Raising Joist Height

  OTT_Buzzard 19:10 15 Aug 2009

After spending most of the last two days emptying my loft in preparation for new insualtion, it's become aparent that the loft is going to need reboarding afterwards so that we can keep our plethora of meaningless boxes stored up there.

The company who is doing the insualtion reckon that they will take it to 240mm depth or there abouts. My current joists are 100mm. This leaves me needing to add around 140mm to the joist height.

So, the question is, what's the best way to do this? Wickes seem to sell some suitable cheapish boards to lay over the result (click here)

Does anyone have any advice on how to do this little project?

  laurie53 19:35 15 Aug 2009

I had a similar problem.

I simply put a framework of 4 x 4s on the joists to support the boards.

  Fruit Bat /\0/\ 19:37 15 Aug 2009

Just had same job done.

They will lay the new insulation across the joists(read the letter carefully it explains exactly hat they will do)

Didn't bother raising the joists just dropped the boards back on top making sure they overlapped where the joists are it will fatten down a lot.

The stuff you put on top will compensate for any loss of insulation caused by "crushing" the insulation.

2 days it took me a week to empty mine and we chucked a load away and still haven't got it all back in yet, job was done last Monday.

  rdave13 21:12 15 Aug 2009

If they lay the insulation across the ceiling joists then do as Fruit Bat /\0/\ suggests. Use only screws to fix the boards as you don't need any vibrations when fixing. Also note any 'junction' boxes for lighting and any other points you can think of if, and when, re-wiring will be needed. Make sure you leave a "hatch" for these points for easy access.
Once you lay 'tongue and groove' chipboard flooring it's a devil of a job to find these junction boxes and gain access.

  Forum Editor 01:14 16 Aug 2009

and you have the time, you can floor the loft without compressing the insulation unduly (which certainly makes it less efficient).

Fix 100x50 timber across the joists on its edge by 'skew' screwing it. This involves slotting the timber in between the runs of insulation; you can tease the material back under the new timbers, so there are no uninsulated gaps. drive the screws in about 25mm from the bottom edge at an angle - with a bit of practice and a good battery drill/driver you'll soon get the hang of it, and the fixing will be very secure. If you want to make the job easier use 55mm 'Floor-Tite' screws from Screwfix - they'll drive easily and fast.

When you're done, fix those cheap Wickes T&G floor panels to the new timbers and you'll have a solid, flat floor without adding too much weight to the ceiling joists.

It's not a quick job, but it's permanent and professional.

  al's left peg 10:06 16 Aug 2009

That's a similar slant on the way I done mine FE. As you rightly say compressing the insulation makes it far less efficient. I was thinking of using Kingspan polystyrene between the joists as I think it is far more efficient than Rockwool, but it would of meant removing the old Rockwool rather than just layin new Rockwool on top that I did.

  OTT_Buzzard 10:35 16 Aug 2009

I'm liking FE's suggestion. I've got a good battery screwdriver. Will let you know how it goes!

  Forum Editor 10:49 16 Aug 2009

Kingspan will be far easier to work with, but the cost will be higher.

  jack 11:13 17 Aug 2009

Perhaps the junk if mainly empty appliance boxes one is required to keep carefully distributed will serve as insulation!


  Grey Goo 11:32 17 Aug 2009

They may well lay it as Fruit Bat has indicated as the insulation can be wider than the joist spacing and it is encapsulated in plastic so that there are no nasty fibres flying around. You will have to find out before deciding on your choice of building materials.

  Stuartli 11:39 17 Aug 2009

Don't forget gloves, face mask and goggles whilst working with loft insulation materials.

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