Loft Conversion Fire resistance

[asteyert]

I am doing a Loft conversion. the new loft floor is 7x3 joists set 3 ins above the existing ceiling, which is underlined with 9.5mm plasterboard.

The fire retardent mat between the joists will be 100mm rockwool flexi.

As the new joists are above the old ceiling does the mat sit directley on the plasterboard with wire mesh if so what happens to the 3ins gap under the new joists or can the mat be fitted between the new joists

Best Regards

 

[DevilDamo]

I'm confused. Where and why is there a 3" gap?

I've never used/specified Rockwool Flexi. What is most commonly used for... thermal, sound, etc...?

You need to achieve 30min. FR to the new floor. This can be achieved with 2 layers of 12.5mm plasterboard, fixed to the underside of the ceiling. However, where that may not be possible due to existing internal decoration, covings, etc... your BCO may be happy with 22mm thick T&G chipboard across the whole new floor, including that into the eaves. This is what we were advised by a BCO on one of our jobs. However, the ceiling was lined with 12.5mm plasterboard and not 9.5mm.

 

[asteyert]

The new floor is suppoted on steels as the existing ceiling has bowed over the years Rockwool has been specified by BC they will not accept mieral wool do not want to underline as will ruin decor

 

[DevilDamo]

How about asking BC if they'll accept 22mm T&G chipboard across the whole new floor to provide the 30min. FR, in addition to the 9.5mm plasterboard?

 

[freddiemercurystwin]

Technically you need 12.5mm board in conjunction with 22mm T&G above to achieve 30 minutes.

 

[asteyert]

Thanks for that bc do not want to know will not visit site and are reluctant to give advice .I have submitted a full plans applicationwhich has been passed the first visit is not until the pre plaster stage which is too late if things are not in compliance they specify rockwool but will not say which spec . The Architect will not answer his phone now he has been paid

Sorry to winge I Thought i was going to enjoy this Project

Best Regards

 

[JohnD]

try to find the rockwool manufacturer's website, they probably have a customer service contact and will know what grade and installation method they recommend for fire resistance.

Manufacturers of building materials often offer very good guidance. Sadly it is often not followed

 

[DevilDamo]

bc do not want to know will not visit site and are reluctant to give advice

You sure you're not getting planning and building control mixed up I have never known the BC department to not give advice, especially if an application has been submitted/passed, etc... After all, that is what they're there for.

the first visit is not until the pre plaster stage

Did they not ask for a trial hole to inspect the existing foundations to determine their suitability to take any additional load? That is usually their first visit if carrying out a loft conversion. However, houses that have been built more recently that have gone through BC approval... those records may still be with the LA so they can refer to those notes as opposed to requesting trial holes.

 

[asteyert]

Thanks for that Rockwool have said 100mm flexi expensive but safe BC will not visit untill the first inspection stage pre plaster due to staff cuts they will give advice but will not be specific .To give you an idea of the cuts they only have two agency Structual Engineers working two days a week . Times have changed at one time they would visit as requested

Thanks for your help

 

[JohnD]

... only have two agency Structual Engineers working two days a week ...

what Local Authority is that?

 

[chappers]

no matter how fire retardant the insulation is, it cant be part of the fire separation requirement, due to the way it fits into the structure.

I was always under the impression that the ceiling itself was to provide 30 min fire separation and not the ceiling and floor as a whole

 

[asteyert]

The ceiling is made up of 9.5 mm plasterboard which does not give the 30 min fire rating by adding 100mm of rockwool on chicken wire the ceiling will comply with regs

 

[DevilDamo]

I was always under the impression that the ceiling itself was to provide 30 min fire separation and not the ceiling and floor as a whole

So was I until recently where the BCO said by adding the 22mm T&G floor boarding will suffice in achieving the necessary fire resistance, assuming 12.5mm plasterboard is below.

 

[geraint]

The ceiling is made up of 9.5 mm plasterboard which does not give the 30 min fire rating by adding 100mm of rockwool on chicken wire the ceiling will comply with regs

adding chicken wire does not help the fire resistence...

it is there for a completely different reason....

 

[cadell]

And that reason is to keep chickens in the loft?

Only joking


It does add to the fire resistance by holding the Rockwool in place if the ceiling gives way during a fire. There are alternative ways of holding it in place, for example using battens or staples.