“Roofline” is the catch-all word for the boards and pipework around the bottom edge of your roof. It is one of those areas where quotes list a lot of terminology and nobody stops to say what any of it means. So:

Fascia

The board running horizontally along the ends of the roof timbers, facing outwards. It is what your guttering is screwed to, which means when a fascia rots, the gutter above it starts to sag and pull away.

Soffit

The board tucked underneath, on the horizontal, closing off the gap between the fascia and the wall. Look up from directly below the gutter and that is what you are looking at. Soffits are usually vented, and those vents are how your loft breathes – block them and you get condensation problems.

Bargeboard

The same idea as a fascia, but on the sloping edge at a gable end rather than the horizontal edge.

Why people replace them

Old timber roofline needs painting every few years, and once water gets past the paint it rots from behind where you cannot see it. uPVC does not rot and does not need painting. That is the whole argument, and it is a reasonable one.

What we would add: replacing roofline is a good moment to look at what is behind it, because it is the only time those timbers are exposed. If the rafter feet are wet, you want to know now rather than in five years.

One thing worth asking

Some installers fit new uPVC straight over the existing rotten timber – it is called capping over, and it is quicker and cheaper. It also seals the rot in. We strip the old boards off. It costs more and it is the right way to do it.