Is our roofer mad? Raise roof height to fit insulation?

[thompster]

Hi experts,

I've been getting quotes on a replacement roof on a 30's semi and wanted to check something I've been told.

We have 4" joists and ceilings that are vaulted round the edge. To meet building regs and stop condensation we need Celotex between joists in the eaves. All ok so far.

Here's the question: one roofer says that we need to have 100mm insulation which then means battening all joists on the outside to lift the roof height and maintain a 50mm air gap. The other we've spoken to says 50mm insulation is fine.

My preference would be to leave the height as is, since otherwise we end up with a joint between us and next door. So - will building control really expect the roof height to be raised or do the regs allow make allowances for older houses that can't meet U values without this kind of work.

Thanks,
James.

 

[noseall]

The last raked ceiling we insulated had 100mm between the rafters and 70mm across, on the plasterboard side.

Our rafters were 150mm deep so they could accommodate the insulation plus air gap. However, you would be expected to add battens to the underside of the rafters if they were not deep enough, in order to create space for an air gap.

 

[pinenot]

Unless your doing any building works i.e.changing the height of your half of the shared roof, the Building control will have no interest, what's being sought here best practices. Put simply you want insulation, but you don't want condensation to be established.

There are two solutions I would offer here, both using Multifoil insulation (a high performance, thin insulation available from a number of merchants)

Fig1 - assumes you can apply the insulation to the face of the existing ceiling, from inside the room in question View media item 60919
Fig2 - assumes the opposite i.e. if the existing ceiling has irreplaceable features, or if you simply prefer not to damage it View media item 60924
Both will provide the relative insulation you require and retain the current air flow, if fitted correctly to the manufacturers instruction...pinenot

 

[tony1851]

Why not just put 50mm board between the bukhead rafters (so maintaining an air space) and 50mm below, and then plasterboard?

Don't bother overdoing it if headroom is a problem because the regs are flexible if the full requirement for insulation is impractical.

 

[thompster]

Thanks all for the helpful comments. I'm pretty certain a complete like for like re-roof does need building control sign-off, though I suspect that stage is often skipped.

I've seen this roofer's work and it's very good. The trouble is our neighbor wants to use a different roofer who is their family acquaintance. They're going to use thinner insulation board and leave the roof height as-is, leaving us with a step between our half and their half plus the additional cost.

My impression of the insulation regs was that it's very much 'where practical'. The question is whether building control would consider battoning everything with 50mm timber practical or over the top, And if its the latter, if they'd accept 50mm between rafters or make me stick an extra 50mm underneath.

Of course I could just go with another roofer, but this lot were our first choice.

 

[tony1851]

Re-roofing with a material of similar weight does not require a building regs submission.
What is this business of raising the roof? It seems crazy and is asking for troube; just put insulation between and below the rafters.

 

[thompster]

Hi Tony,

According to Planning Portal, replacing less than 25% of a roof doesn't need approval but 'You are replacing/ repairing more than 25 per cent of the roof area, in which case, the roof thermal insulation would normally have to be improved.' So does that mean the roof itself doesn't need sign-off but I'm supposed to get just my insulation inspected/signed off?

I'm glad it's not just me that thinks the 50mm baton thing sounds excessive, but not begin a roofer I wasn't sure if that was normal. I'm guessing not...

 

[noseall]

50mm roofing battens?

19mm or 25mm is a typical thickness roofing batten or lath.

 

[thompster]

Sorry, maybe I wasn't clear about the battening. The 50mm timber will be fixed onto the outer surface of all rafters to increase the thickness of the rafters from 100mm to 150mm. The reason given is so that they will accommodate 100mm insulation plus a 50mm air gap in the eaves.

 

[dhutch]

Why not just put 50mm board between the bukhead rafters (so maintaining an air space) and 50mm below, and then plasterboard?

This is what ive just done in all of my rooms, where this is a 2ft of sloping ceiling.

Large hammer, plasterboard off. Then cut to fit between the rafter, two peices per space, one poking down to the top of the wall, one up into the attic space which also holds back the glasswool on top. Spray foam all round for airtightness, one board over the top, sprayfoam, then I put up a polythene vapour barrier over the lot, plasterboard, and skim and paint.

Just done the painting of last of three bedrooms, doing each one at a time, I can rip and get it all back up in a weekend, or a week of evenings, including boarding the flat section of the ceiling while I was at is as this was also shot.



Daniel

 

[tony1851]

thompster;

The thermal regs don't apply unless your room is in the roof. I suspect that you only have a small sloping bulkhead in your bedroom, so that rule won't apply.

Don't put battens on the upper surface of the rafters! You only need to ensure adequate fibreglass over the flat ceiling.

Whatever your arrangement, building control would not expect you to raise the level of the roof just to put extra insulation in.

Just re-roof at the same level; put some insuation in the bulkhead from below, and don't bother advising building control.

 

[thompster]

Thanks Tony. It seems odd to me too. The schedule of work has come from a reputable local firm who I would have thought would have known what they were doing.

I'll have a chat with them and ask why they think its necessary go to this length.

Cheers,
James

 

[^woody^]

Unless your doing any building works i.e.changing the height of your half of the shared roof, the Building control will have no interest,

Jeeezus, when will you start posting correct information?

This work is notifiable under Part L1b

 

[^woody^]

Here's the question: one roofer says that we need to have 100mm insulation which then means battening all joists on the outside to lift the roof height and maintain a 50mm air gap. The other we've spoken to says 50mm insulation is fine.

The applicable building regs require you to "consider" insulating as part of the re-roofing. They do not state a thickness, however you would be expected to try and insulate as much as possible.

With this type of job you could insulate the sloping part with as much celotex as possible so as to leave a minimum 25mm gap under the felt. Depending on the state of the plaster snots on the back of the ceiling this can be anything from 50 to 75mm of celotex.

Then normal quilt in the loft on top of the flat ceiling. No need to raise the tiles

If you specify a breather membrane, then you wont need any additional vents in the roof, but do specify eaves vents (soffit or over fascia), eaves felt support tray and eaves 'rafter roll' to duct the air up to the main roof past the eaves

 

[thompster]

Hi Woody,

I did think it was notifiable from reading the regs. I'm not 100% sure whether the U value is meant to be 0.16 or 0.20 but either is more than 50mm celotex.

The job spec lists a breathable membrane so that's covered. As far as insulation goes, if it's a case of 'consider increasing insulation', and I add additional 270mm fibreglass across the flats and 50mm in the eaves that sounds like it should be enough to show willing. I'd rather not add insulation under the rafters as a couple of the rooms have been recently decorated so I need an acceptable amount between rafters.

That leaves me wondering why the roofers would propose something apparently unnecessary. As I said before, they're a reputable firm so perhaps a case of gold plating?