few bricks, but of the individual joint, the theory is as the lime absorbs moisture from inside it will be blocked from exiting the house by the cement pointing will try and pass into the bricks instead damaging the face of the brick when it freezes in winter or pushing the cement mortar out as witnessed by another poster on his house above.

How much moisture passes through an individual joint/brick from inside? Hey just do what you want, you clearly want lime mortar so ask them to put the **** in. Simples.

If mudmuncher refers to me I don’t know what pushed out the cement mortar (I suspect lead movement) could have done it) and if I was doing it again I’d be very tempted by the mastic which I didn’t know about at the time.

Yes lime is more breathable than cement mortar (particularly when the roofers use 1:4 cement: sand mixes). The purpose of a lead flashing is to keep water out. The main reason they fail is because they aren’t dressed far enough into the mortar joint (25mm is a minimum if the bricks are porous or exposed locations 35 of 45mm might be more appropriate) – the idea is to intercept water soaking into the brick during a storm to direct it back out of the wall. In places like chimneys this might not be enough and you need a top hat dpc in addition to the flashing.

People get hung up on lime, if you ask a builder or roofer to mix and place lime mortar without prior experience they will almost certainly mix it too wet and won’t tend it as it cures, which will then crack and fall out. I’ve worked with a number of the people who wrote the Historic England text books on mortar, leadwork conservation etc. At least one of those recommends that you chat out joints for flashings to 40mm fix the flashing and then seal the back of the joint with leadmate then post the front of the joint with lime to keep everyone happy.

The pointing to flashings is a nightmare, particularly on the southern side of buildings, even when it is done by people who know what they are doing. Personally if it was my house and it wasn’t listed I would for leadmate every time.

If you do go for lime mortar and you want to do it properly cover the lead which is dressed into the joint and which will be in contact with the lime with a later of masking tape. The free lime in a wet mortar can attack the lead, it won’t do much damage but it can roughen up the surface enough for the lead and mortar to stick together, which can make the mortar more likely to crack as the lead expands and contracts. Its not proven but I trust the experience and skill of the contractors who swear by it.

If you want to be really on trend, use hot mixed lime (according to the true acolytes hot mixed lime cures all problems, including world poverty).