Weight of new plasterboard on old roof

We have had the roof of our outbuilding redone with new slate (replacing old slate) and it has been felted and insulated with between rafter insulation at the same time. It has never been boarded before.

I now want to board it to finish it and was thinking of plasterboarding and skimming. However one thing that is concerning me is the additional weight of the plasterboard on the roof. I have calculated that I need around 22 sheets of 1.8x0.9m plasterboard. If I use 9.5mm thickness this will be about 250kg I will be adding to the roof. Should I be concerned about this - ie. does the roof need reinforcing?

From looking at it my gut feeling is no, but I thought I would check with some people with more experience first!

If it helps, I have attached a photo. The span of the roof is about 5m and the smaller rafters are about 7cm x 8cm. It is an old building, Victorian or Georgian I think but seems to be very sturdily made.

I had also considered boarding it with hardboard to keep the weight down but I don't think this will be anywhere near as neat or durable, if anyone has any opinions on this I'd be interested.

Thanks in advance!

 

The additional weight of plaster and boards will be fine.
I would suggest you use 12.5mm boards rather than 9.5mm though, as they are more rigid and won't warp and move as much after being fitted. So will prevent cracking on joists.
I prefer to use the 2400X1200mm boards, less joints, less screws, area covered quicker and thermal resistance is greater.

Going be your calculations you will need about 2400x1200x12.5 boards to cover the same area.
Coming at about 300 kg for the boarding
and 135 Kg for plaster

Your method would give you 139 Kg for boarding and 135 Kg for plaster.
But I still would not consider the extra weight an issue.

You could reduce the weight but using taper edged boards and joint taping rather than plastering.

 

I prefer to use the 2400X1200mm boards, less joints, less screws, area covered quicker and thermal resistance is greater.

how is thermal resistance greater.

 

Thicker boards

 

No, I think PBD is alluding to a theoretic [thermal] advantage with the fewer joints

But the main problem with 8x4 sheets is getting them up into the loft and then humping them around trying to hold them up and fit them.

 

I stated that thermal resistance is greater, as 12.5 boards are thicker, as correctly mentioned by alastairreid

12.5mm= 0.07 m²K/W
9.5mm=0.05 m²K/W

 

Thanks for the information and opinions, you have confirmed what I thought and I will go ahead with plasterboarding.

Think I will use the smaller sheets though for ease of handling.

Cheers

 

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