Boarding a warm roof / cathedral ceiling

[markocosic]

How should a warm roof / cathedral ceiling be boarded and plastered?

 

 

We have:

 

- Stick built timber frame

- Roof structure is a 12 metre ridge beam in 3 parts with 2 posts (visible width of ridge beam 245 mm; post width 145 mm)

- Rafters are 245 x 45 on 600 mm centres; 12 mm OSB internally for racking; then "45 x 45 mm battens" on 600 mm centres

- Each "panel" is effectively 12 metres long / 4 metres tall

 

The design intent for these 45 x 45 mm battens was:

 

- space for cables

- space for final 45 mm of insulation

- to mount plasterboard to

 

At the minute they're nailed on with a 90 mm nail every 600 mm into the rafters. 

 

 

We'd previously planned timber cladding that could tolerate flex. She thinks that she'd now like plasterboard. 

 

How should we board this for (1) ease, (2) to avoid cracking, and (3) to avoid overloading the foundations?

 

(double boarding would the norm here but I'd rather NOT double board this for weight reasons after she swapped a wriggly bitumen / metal roof for a wooden roof)

 

 

It would be emulsion painted a light colour. There may be uplighting; particularly on the mezzanine where you don't want to be walking into light fittings.

 

 

 

My thoughts:

 

- Get boards with recessed edges and run them in landscape so that they're supported on the longest edges / between rafters by the battens where there is the most movement potential

- Use floating battens to butt-join the short edges and set all the joints under a rafter so that there shouldn't be too much movement here

- Tape and joint it rather than skimming it for ease on such a big area

- if taping and jointing it then line up the joints rather than staggering the boards in 'stretcher bond' to make the taping and jointing less visible

 

I could also add metal profiles and fasten to those; or use them as the board joiners. They're fairly readily available. Or indeed remove the 45 x 45 mm timber if metalwork would be better. (though I'd prefer not to!)

 

 

 

- Edges...are probably where it will crack...what to do at the eaves / ridge / gables in terms of (a) support and (b) finish? 

 

It's easy to add support for one or other or both ceiling and wall. We deliberately did nothing for now on the basis it's easier to add later than to remove what's not needed. They'll be joining to more plasterboard.

 

The side walls on the open end are 3 metres tall. I'd planned to do with single vertical boards and all tape in the recess. 

The side walls under the mezzanine are 2.5 metres all. All vertical boards again and all tape in the recess.

The ceilings under the mezzanine are 3 metres wide. All single boards again joined on the joists.

 

 

- Posts will be fun. We'd like them to remain as wood. Stop the plasterboard short and mask with a trim piece? Any other neat tricks? This isn't the airtight layer.

 

 

The sheds have a decent choice available. ~€1.50/m2 for the boggo stuff or ~€3/m2 for knauf moisture resistant; 3 metre lengths available for doing the walls in one length with no butt joints. I'm tempted to 

 

https://online.depo-diy.lt/products/6275

https://www.senukai.lt/c/statyba-ir-remontas/izoliacines-medziagos/gipskartonio-ploksciu-montavimo-sistemos/gipskartonio-plokstes/6ys

 

 

There's a bewildering choice of plasters and fillers available too. What's the difference between EN 13963, type 3B and 4B?

 

https://www-knauf-lt.translate.goog/produktai-ir-sistemos/produktu-a-z/uniflott.html?_x_tr_sl=lt&_x_tr_tl=en&_x_tr_hl=en&_x_tr_pto=sc

https://www-knauf-lt.translate.goog/produktai-ir-sistemos/produktu-a-z/q-filler.html?_x_tr_sl=lt&_x_tr_tl=en&_x_tr_hl=en&_x_tr_pto=sc

 

The surface preparations are covered here FWIW:

http://www.eurogypsum.org/wp-content/uploads/2015/04/EUROGYPSUMFINSHINGUK.pdf

 

Q3 probably ok given roof pitch of 9:12 (36 degrees ish) - tape and joint and smooth it but don't go so far as to skim the entire thing with a 1mm+ layer because you're going to see the variation in the rafters or lines between boards anyway?

 

 

What would you do?

 

 

 

[ProDave]

Very much like our roof single boarded with 12.5mm plasterboard, no cracks.

 

The only thing I did different was before fixing the battens to create the service void, I lined the entire roof and all the walls of the house with air tight membrane.

 

 

Only just started fitting the service void battens then in that photo

[JohnMo]

Similar to ours we used taper edge plasterboard, which taped and jointed, not skimmed.  9 months since jointing and taping no cracks.

[markocosic]

Thanks both

 

What edge detail did you use at the walls / gables to support the PB there?

 

Am I right in thinking that your battens for the PB are "vertical" John?

 

[Gone West]

2 hours ago, markocosic said:

What would you do?

We had a portal frame so no ridge beam. Everything had dried out well before plasterboarding so no cracking.

 

 

 

 

 

 

[JohnMo]

We just used battens.

 

The battens are vertical, but mainly because I had counter battens at the rafters to increase insulation thickness and kill off repeat bridges.

[nod]

Which option you choose 

I would always use an insulated plasterboard on the underside 

As a contractor I’ve been back to so many that have cracking and nail popping issues Usually South facing

Also it will insulate across the rafters  

[markocosic]

Thanks @nod - that's not what I wanted to hear (many with cracking and nail popping) but it's what I am worried about.

 

Insulated plasterboard isn't a thing here. They glue 50-100 mm thick sheets of EPS on; mesh it; then render it much like an external wall. Lightweight and unbelievably resilient (when you need to dig the heating pipework back out again...) but I'm not keen on the time / mess / cost of that vs board / tape / joint.

 

 

 

Is there some magic to insulated boards from a fixing / flexibility perspective (glued on? extra thickness lets screws wobble more within the foam to accommodate movement in the rafters?) or is it purely to avoid heat? The latter shouldn't be awful (350 mm mineral wool all told) so unless there are other subtleties I might listen to the warning but ignore the warning...