Additional joists for loft boards

9 May 2006
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United Kingdom

I want to board my loft for storage. The house is only 4 years old. I am only looking to board the central section of 6m x 2.4m. The joists in this section, 11 of them are at 600mm centres running 2.4 m in length.

The insulation is in 2 layers both running between the joists and is about 10cm or more above the joist level. I have seen elsewhere that it is OK to simply compact the insulation into the gap and board on the original joists. I am really not sure about this - surely the thermal property of glass fibre is dependant on the volume of trapped air?

So I am thinking of adding 38*89mm joists to give the insulation room. Question is should I:

1) Lay the raising joists in 2.4m lengths directly on the original joists then using 600x2400x18 TG Chipboard sheets:
a)at 90 degrees with staggered joints
b)10 of them all in a line

2)Lay the raising joists at 90 degrees to the joists thereby squashing the insulation at intervals. If I am doing this should I use a smaller gap? 400mm perhaps if using 2400x600 boards?

Fixing the raising joists - If they are across then I can screw in from the side at an angle and still hit the joist at inter-section. If directly on top in line can I still go in the side. Or am I simply best using 140mm screws straight through the top.

Advice much appreciated.
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