The short answer
Either can be right. Mortar is the traditional choice, especially on old lime-built stone and brick, because it matches the wall and can be made to the same mix. A flexible sealant suits modern walls and awkward joints, and where a mortar joint would keep cracking. In both cases the lead should be wedged into the chase first, so the pointing seals and does not carry the weight.
What the top edge of a flashing does
A flashing runs down from the wall over the roof covering, and its lower edge lies on the slates or tiles. At the wall end it does something different: the lead is tucked into a slot raked out of a mortar joint, called the chase or reglet, and secured in that slot. If that top edge lets go, rain runs behind the lead and the flashing is no use at all.
Pointing is the filling that closes the chase once the lead is in. It keeps water out of the groove and stops the lead from slipping. It does not do the main job of holding the lead; the wedges do. Treat pointing as the weather seal and the wedges as the grip.
That division matters because it explains most failures. Lead that has been held only by pointing drops when the pointing perishes. Lead held by wedges stays put, and the pointing can be renewed without disturbing it.
Mortar pointing: when it is the right choice
Mortar matches the wall. On a soft stone or old brick building, a lime-based mix gives a joint that moves and breathes at much the same rate as the masonry around it. A hard cement mortar, by contrast, can crack away from the softer stone and trap water behind it.
Old and listed buildings
For a listed or period building the mix is part of the building's character, and mortar is the like-for-like repair. Changes to a listed house need the right consent, so ask the conservation department at the council before you change the style of pointing. Stroud District Council's guidance gives legal protection to the entire listed building, whatever its grade.
What goes wrong with mortar
Mortar is rigid. Once it has set, a gap between the wall and the lead can open as the roof and the masonry settle at different speeds, and water then runs into the chase. It also shrinks as it dries, so a joint packed too wet can leave a hairline at the lead.
The right sequence is to rake out the old mortar, wedge the lead, then fill in layers and finish flush or slightly recessed so water sheds off rather than pools. Mortar needs mild, damp-free weather to cure well, and fresh work is protected from hard frost and from strong sun, which is one reason a pointing job may be timed around the forecast.
Sealant: where it earns its place
A flexible sealant made for the purpose stays elastic as the joint opens and closes, and that suits places where movement is expected: a long run on a modern wall, a junction between lead and a metal or timber component, or a chase that has been repaired before and keeps cracking.
Sealant is usually quicker to apply and is useful when the weather is not right for mortar. It has drawbacks, though. It is a surface seal with a finite life, and it does not match the character of an old stone wall. A thick bead smeared over a failed joint can hide a loose flashing for a season or two.
We keep sealant for where it suits the wall, not as a shortcut over a failed joint. Our answer on repairs, in the guide to stepped, cover and apron flashings, shows how a full repair is planned from the lead down to the pointing.
Wedges, lengths and getting the sequence right
Lead wedges, small folded strips of lead, are tapped into the chase at intervals to hold the flashing against the back of the groove. They are made of lead so that nothing in the joint reacts with the metal. Only then is the chase pointed.
The length of each piece of flashing affects the pointing, too. MyJobQuote gives the correct code in 1.5 metre lengths as the guide. A longer run moves more and puts the pointing under more strain, which is a common reason a joint opens up. Our answer on why lead is fitted in short lengths explains the movement.
Code matters at the top edge as well. Leadworx lists Code 4 as its everyday flashing sheet. A thin sheet in the chase will fatigue where it turns, whatever is put on top.
Deciding between them, and what to expect on the quote
The deciding factors are the wall, the joint and the history. Old lime wall, listed building, joints that have always been mortar: use mortar. Modern wall, long run, a joint that has cracked more than once: a flexible sealant may last longer. Where we are unsure, we tell you why, with photographs of the chase from the survey.
Nothing in this answer is a promise about how long any joint will last, as that depends on the wall and the weather. When cover is stolen, as can happen with leadwork, the chase is left open and needs urgent attention. See the answer on stolen lead and insurance for how that is treated.
Pointing is a small line on a quote. MyJobQuote puts a chimney flashing patch at £150 to £250, which gives a sense of scale. Your itemised quote shows the lead, the wedging and the pointing separately, and the guarantee on the work is our 10-year workmanship one, given in writing. Start at lead roofing.