Increasing joist height

[Dippy]

I want to board part of my loft just for storage reasons, however I also want to improve the insulation. Looking at info about insulation I believe that the recommended depth of glass fibre type is 170mm. My joists are only 70mm high. So this means that I'd need to screw down lengths of 100mm wood so that I can lay boards and have 170mm gap for the insulation.

I was rather surprised that the joists are only 70mm. Is this typical?
I what I propose to do OK?

 

[catlad]

They are not 70mm joists they are ceiling ties. Put insulation between your ceiling ties then put 75x50mm timber perpendicular to ceiling ties at the spacing that your insulation comes in, normally 400mm then you can board over the top with 2400x1200x18mmt&g chip board. Don't forget to put the heavier items in a place were you know corresponds to a wall below.

 

[jabuzzard]

I want to board part of my loft just for storage reasons, however I also want to improve the insulation. Looking at info about insulation I believe that the recommended depth of glass fibre type is 170mm. My joists are only 70mm high. So this means that I'd need to screw down lengths of 100mm wood so that I can lay boards and have 170mm gap for the insulation.

I was rather surprised that the joists are only 70mm. Is this typical?
I what I propose to do OK?

Save yourself a bunch of time, and get better insulation; buy 100mm celotex/kingspan, cut into strips feed through loft door, lay over joists and place loft boards over the top. That way you will get 270mm equivalent of insulation which is the current recommended level. The celotex also adds virtually no weight to the existing "joists" unlike the wood option. You do however need 100mm wood around the hatch.

It's a bit more expensive, it is a lot easier and a better result. Trust me I have done the joist method twice and the celotex method once, and would not even consider the joist method anymore.

 

[Dippy]

jabuzzard,

I'm looking into this again and your idea appeals to me - primarily for not having the extra weight of wood and getting better insulation for my additional 100mm depth. However I have a few questions:

1) I only have limited experience of insulation boards. Is there any one particular type that you would recommend for this application or are they all much the same?

2) I presume since you have done this the answer would be no or 'not much', but doesn't the insulation board get 'dented' where it lies over the joists and has weight put on the floorboards?

3) Have you used 'standard' t&g loft boards on top of the insulation board? If so how have you fixed them down to prevent them coming apart when walking on them?

 

[big-all]

do not overboard unless the timbers are taking the weight
i am sure the 100 against 70 deep timbers was a genuine mistake

 

[jabuzzard]

jabuzzard,

I'm looking into this again and your idea appeals to me - primarily for not having the extra weight of wood and getting better insulation for my additional 100mm depth. However I have a few questions:

1) I only have limited experience of insulation boards. Is there any one particular type that you would recommend for this application or are they all much the same?

They are I believe all pretty much the same. I purchased them from Wickes because they would also deliver the T&G loft panels at the same time as well and some extra fibreglass rolls for the areas not boarded out.

2) I presume since you have done this the answer would be no or 'not much', but doesn't the insulation board get 'dented' where it lies over the joists and has weight put on the floorboards?

I have boarded three lofts in my time. My sisters flat and my house using extra timber to jack the floor level up. My sister then moved to a house and asked if I would board out her new house and fix the loft insulation, as it was only as deep as the joists and missing in places. I decided to go for the Celotex/Kingspan method as something of a punt for speed and minimizing the loss of height.

You need to be careful when installing it, always make sure you use a board to kneel on it, but I did not find it has dented significantly, and it still seems to be doing fine a year later.

3) Have you used 'standard' t&g loft boards on top of the insulation board? If so how have you fixed them down to prevent them coming apart when walking on them?

Yes I used standard T&G loft boards on top of the insulation board. They are fitted tight between the gable ends of the house and two thicker joists that ran the length of the house, so it cannot in effect "move" I is however screwed to the 100mm timber I used to jack up around the loft hatch. You could if you want get 150mm screws and screw the loft boards to the joists through the insulation board around the edge if it is not butting up against something solid.

This is a picture of the job in progress.

Untitled

• jabuzzard
• 15 Oct 2012
• 1

I have just had a report from my brother who reckons that the plastic loft legs are good, so if you have the insulation already and have plenty of headroom, then that is reasonable solution. If you look at the picture of my sister's loft she does not have much headroom, and did not have any insulation to begin with so the celotex route turned out to be a no brainer.
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[chickenlips]

Wickes are cheaper than a local Builders merchant or timber merchant ?

 

[big-all]

i personally would avoid the plastic legs as they point load rather than spread the load
they also increase the gap between supports by around 40% as the span is now the diagonal between legs rather than 2 parallel surfaces so in general bad news

 

[jabuzzard]

Wickes are cheaper than a local Builders merchant or timber merchant ?

In my experience they are often just as cheap if not cheaper. All depends what you are buying, where you live, is it on offer and what delivery charges might exist.