Should old roofing felt be removed when you re-roof?

Many American articles call this layer tar paper and talk about sheathing and asphalt. On a British pitched roof the layer is the underlay: a sheet that runs across the rafters, under the horizontal battens that hold the slates or tiles. Older houses often have bitumen-coated hessian felt, tacked on loosely. Roofs from the later twentieth century commonly have a black reinforced felt. Some Swindon terraces and Cotswold-edge cottages have no underlay at all, just tiles hung on battens, with the loft open to a driving storm.

Think of it as a second line of defence. Slates and tiles shed most of the rain, but wind can push water up and under the laps, and snow can blow in as powder. The underlay catches whatever gets through and carries it down to the gutter. It also keeps dust and draughts out of the loft, and it stops loose mortar and debris dropping onto stored belongings.

Bitumen felt stiffens with age. It dries out, cracks along the folds and splits where it was stretched over the rafter tops. Where it sagged between rafters, water pools and rots it from above. The nails that held it have usually rusted, so it flaps under the battens in a gale. Squirrels and rats use rolls of it as nesting material, and holes appear where pipes and cables were pulled through years ago.

Leaving the felt in place would be false economy. You would be paying for a new covering and putting it on a layer nobody can trust. The first time wind-driven rain gets under the tiles, the old felt lets it through, and the damp turns up in a bedroom ceiling years later with no obvious cause. There is also the matter of what you cannot see. Rotten battens and decayed rafter ends hide behind old felt, and stripping it is the quickest way to find them before they become a bigger bill.

Modern underlays come in two broad families. Non-breathable underlay, sometimes labelled HR, blocks moisture in both directions. Breathable membrane, often labelled LR, lets water vapour from the house pass through while keeping liquid water out. The choice is tied to ventilation. Marley's guidance, which cites BS 5250 on moisture in buildings, asks for 25 mm of eaves ventilation where the underlay is non-breathable and the pitch is 15 degrees or less, and 10 mm above that, with extra ventilation at high level on steeper or longer roofs.

Combine a breathable membrane with an airtight ceiling and tiles that let vapour escape, and the same guidance needs far less purpose-made ventilation. That is why a roofer should look at the loft, the eaves and the covering together, and not just pick a roll off the van. Warm, humid air from kitchens and bathrooms needs somewhere to go, and which underlay suits the roof depends on the answer. A loft packed with insulation right to the eaves can choke the airflow, so we check that as well, because a new underlay cannot cure condensation caused by blocked ventilation. Often the answer is a simple baffle at each rafter bay, which keeps a clear channel above the insulation.

Once the covering and underlay are away, the battens are exposed, and old ones are normally replaced too. They cost little compared with the labour of fixing a covering twice, and splitting or rot in old softwood is common. New battens are set to the gauge for the chosen tile or slate, which takes account of the headlap and the exposure. Eaves trays and tile vents keep the underlay draining into the gutter rather than tucking behind the fascia.

While the roof is open, the carpentry gets a proper look: rafter feet, wall plates, and the timbers around chimneys and valleys. Our page on replacing roof timbers explains what happens when we find rot or insect damage. Relaying tiles over old felt skips all of this, and that skipped inspection is the real cost of leaving it.

Flashings meet the underlay too. Where the roof abuts a wall or a chimney, the new membrane is lapped under the lead, so water running down the surface is steered out over the covering and never behind it.

The felt is removed in sections, never all at once. A slope is stripped, repaired where it needs it, and covered with new underlay and battens before the light goes, so the house stays closed up overnight. Even so, a wet week is the wrong time for it, and the programme is built around the forecast. The reasons are set out on our page about roofing in the rain.

Do not try to strip felt yourself from inside the loft. Old felt hides nails and loose tiles that can fall, and the work needs a scaffold and edge protection. If you are weighing it up, our note on doing a re-roof yourself explains the risks, and our timings page shows how the sequence runs. The whole job from scaffold to ridge is covered under roof replacement.

If the felt turns out to be the only fault, the job is sized to match. Photographs of your underlay and battens come with the written quote that follows the survey, and the workmanship carries a 10-year guarantee in writing.