Whatever roofing material you choose to ward off rain, snow and wind from your home, you are putting all of your eggs in one basket – if you do not add the failsafe measure of roof sheathing. In some cases, the sheathing is integral to the construction itself, as with felt or asphalt shingle roofs, where it acts as the underlay for the roofing materials. But it also has another role – it serves to drain water from the rafters or trusses, should a leak occur in the roofing material.
With felt roofs, there is no way around it: The sheathing must be made of some kind of solid material, like plywood or wooden planks or boards. The bituminous products can’t support themselves, so a strong underlayment is needed. With heavier roofing materials like slate or roofing tiles, however, the load is carried by battens, sometimes supported by an underlayment, but not always. Where the battens carry the roof material alone, there is no theoretical need for a solid underlayment or sheathing, but we still want something there for drainage – just in case.
At some point, the idea was born that quite soft or thin materials could be used for this purpose – like tarpaulins or rolls of plastic sheet. Why not – it is both light and cheap. Later came the invention of a permeable membrane, which permits humid air to escape through it – but in one direction only: Outward (if you install it right). Breathable roofing felt was born. Now, it seems, there is no longer a need for additional ventilation in the roof. The breathable membrane gets rid of humidity from inside, while keeping moisture from the outside – on the outside.
There are several problems with that mindset though:
- Breathable roofing felt can only stand alone on the task of ventilating your roof under very specific circumstances – of which there are several. In most cases, you will probably need vents in the eaves or the ridge anyway.
- This material is not the same as the tough top felt you put on the outside of a roof – it is more like a soft fabric. This means it can be torn, which happens when it is not tightened enough over the rafters and incoming wind can somehow cause it to flap and oscillate over the nails used to fix it with. In time, it will cut itself open over those nails. Sometimes it is even torn when it installed, due to a clumsy roofing service person.
- Some types of plastic and synthetic fabric tend to decay over time, when exposed to direct sunlight. This means, that if any sunlight can somehow shine through a tiny hole in the roof, the membrane it hits underneath, will develop a brittleness which eventually will become a leak in itself. There doesn’t even need to be an actual leak in the roof for this to occur – some older slate or tile roofs often let in a little sunlight, without letting water in.
Do I recommend against using breathable roofing felt? Yes and no. If you are repairing an existing breathable membrane, then it makes sense to continue down that path. But with a major re-roofing or even a completely new roof, it wouldn’t make sense to risk shortening the lifespan of it, by choosing inferior materials. By far the best solution I know of (and what is used here in Denmark, where I am located) is to use a solid underlayment with a heavier roofing felt on top (the kind you would use as bottom felt on a torch-on felt roof). Then, you install battens and tiles. Expensive? Only if you are short-sighted.
