breathable or non-breathable membrane for clay double romans?

[EdStan]

Hi, I'm new to the forum...thanks in advance for any feedback and advice.

We recently bought a 1903 non-listed 3-story victorian house in the Bristol area. We knew when we bought that the soffits and fascias needed TLC, but the roof looked fine for a while (has had a few fixes over the years). It's got clay double romans with a bitumen underlay, and I'm guessing this was laid around 1950-1960, based on the underlay packaging we found and the many old slate tiles in the voids (we assume it was welsh slate initially, but most people on the street have cement). On exposing the voids internally to prepare to insulate the top rooms, the lower areas of the roof have felt that getting brittle and some holes. We also found some areas of rot which can be fixed from inside if needed (the roof is mostly dry), and condensation is present on the felt in the top of the roof void.

I've had a couple of roofers over for advice. Two of these recommended new tiles with breathable membrane, however another who appears to work on more period properties, suggested we'd be mad to lose the old clay tiles, and that breathable membrane is not right for double romans. He even suggested we replace an area that has had breathable membrane used to non-breathable. He would use F1 bitumen underlay (like we have) if we wanted the roof relayed, however he was suggesting it wasn't an absolute need to relay now...unlike the others. As others have suggested here on posts I've read, the cost of the scaffold to fix soffits and gutters may make it worth insulating from the outside, relaying the roof and I'd get a 15 year insurance guarantee. I like this alternative approach, however I cannot seem to find much to back up the use of non-breathable membranes. His explanation is that double romans are designed to be breathable themselves and allow ventilation, and then vents are added to ventilate the internal side of the felt.

I've attached some photos. It's worth noting that I'm probably sharing the poorest tiles here, as they are in a shaded north west facing front. the ones at the back appear to be much better

Hope to get some advice. Thanks

 

[andyrew1]

Welcome to the forum from a fellow Bristol Victorian homeowner.

This is purely my anecdotal experience, I'm no expert.

We have the same clay tiles, when we moved in the roof had breathable on the front and bitumen membrane on the back.

We had the back replaced with breathable membrane about 2 years ago, I haven't noticed any issues since.

Logically speaking I can't see a reason not to use breathable membrane with clay tiles. If there was an issue with water vapour behind the tiles I would have thought that would be worse with concrete tiles that fit together much more snuggly.

We have a cold roof setup, it looks like you might be in a warm roof setup, maybe this makes a difference...

 

[EdStan]

Thanks for the reply. Yes, we have a warm roof, and we intend to insulate the top rooms further, which will create a higher level of vapour barrier. From reading up, it doesn’t seem that breathable is always necessary and the right choice, and non-breathable are becoming more common again. We’ll keep everything well ventilated both internally and in the roof, so I really am questioning what’s the right membrane (and if it’s worth investing £20k to get a new roof covering)

 

[Stevers]

We have a slate roof replaced in 1997. For that the builder used a trad bitumen felt except at the eaves where he used a plastic membrane that wasn't breathable either. We vented the eaves and ridge, but have only a very nominal 100mm of fibreglass insulation. No problems with condensation, but our bathrooms both have Passivents ducted up onto the roof, virtually eliminating them as a source of moisture. Recently I have had scaffolding in for me to re-slate both verges, and it struck me how brittle the bitumen felt had become, although it still seems to be doing it's job. The plastic membrane along the eaves is very flappy in high winds, but has lasted better at the eaves than the bitumen felt that I fitted on my porch a few years later. We can barely stand up under the ridge, so the loft will only ever be used for storage, but we do have water tanks up there to consider when we add insulation. Both eaves and verges of my roof would have benefited with much better detailing back in 1997, but no regrets about the roofing felts chosen.