The short answer
A new roof is joined to an old one at a junction, and each kind has its own detail. A valley carries water where two slopes run together. An abutment, the point where a slope butts against a wall, is sealed with lead flashing. Underlay and battens are lapped so water runs over every joint, and the covering is matched to the old.
The join between new work and old is where an extension roof is tested hardest. Getting the junctions right matters more than the new slope itself.
Where new meets old: the types of junction
When a house is extended, the new roof can join the old one in three ways, and a given job often has more than one.
What is a valley, and what is an abutment?
A valley is the inside corner formed where two slopes run together, such as where a rear extension roof runs into the main slope. It collects water from both sides and carries it to the gutter. An abutment is any spot where a slope runs up against a taller wall, such as the side of a chimney or the wall of the main house above a flat-topped single-storey extension. A third junction is the ridge or hip, the external angles, where the new roof's high line is brought up to meet the old one.
Before any of these are formed, the old roof is opened up at the line of the join. Covering is stripped back, and the timbers behind are checked, because a junction is only as sound as the rafters it is fixed to. Our survey photos show the line where the new work will meet the old, before a price is agreed.
Flashings, soakers and lead codes
Most junctions are made watertight with lead. Flashing is lead sheet worked into the wall and over the covering, and a soaker is a small lead piece slipped beneath the covering at the wall and turned up its face. Together they let the covering lap over the lead and the lead lap behind the masonry.
Which lead code suits which junction?
Lead sheet is sold by code number, and the code sets the thickness. The cast-lead brochure from Royston Lead gives Code 3 (1.32mm) is for soakers only, Code 4 (1.80mm) suits soakers, flashings, valley gutters and dormer work, and Code 5 (2.24mm) adds parapet and box gutters. Heavier codes, 6 to 8, suit gutters and larger areas. A common choice for a standard flashing is Code 4 or 5, and the roofer decides after seeing the length, exposure and the wall behind.
Lead "can last for over 80 years" (MyJobQuote, Sep 2026). That is a lower bound, not a promise, and it holds only if the lead is fixed correctly with room to move.
Matching tiles and keeping the lines straight
New and old coverings must meet without a step, so the battens on the new roof are set to line up with the courses of the old. Gauge, the distance between batten lines, is taken from the existing roof and carried across.
Can I match my old tiles?
Sometimes. If the old roof is double Roman, plain clay or natural slate, a new product of the same profile and size is usually available. Expect a colour difference to begin with, and reclaimed tiles can be used on the face the street sees. Pitch matters too: Marley's smooth Double Roman needs a pitch of at least 22.5 degrees, so a shallow extension roof may need a different covering. Always check the product data sheet.
Changing the material across a whole roof is a different matter and a planning one. Class C of the permitted development order normally covers like-for-like re-roofing, and in a conservation area or on a listed building, put the question to Somerset Council's planning team before you commit. Our answer on changing slate to tiles says more about that.
Regulations, sequence and what to ask your builder
An extension roof is new building work, and it needs a Building Regulations application with your building control body. The roof's tie-in to the existing structure forms part of it, with checks on the rafters, the bearing and the insulation. Somerset Building Control is on 0300 303 7790.
In what order is the join made?
The sequence for a junction is: structure checked, underlay lapped over the old, battens aligned, lead flashings and soakers fixed as the covering rises, then the covering is finished against the lead. Doing it out of order, covering first and lead afterwards, is how leaks are built in, and it is why we agree the order with the builder before the first tile goes on. Our roof replacement page explains how a whole roof is planned, so the junctions are decided early.
If a builder is making the extension and a roofer is covering it, ask who is responsible for the join. Ask also who supplies the lead and what code it is, how the valley is formed, and whether the old roof around the join will be stripped back and rechecked. Our guide to leadwork during a re-roof sets out the lead details further. After the survey we give one written price that states the junctions covered.
Junction details
Why does a leak often appear where an extension meets the house?
Because that line is where two structures, built at different times, move at different rates. A flashing that is too short, a soaker missed or a valley not lapped properly can let water in. The fix is to open the junction up and check each lap, not to smear sealant over it.
Can I use sealant instead of lead flashing?
Sealant can be used as a temporary patch, but it cracks and peels as the roof moves in sun and frost. Lead is dressed to the wall and the covering, and it flexes with them. For a lasting joint the lead work is the right detail, with sealant at most as a finishing touch.
The full picture: Roof replacement and new roofs in Taunton ยท More short answers