Boarding loft to 200mm above existing joist level

[aarondown]

HI there,

So, as part of the EDF free insulation scheme I've just had my loft insulation increased. The original was about 80mm laid between joists and the new stuff is 200mm laid perpendicularly over the joists (see attached photo). I know need to board out the centre section for storage.

Was originally going to install 7" or 8" x 2" cross battens onto existing joists and affix loft boards to these. Having read another post here I'm thinking that I'll use a smaller section timber (to save weight and cost) and simply cut sections of the timber off to use as legs at each joist (see attached drawing ).

Now have the following queries:

-Which option of attached sketch should I use for the 'legs'?
-Intend to screw the boards into the cross battens but how should I connect A) legs onto joists and B) battens onto legs?
-What height above the joists should I aim for given the 200mm of insulation? Should I clear the whole 200mm or would I get away with compressing it by 20-50mm?

Would really appreciate people's thoughts on this as I hope to do it this weekend.

Thanks in advance

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• aarondown
• 18 Sep 2012
• 1

Untitled

• aarondown
• 18 Sep 2012
• 1

 

[JohnD]

if you squash it down, you will reduce its insulating ability, and the pressure will be transmitted to the ceiling below, which may bulge or crack.

you will also prevent any air circulation under the board, which may lead to condensation.

 

[jabuzzard]

HI there,

So, as part of the EDF free insulation scheme I've just had my loft insulation increased. The original was about 80mm laid between joists and the new stuff is 200mm laid perpendicularly over the joists (see attached photo). I know need to board out the centre section for storage.

Was originally going to install 7" or 8" x 2" cross battens onto existing joists and affix loft boards to these. Having read another post here I'm thinking that I'll use a smaller section timber (to save weight and cost) and simply cut sections of the timber off to use as legs at each joist (see attached drawing ).

Now have the following queries:

-Which option of attached sketch should I use for the 'legs'?
-Intend to screw the boards into the cross battens but how should I connect A) legs onto joists and B) battens onto legs?
-What height above the joists should I aim for given the 200mm of insulation? Should I clear the whole 200mm or would I get away with compressing it by 20-50mm?

Would really appreciate people's thoughts on this as I hope to do it this weekend.

Thanks in advance

Untitled

• aarondown
• 18 Sep 2012
• 1

Untitled

• aarondown
• 18 Sep 2012
• 1

Having boarded out three lofts now, my recommendation is to remove the extra insulation from the area you are going to board, lay 100mm cellotex/kingspan down and just lay loft boards over the top. It is way way quicker than adding extra joists, and does not reduce the height as much as mineral wool/fibreglass, which makes getting into the loft easier as well, and does not add the weight of the cross joists to your ceiling. Costs more but if you plan on staying there well worth it. The first two I did involved cross joists, and it takes much longer than laying down cellotex.

My other top tip is around the hatch. Obviously you need to add a box around the hatch to the new floor level, so use 100x47mm sawn timber. When fitting the boards leave an overhang of say 10mm, then when finished line the hatch with say 4mm MDF or similar. Finally take a router with flush cut bit and trim the flooring back to the MDF lining precisely, to finish off give it a quick sand on the cut chipboard edge. Might seem overkill but saves your clothes getting snagged

Another tip is to screw batons around the edge of the boarded out area to stop you pushing boxes off.

 

[aarondown]

lay 100mm cellotex/kingspan down and just lay loft boards over the top.

Thanks for your advice jabuzzard but I'm happy with the way I'm proposing to do it already. I was just looking for answers to the specific queries. The Celotex method would be circa £120 more than the method I'm proposing and time isn't really an issue as I have 3 guys and a whole weekend to do it if necessary. Not to mention that I'd be cutting every single Celotex sheet to get it up there. Long term the roof will be converted so I'm happy with this approach.

Any advice on the dimensions/connection queries would be appreciated.

 

[jabuzzard]

lay 100mm cellotex/kingspan down and just lay loft boards over the top.

Thanks for your advice jabuzzard but I'm happy with the way I'm proposing to do it already.

That's because you have not done it yet From personal experience of doing it both ways cellotex is by far the easiest method, and I would never contemplate doing it the extra joists way again.

As you have already laid the 200mm of fibreglass technically option B is the only one you can take. With option A you be reducing the level of insulation from one that meets current regulations (min 270mm) to one that does not which is illegal as you would be making it worse.

You can buy custom plastic legs to fit over joists and lift you up 200mm, but I worked out they where more expensive that the celotex, though that did include the cost of the fibreglass as well.

You could do a variation on A by getting a couple of lengths of 100x100mm timber to make the legs and then use 47x100mm or perhaps 38x100mm timber for the joists. 100x100mm timber is common for fence posts. That way you would keep the 200mm for the fibreglass but have more stable joists for laying your floor on. The joists in option B won't be laterally stable till you screw the floor down. Did I mention that you don't need to bother screwing the floor down with the cellotex method other than at the hatch

I strongly recommend my tips for the hatch and edge of the boarded area. The only other tip I have is if you are fitting a loft ladder extra length in the hatch is important otherwise getting bulky items in is impossible and you will keep scraping your back getting in/out.

 

[aarondown]

With option A you be reducing the level of insulation

I should clarify that both options still clear the 200mm insulation. Option A uses 150x50mm timbers and Option B uses 100x50mm.

to one that does not which is illegal as you would be making it worse

Not technically true even if I was squashing down the rockwool. In existing buildings, amendments to the thermal fabric of the building only have the requirement that it is not worse than when you started. As I consider the boarding out of the loft as one complete job, including the insulation, it cannot end up worse than the existing 70/80mm currently installed.

You could do a variation on A by getting a couple of lengths of 100x100mm timber to make the legs

As per attached? How would you recommend fixing the 100x100 to the existing 4x2 joists and subsequently the cross battens to the 100x100?

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• aarondown
• 18 Sep 2012
• 1

I strongly recommend my tips for the hatch and edge of the boarded area

I like the sound of this, although my router skills are a little shabby so we'll have to see!

The only other tip I have is if you are fitting a loft ladder extra length in the hatch is important

Thankfully the existing hatch is very long and the ladder I have to fit stores almost completely out of the perimeter of the hatch, thus freeing up the whole void to get big stuff up/down.

 

[jabuzzard]

Not technically true even if I was squashing down the rockwool. In existing buildings, amendments to the thermal fabric of the building only have the requirement that it is not worse than when you started. As I consider the boarding out of the loft as one complete job, including the insulation, it cannot end up worse than the existing 70/80mm currently installed.

That would be stretching it a bit.

As per attached? How would you recommend fixing the 100x100 to the existing 4x2 joists and subsequently the cross battens to the 100x100?

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• aarondown
• 18 Sep 2012
• 1

Get some 38x63mm CLS and screw it to the existing 4x2" joists either side. Then use some 150mm screws to screw the 100x100mm legs to the CLS and 4x2", repeat to hold the joists down.

I like the sound of this, although my router skills are a little shabby so we'll have to see!

You would be using a flush cut bit like this

http://www.screwfix.com/p/erbauer-pro-flush-trim-bit-1-4/74505

It is idiot proof. You put the router flat on the loft floor, push it to the edge where the guide wheel stops it exactly where required. You then just run it around the hatch for a perfect finish. It makes a right mess so if you can shut the loft hatch first or have someone hold a board or piece of corrugated cardboard over the hatch it will save a lot of mess.