# Vaulted ceiling

#### BarnabyT

He isn't..... what he said was that the maximum floor to ceiling height is 2.7 m. His contention is down to the gable wall being restrained to the trusses. He believes that the maximum height between the first floor joists and the rafters is 2.7m because this is where the gable wall will be laterally restrained. He thinks you can't go above 2.7 meters because you can't provide the required lateral restraint. However, from my understanding of the above diagrams, there is no such limit. He believes that a measurment from 1st floor joists to rafter height of more than 2.7 m will mean the gable wall cannot be adequately restrained and will need other means of restraint..... in his view this would be by way of one (or more) windposts. But the second diagram seems to contradict the 1st.

#### Mr Chibs

From memory mates roof was restrained with straps built into gable, you can restrain as the ‘black dots’ on diagram as the ceiling binder/chord is not straight across its angled.

Restraining scissors is not as straightforward as finks etc.

This must be the fact that it’s two storey.

Mates was single, but it’s tall must be just under 3m to eave height.

#### BarnabyT

Yeah.... doc A shows how gable walls are tied to roof. Basically, the roof is strapped up its slope to 1 bent straps that are built into the gable wall. But there is no requirement for restraint straps at ceiling level unless the height from 1st floor level to ceiling level is greater than 16 times 't' where t is the thickness of the 2 leaves added together plus 10mm. Given that a normal cavity wall is 102 mm brick and 100 mm block thats 212mm total.... multiplied by 16 equals a total wall height (without restraint at ceiling height) of 3.392 meters.

If I've understood that correctly, then there is no requirement for additional restraint other than what is provided up the slope of the roof. And if that is indeed the case, then I cant see any requirement for a windpost, or any other kind of additional restraint.

Also.... I cant find any reference to the 2/3 wall thickness X 16 that the BCO was talking about. Either im missing something, or he is. Given that it is his job to 'know' I'm assuming I've got something wrong..... id just like to know what it is. I need to order the rafters this week and so need to resolve it one way or t'other. I want to vault the ceiling, but can only do it if its as simple as using scissor trusses rather than finks. I cant see any reason why I cant, but the inspector has a different view but I think his view might be based on flawed calcs.

#### Dereekoo

To me diagram 16 is all about how things need to be tied in and diagram 8 sets the maximum height of your top storey which seems to suggest that this would be the height of your side walls plus half the height of the gable (and the maximum height of sides walls that can be taken into account is 2.7m).

#### BarnabyT

Thanks Derek.....

The point that I don't get is that the reason I cant just use scissor trusses is due to lateral restraint of the gable wall at the ceiling height. But from my reading, it appears that restraint at the ceiling level is ONLY required when the height between the 1st floor joists and half the roof height is more than 16 multiples of t where t is, in my case 210mm.

#### Mr Chibs

Just thought, Could you not get a Hybrid scissor truss made with a straight chord for the one nearest the gable End, then restrain as needed?

You might have to get creative with the wall boards to hide this truss.

#### BarnabyT

I wonder if raised tie roof trusses could be used?

I've had a reply from a designer who reckons its to do with 'bracing' the gable wall and thats why scissor trusses can't be used without a windpost. I cant find any mention in document A about bracing...... just restraint.

#### Mr Chibs

Document A isn’t specific about roof construction. My mates roof came with a sheet showing layout and bracing.

To be fair, we had to adapt it here and there as it wasn’t practical to follow the plan 100%, as threading through the trusses got harder as more bracing was added.

Curious... What is the size of the roof, spans etc.

Been reading about wind posts... take it these only apply to cavity walls?

Also... What did folk do before these became all the rage

Raised collar might be a good compromise for ease etc.

#### Dereekoo

Thanks Derek.....

The point that I don't get is that the reason I cant just use scissor trusses is due to lateral restraint of the gable wall at the ceiling height. But from my reading, it appears that restraint at the ceiling level is ONLY required when the height between the 1st floor joists and half the roof height is more than 16 multiples of t where t is, in my case 210mm.

Yes I agree, it does imply that as long as h is less than 16t ceiling ties are not required and only roof ties need be provided. The BCOs point about 2.7m max height to ceiling level I think is a bit of a red herring and relates to wall heights (not ceilings as such) and by virtue of D in diagram 8 you could have a top room with 2.7m high side walls (minus floor joist thickness) with a sloping roof that as long as h in diagram 16 did not exceed 16t building regs would be satisfied. Similarly if room side walls were 2.1m high then again if h did not exceed 16t building regs would be satisfied (in the 2.1m case the gable could be 0.6m higher than the 2.7m case). The fact that that your vaulted actual ceiling has a lower trajectory than the roof line (due to the scissor trusses) should not affect the reasoning that building regs are being complied with.

As for the 2/3 wall thickness factor, at a guess I imagine this comes from stability of brick walls and keeping various overturning forces contained within the " middle 3rd" to prevent undue stresses occurring. But everything is simplified and made fail safe and by using a factor of 16 gives a "deemed to comply height". As with all the building regs any figures used are so Joe public can just follow and everything is deemed to be OK. If they felt so inclined different values could be used if they are supported with calculations.

I'd be inclined to ask the question of them re Diagram 16 but obviously time is running out

#### Dereekoo

I wonder if raised tie roof trusses could be used?

I've had a reply from a designer who reckons its to do with 'bracing' the gable wall and thats why scissor trusses can't be used without a windpost. I cant find any mention in document A about bracing...... just restraint.
Bracing? lateral support? I take those to be the same thing, no doubt your trusses whichever type used will need some sort of timber bracing to stop a domino type of collapse.

#### BarnabyT

Thanks again. I'm gonna try and speak to him again tomorrow.

Could you not sheath it on the inside before the plasterboard?
Could that be a solution instead of wind posts.

#### noseall

I wonder if raised tie roof trusses could be used?
You can but they become stronger and much heavier (crane hire) and are a ball ache to brace. Then you'll have the additional insulation detail faff on the slopey bit.

#### BarnabyT

Spoke to BCO again today..... to be fair to him, he is trying to agree it. The issues comes from the fact that structural engineers use British Standards...... which are much more restrictive. Document A gives much more flexibility. He said building control are kind of stuck in the middle. He asked for the truss plans from the truss company and said he would try and get it approved by one of the structural engineers. But, he agreed that my interpretation of doc A was correct - but he doesn't think the councils structural engineers will agree because they apply much tighter numbers. Ill update tomorrow with their decision. Whatever they decide will be the end of the matter. I've lost too much hair over it.

#### Dereekoo

Out of interest what dims are the gable wall, outer width, height of side walls from underside of floor joists to beginning of gable slope and height to apex of gable from underside of floor joists. Its odd that their SEs use more specific numbers, thats usually done by the applicants REs if they are trying to circumnavigate the simplified approved docs figures to validate a "non deemed to comply" proposal.

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