having a huge dormer built.

Roystone · 18 Aug 2021

We are in the process of having a huge dormer built. The approved drawings the builder is working from says single layer membrane roof covering bonded to19mm plywood, also sides from 25mm plywood (exterior grade ply). The builder has used 11mm OSB3 board all over (also gaps all over between boards - you can see daylight) and has purchased a load of GRP.
We are not living in the house so we didn't know - only the boards are up so it is not too late to change, but obviously an unpleasant conversation for the builder who has been great up to now. It is (what I would class as) the world most expensive roof (already feel like we are paying a ton for it - so has to be right).

I know nothing about building - is OSB any good, Google says not (so it must be true....).

RandomGrinch · 18 Aug 2021

At the very least, if the builder is working from the drawings, I would expect that would have formed the basis of their quotation. I may be wrong, but I have seen other threads on here (sorry, but I can't place them now) that would suggest changes that hadn't been agreed would amount to a breach of contract.
Plywood is stronger, more moisture resistant and costlier than OSB.
The price of timber is high at the moment and availability is often limited. Your builder may be trying to claw back some profit by using the cheaper OSB.
Good luck!

Roystone · 18 Aug 2021

@RandomGrinch Thanks - I wouldn't mind if they checked in with me first. There are loads of gaps between the boards. Would it be advisable to double board (they said gaps are because they ended up getting board from two different places - and they didn't fit together well so they had to remove the tongue from some).

sxturbo · 18 Aug 2021

I prest there is cladding and insulation to over the outside and inside?

Notch7 · 18 Aug 2021

Roystone said:
@RandomGrinch Thanks - I wouldn't mind if they checked in with me first. There are loads of gaps between the boards. Would it be advisable to double board (they said this be they ended up getting board from two different places - and they didn't fit together well so they had to remove the tongue from some.

what’s the roof construction?

is it warm roof, cold roof, what insulation thickness etc.

Roystone · 18 Aug 2021

@sxturbo @Notch7 These are cold dormers finished with GRP (didn't know the difference until recently - too late now as the job is well underway) - the dormers were specified by the architects as part of a bigger job. They will be tile hung fronts and cheeks - they are not finished yet - we have a couple of large OSB boxes at present (front dormer has not got gaps, rear larger dormer has load of gaps between boards).

Spec for roof was 19mm ply, insulation on the inside of 100mm Kingspan between joists, then layer of 45mm PIR insulation on underside of joist and a layer of 12mm plasterboard.

Builder has gone with single layer OSB3 instead of ply, they will have the 100mm Kingspan, but saying no such thing as 45mm PIR so going with 25mm PIR. Instead of 12mm plasterboard, I'm opting for 2 layers of 12.5mm acoustic board off clips and metal furring strips - to reduce noise.

Spec for walls and cheeks was 25mm ply, insulation CELOTEX Tuff-R GA 3075 between timbers, one layer foil backed plasterboard (builders say 25mm PIR that standard 12mm plasterboard is better than foil backed plasterboard). I have asked them to also add space before plasterboard - and another layer of acoustic on top of the standard board.

The requirements from tuther-half (no condensation, no damp, very warm, as little noise as possible - rain noise and birds dancing on dormers).

I had considered asking the builder to double board the OSB to get rid of the gaps (they already said they are doubling up on the cheeks - gaps are worst there.

Also, do we need an air vent.

catlad · 19 Aug 2021

You need foil backed plasterboard to help with the vapour control layer.

oldbutnotdead · 19 Aug 2021

11mm OSB is too thin for using as a deck for GRP. Daylight between boards indicates square edged rather than T & G so all the joints will have to be supported and taped (otherwise you'll get holes in the laminate).
GRP deck also has to be very solid or you'll get a lot of thermal noise from it (search on this forum for noisy grp roof).

SpecialK · 19 Aug 2021

Builders are taking you for a ride.
Tell them to build it to the spec you asked for and assume are paying for...

No such thing as 45mm PIR?? There is and 40mm PIR is available everywhere. Building control insisted on 40mm over my vaulted veiling. I would be inspecting after installation of the 100mm PIR to ensure there are not too many gaps if they cant even install OSB without gaps...

HERTS P&D · 19 Aug 2021

Is the gaps between the boards for expansion, due to the fibre glass roof?

Andy

Roystone · 19 Aug 2021

no, gaps simply because they had to cut the tongue off as ended up with two types of T & G that did not fit together - one dormer has no gaps - as it was all the same board - the other has loads of gaps - some examples - from smallest to worst.

oldbutnotdead · 19 Aug 2021

Sounds like your builder has hit supply problems (like everyone else) and instead of having an honest conversation with you he's chucking up whatever he can get hold of.
Your project manager should be all over this, BCO may well twist about OSB instead of ply (very different structural strength and properties) and will almost certainly reject 25mm PIR instead of 45mm (presumably 45mm was specced to make the SAPS sums work).
Time to get in builders face (if you are the project manager), hopefully you're on staged payment schedule. Trouble is if he walks you'll have to pay current prices for materials (which are probably double what they were when he quoted) to whoever takes the job over

Roystone · 19 Aug 2021

@oldbutnotdead, hit the nail on the head. There is a lot of glass so it is all about the SAP... I am PM (and my first go in this trade) - I am an IT PM by trade... This is very different - lots to understand!

oldbutnotdead · 19 Aug 2021

Mmm.

Roystone said:
@oldbutnotdead, hit the nail on the head. There is a lot of glass so it is all about the SAP... I am PM (and my first go in this trade) - I am an IT PM by trade... This is very different - lots to understand!

Unlike with IT stuff you don't have a test area where you can mess up without any significant costs. Your build is working on the live production system- you have full plans by the sound of it (with full construction detail) so you're not in a bad position. You have to stop him deviating from spec. If he insists there's no problem then he needs to pay for professionals (real ones with all the right letters and indemnity cover) to justify his variations.
And then the proposed variations need to be waved at BCO (who is your friend in this scenario) before stuff gets cut up and nailed down.
If builder won't do this then go legal. Quickly.

Roystone · 19 Aug 2021

@oldbutnotdead, thanks - yes, I had a good call with BCO today - they have accepted all changes saying the architect over spec'd - the builder is still proposing minimum or higher than minimum.

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having a huge dormer built.

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