The fascia is the board across the ends of the rafters that carries the gutter. The soffit is the panel closing the gap underneath, between that board and the wall. Together they finish the eaves, keep weather and birds out of the roof space, and in most modern houses they also carry the ventilation that stops the loft becoming damp. They are treated as decoration and they are not.
What each board is doing
The fascia takes the gutter and its load, which on a long run full of water is not trivial, and it closes the ends of the rafters against weather. The soffit seals the underside and is usually where eaves ventilation enters, either through a continuous vent strip or through individual soffit vents. The bargeboard does the equivalent job on a gable end. If the soffit is solid with no ventilation and the loft is insulated at ceiling level, that is worth investigating.
How they fail
Timber fails by rot, usually starting where a leaking gutter joint has been dripping down the face for years, or at the ends where the board meets a wall. Plastic fails differently: it does not rot, but it chalks and discolours in sunlight, and the fixings or the timber behind it can still be failing while the surface looks acceptable. A plastic fascia fitted over rotten timber, which was common practice for a while, hides the problem rather than solving it.
Repair, overclad or replace
Sound timber with failed paint is a painting job. Localised rot in an otherwise good board can be cut out and spliced. Widespread rot, or rafter ends that have gone soft, means replacement, and that is the point at which the rafter ends themselves need looking at because they are structural. Overcladding existing timber with plastic is quick and cheap and is only honest when the timber underneath has been inspected and is sound.
Painting still has a place
On a period property where plastic would look wrong, and on sound timber generally, keeping the boards painted is the cheapest way to make them last. The work is preparation rather than paint: cutting out any soft timber, treating, priming bare wood including the back edges and ends where water sits, and dealing with whatever gutter fault caused the problem. Painting over damp timber locks the moisture in.
Questions people ask about fascias and soffits
Do fascias and soffits affect loft ventilation?
Very often yes. Eaves ventilation usually enters through the soffit, so replacing ventilated soffits with sealed ones, or blocking the path with insulation, can cause condensation in the roof space. Ask what ventilation provision is included.
How do I know if the timber behind plastic boards is sound?
You generally cannot from the ground. A contractor should be able to tell you after getting up there, and reputable ones will say if they are overcladding rather than replacing. Soft spots, sagging gutter lines and staining on the wall below are clues.
Is a rotten fascia a structural problem?
The fascia itself is not structural, but rot in a fascia usually means prolonged water where it should not be, and the rafter ends behind it are structural. That is the reason to look rather than to cover it up.