Use of felt under tiles in pitched roofs.

[AL_n]

A layer of bitumous felt immediately under the tiles or slates on a pitched roof is normally found in houses.

Is this purely a precaution against rain ingress due to broken tiles or is there another reason for it?

The felt generally projects a few inches beyond the edge of the lowest course of tiles and ends within the reach of the guttering. However, this projecting portion typically perishes and didintegrates within about 20 years. Why don't they use something that lasts longer? Apart from softwood window frames, it's hard to think of any building material that is so short-lived.

Al

[Geoff Pearson]

In Scotland we have solid timber roofs which are covered in slates (with felt on the wood too). English roofs seem so flimsy.

[John Williamson]

It's there mainly to stop the wind blowing in and removing all the heat from the loft area as far as I know. When I had the roof done on one house I owned, putting felt in reduced heat loss quite a lot. I'd say it was worth about the same as an inch of rockwool.

The downside is that you need to make sure there's still enough controlled ventilation to keep things dry, or it encourages rot.

[Jon Fairbairn]

It also reduces movement due to wind. In Scotland wind is more of a problem and sarking is general wooden boards.

I think it?s not uncommon nowadays to use Tyvek sarking, which might last longer and is semi-permeable so that water won?t collect on the battens.

[NT]

also acts as a backup in case of tile/slate loss

NT

[Roberts]

also acts as a backup in case of tile/slate loss

NT

I once lived in a house which did not have any felt under the roof tiles. It was not a problem until it snowed and the snow drove in through the tiles and laid on the floor.The snow then melted and leaked through the ceiling making the bedding wet. After the first time we swept up the snow before it melted. This was not way up north but down south on the Isle of Wight. It was not our house but came with the job. Robbie

[Kipper at sea]

The bitumen felt is purely a vapour barrier against condensation ingress from the out roof covering. Bitumen felt replaced touching morter which was spread on to the back of the roof lath, slates or tiles this which stopped any ingress of dirt and wind.

There are special units called "over fascia felt support trays" Theses are fixed on top of the fascia board and under the bottom of the felt, the bottom of the tray hanges into the gutter. Unfortunatly the are not suitable on rose mary type tiles.