Insulating roof.

[Ian Stirling]

I have a roof. I suspect others may too, so am asking advice.

It's on top of an attic, and has tiles on top of a felt paper layer, attatched to rotting bits of wood.

The underlaying planks (I'm sure there is a technical term) are sound, but won't be for that much longer.

Hence, I plan at some time in the not-too distant future to remove all the tiles (section by section) the old bits of wood which the tiles are hung on, replace the tar-paper, with nice thick tar-paper, and then attatch more wood, and replace the tiles.

The only dodgy bit of the existing roof is the flashing at the edges, which is crap, and that at the bottom of the roof, the tarpaper actually goes up a bit and forms a valley against the lowest plank at the edge of the attic, as the brickwork has lots of debris on top of it. This has caused that plank to partially rot, so this will need replaced along the length.

At that time, I'm planning to add some exterior grade ply along the edge, to carry the tar-paper to the edge, in a nice smooth manner.

However, I was wondering about adding insulation at that time. What would be involved in adding 100mm of polystyrene sheets to under the tiles? Obviously, the flashing needs redone (but needs doing anyway), and tiles at the ridge need raised, and the gable-end needs 50-100mm added on (the attic beams don't quite come high enough to make the same angle as the gable end).

I was thinking of removing all the tiles, tar-paper, and bits of wood that are existing, and then tiling 100mm*8'*4' sheets of polystyrene all over the roof up to the ridge where it's mitred, using expanding polyurethane glue. Then, 4mm exterior grade plywood over the whole lot, followed by gluing battens on to hold the tiles, then replacing.

Access is awkward, as it's a bit high up, being a roof, but possible safely. I'm not in a conservation area, have no near neighbours who tiles will fall on, and the roof is not shared.

Is raising the ridge-line 10cm likely to need planning consent? The gable-ends are already 15cm higher than the ridge.

It's a fairly repetitive job, that involves working on a roof, exposed to the weather, and vulnerable to rain, but I don't see anything that I fundamentally don't know how to do. Thoughts?

[Pete C]

Hi,

Tar paper might act as a vapour barrier, leading to condensation underneath the roof and making the tile battens more likely to rot. You can get synthetic vapour permeable roofing membranes or sarking which will last much longer than felt paper, also use treated wood for the tile battens.

Be careful you don't block ventilation to the roof space, which would make condensation and rot more likely.

Sounds like you are trying to turn a tiled roof into a 'warm roof' which might not work. It's easier and cheaper to lay insulation on/between the rafters.

Falling off the roof too would be pretty dangerous to yourself.

Either you need the services of a roofing expert (I'm definately not one BTW) or to become one yourself.

It might just be better to get someone to do the job for you, but learn enough to judge whether they will do the job right or not. If you do a lot of DIY it might be possible, but if not it's not the easiest thing to start with.

cheers, Pete.

[Terry]

If you have rot it sounds as though the attic may not be sufficiently ventilated. I.e. moisture is getting bottled up/trapped in the attic/roof space and is condensing on the inside of the cool roof. Presto rot, mould etc. If you use anything at all on the inside of the roof it should be a 'permeable' material that will allow moisture to breathe out through it along with adequate ventilation. The other question is where the moisture is coming from? All heated/insulated living spaces should have a non-permeable vapour barrier on the warm side of the insulation. At an extreme some types of oil based paint can be used as a vapour barrier but are not ideal. BTW the recommenmndation in some jurisdictions is that there should be a MINIMUM of 3 sq. feet of venting, evenly distributed to permit cross ventilation of the roof space for every 1000 sq. feet of ceiling area. Are there any openings into the attic that allows warm and therefore humid air to get up there; or is there anything vented into the attic that should not be?

[Ian Stirling]

Nope. Most of the attic boards are sound. The problem isn't condensation, but water getting under defective/slipped tiles, running down the tar-paper, and pooling against the 'gutter' of tar-paper that's against the bottom board.

Unfortunately, not. It's plasterboard, with no barrier that I am aware of, just 150mm of loose glass-fiber, with boards on the top.

Is there special paint that can be used instead, on the insides of the house, and then painted over with normal magnolia/...

I really doubt that there is this much.

Would simply leaving the bottom board off the attic, and replacing with a shorter one, so there is a small gap work? (obviously there would have to be a carrier over this to keep the tar-paper off and provide a vent.)

If the roof was insulated with polystyrene or similar boards on the top, would this not greatly help?

There is only light mold on the inside of the timbers, they are fundamentally sound, but it should be fixed.

I don't really think so. There is a recent 3" hole, for cabling, but this is not a recent problem.

This house was rebuilt from an old stone cottage by a builder for his mother, who subsequently died. I suspect it was not built with longevity in mind.

Thanks everyone for the comments.

[Pete C]

Hi,

You definately want to avoid this kind of thing.

Not really worth worrying about condensation if it's not a problem now. You might get some if you replace the felt paper with something less permeable to water vapour though.

Difficult to know without seeing a picture. It might be well worth finding a local roofing specialist and paying them for an hour of advice, if you're going to spend a lot of time and money on this.

It's easier to add insulation above the ceiling if you're not using the loft space as a room.

cheers, Pete.

[Ian Stirling]

Probably wise, thanks.

The entire roof (at the very least one side) needs re-doing. So, all the slates/tar-paper are going to be up, and the bare boards are going to be available.

That was why I was wondering if it might not be a good idea to stick a layer of insulation in at that time, to allow the option of using the attic as a room, plus to increase the insulation of the whole house by the attic warming up a bit.

All that needs doing is relatively minor work round the edges...

If the attic was to then be converted into a room, there would be the option of also adding insulation inside, for really good insulation. (paying attention to proper ventilation to avoid condensation)

[Pete C]

It it possible to create a 'warm roof' with a tiled roof, but usually the insulation is put on the inside between the rafters so no space is lost. A roofing expert would be able to give all the pros and cons. Eg if you had old roof beams that you wanted exposed in the room then having the insulation on the outside would give you that.

cheers, Pete.