The short answer
Felt rots at the eaves because the edge of the roof is where water gathers, where the underlay is most exposed to sun and weather, and where it often sags into the gutter. Over time it cracks, goes brittle or turns to mush. The fix is to renew the underlay along the edge and fit an eaves tray, which carries water into the gutter. A new fascia is often done in the same visit.
Why the gutter line is the first place underlay gives up, and how a tray and a new fascia put it right.
Why felt rots at the gutter line
Eaves is the name for the bottom edge of a pitched roof, where tiles finish above the gutter. Underlay is meant to catch any water that slips past the tiles and lead it down into the gutter, which means it has to hang a little way over the fascia board. On many older roofs the felt simply stops at the fascia or droops into the gutter, and that is where trouble starts.
Drooping felt sits in damp, collects leaves and moss, and gets soaked by every shower. Sun and frost weaken the exposed edge, and a blocked or leaking gutter holds water against it. Over the seasons the felt cracks and turns brittle, and its lower edge can rot away entirely. You may see black or slumped felt poking out beneath the first course of tiles, or damp marks on the soffit and fascia.
Nobody looks at this part of the house, so the first sign is sometimes a stained ceiling at the outer wall, or soft timber at the rafter feet. Our answer on torn roof underlay covers tears elsewhere on the slope.
What happens if it is left
Rotten felt at the edge stops doing its job. Water reaching the underlay is no longer led into the gutter; it runs back under the tiles or behind the fascia. Rafter ends, the wall plate and the soffit all sit in that dampness, and timber that stays wet will eventually decay.
Ventilation can suffer too. Marley's reading of BS 5250 puts the air gap at the bottom of a cold roof at 25mm for a pitch of 15 degrees or less when the underlay is the older non-breathable kind, and 10mm for anything steeper. Slumped, rotten felt can choke that opening, so moist air stays in the loft and condenses on the timber. Our answer on loft condensation and roof ventilation goes into this.
Damage of this kind rarely heals or stays small. The longer it runs, the more of the roofline has to be replaced. A visit while only the felt is affected can mean a limited repair, and later the job widens to timber, fascia and gutters, each adding to the bill.
Eaves trays as the fix
An eaves tray is a shaped plastic strip that sits along the roof edge. It props the underlay up so it cannot sag, and guides any water that reaches it across the fascia and into the gutter. Fitted properly, it keeps the felt dry and stops it hanging in the gutter. Our gutters, soffits and fascias page covers the roofline it belongs to.
Fitting one means lifting the bottom rows of tiles to reach the batten line. The rotten felt is cut away to sound material, the tray goes on, new felt is lapped over it, and the tiles are relaid or replaced where cracked. With the lowest courses open, the roofer can also inspect rafter feet and wall plate for damp.
It is a modest job over a short length, but it needs access along the whole edge, so a tower or scaffold is common. Scaffold hire and VAT can swell a total, so see that both are named in any figure you are given. A firm price can only follow a visit that shows how far the damage runs along the edge, because the length of rotten felt is the main thing nobody can judge from the ground.
Tying it in with new fascias and gutters
Two boards matter here. The fascia runs across the ends of the rafters and holds the gutter, while the soffit closes the gap underneath the overhang. If the felt behind them has rotted, the fascia is often damp too, so it makes sense to deal with them together. With the gutter and fascia off, the roofer can see the rafter feet, fit the tray and felt, and then fix new boards and guttering, avoiding the cost of putting scaffold up twice.
Check that the new fascia is set so the tray overhangs the gutter and the gutter sits low enough to catch the water. Our pitched roofing page covers the tiled slope above.
Another point is the membrane itself. If the new underlay is a breathable type, check it suits the roof and its ventilation; our answer on breathable membrane versus roofing felt helps. We inspect the edge first, send you pictures of the felt and timbers we reach, and give one overall price on paper for the whole edge, including the new boards if you choose them.
More on this in Pitched Roofing Services in Crawley
Two more eaves questions
Can I see rotten eaves felt from the ground?
Sometimes. Look just below the lowest row of tiles for black, curling or hanging felt, and for stains on the fascia or soffit. A torch from the loft may show damp on the rafter feet. Many cases are only found when the first courses are lifted, so a roofer's survey with photos is how it is confirmed.
Do I have to replace the fascia when fitting an eaves tray?
Not always, but it is worth considering. If the fascia is sound and straight, the tray can go behind it. If the fascia is soft, split or stained, replacing it at the same time avoids paying for access twice. The survey should tell you which applies, and the written price will cover the chosen job.