Garage roof

[OldPosty84]

Hi all, I have a 9mtr x 4.1 mtr garage with a cold flat roof (felt)
It was reborded and refelted a couple of years ago.

The issue I have is I want to plasterboard the ceiling and add downlight so i measured the joists and here is the issue, the joists are 5x2 (47mm x 120mm) at 400 centres and according to my span tables they seriously undersized and should be 7x2 ( 47mm x 170mm)
my span tables are showing the current joists good for 2.6m max.

What would you guys do? I was thinking 7x2 under each joist to support the existing roof? just means i loose 7" of headroom.
Thanks in advance guys

 

[DAZB]

If you have 5 X 2 joists and should have had 7 X 2 joists then screw and glue 2 x 2 timbers to the 5 x 2's to make them the correct depth. Use the foaming glue and good quality 125mm screws @ 400 centres and you should be all good.

 

[Dereekoo]

All depends on your actual roof loading, if your boarding/felt and plaster weigh 25kg/m2 then if you ignore any snow loading the deflection is 5mm as opposed to an allowable of 12mm. A snow loading of 30kg/m2 as against a standard design one of 75k/m2 would put the deflection at the 12mm limit. Both the foregoing bending moments in the joists are well within the allowable limit. Using a snow load of 75kg/m2 would give you a deflection of 21mm and the bending moment force in the joists would just be at their allowable limit. So based on the forgoing loading criteria its not going to fail anytime soon. Only proviso is if you need to go on the roof do it when there is no snow loading. If you really wanted to put any doubts out of your mind you could put extra 5 x 2 joists in so the centres are 200mm as opposed to gluing in 2 x 2