Another Loft floor raising question

[slangivar]

Hi,
I am having the inadequate 100mm of insulation increased to somewhere nearer 300mm in my loft. and want to retain the storage space.

I have considered increasing the insulation with celotex but the cost of this is prohibitive and so I am left looking at adding new joists around 20cm above the existing ones.. The span is 4.6m which I guess is too much for a suspended floor to be a viable option.

I see from previous threads that there is some concern about the weight of adding new joists (at 40cm centre to centre) to the loft, with this in mind I would like to keep the size of the perpendicular joists down and plan to use the off cuts to make spacers to raise the new joists off the old ones.

My two questions are firstly are 40cm centres excessive or can I use 60cm centres for the new joists. I will be using something similar to B&Qs 18mm loft flooring packs which are already installed.

Secondly what size of joist would be recommended I am considering either 38x75mm or 38x100mm but not sure if this will be strong enough (I guess this partly depends on the distance between joists)?

The existing joists are 37mm x 138mm (1.5"x5.5")

 

[catlad]

I have seen that you get plastic spacers that sit on the joists, that will allow you to get the insulation in and still board it for storage.

 

[slangivar]

Thanks catlad,

The problem with them is that the unsupported span becomes the diagonal so even if you use a 60cm span for your flooring a pack of 12 only does a little over 1 square metre. At £15 to £20 a pack that means upwards of £300 for a 20sqm loft compared to £50 for the timber.

At that price the Celotex, or B&Qs polystyrene backed loft boards becomes an attractive solution.

 

[RonnyRaygun]

The span is 4.6m which I guess is too much for a suspended floor to be a viable option.

Yes, there is no need to do a suspended floor, more work, cost, and hassle.

I see from previous threads that there is some concern about the weight of adding new joists (at 40cm centre to centre) to the loft, with this in mind I would like to keep the size of the perpendicular joists down and plan to use the off cuts to make spacers to raise the new joists off the old ones.

The weight of new joists will be somewhere in the region of 4Kg/m2. There might a bit of spare capacity in the dead load assumption anyway. The design imposed load for loft storage is 25kg/m2, so assuming no spare dead load capacity your storage limit would reduce to around 20kg/m2. Which you probably aren't going to exceed unless you fill your loft with books...

My two questions are firstly are 40cm centres excessive or can I use 60cm centres for the new joists. I will be using something similar to B&Qs 18mm loft flooring packs which are already installed.

18mm board is generally recommended for 400mm centres, with 22mm board for 600mm centres. But as that recommendation is for domestic loads, 600mm centres will be fine for storage loads.

Secondly what size of joist would be recommended I am considering either 38x75mm or 38x100mm but not sure if this will be strong enough (I guess this partly depends on the distance between joists)?

Your joists are perpendicular to the load bearing joists. As they will be spanning a maximum of 400mm at 600mm centres, 38x75 will be fine...although it leaves less room for insulation.

The existing joists are 37mm x 138mm (1.5"x5.5")

Do these joists span 4.6m? 4.6m is a long span for that size joist, even under storage loads only.

What I did in my loft was use recycled plastic around the edges and just kept a central area for storage where I laid 175mm Kingspan with boards on top.

 

[slangivar]

Thanks Ronny,
The existing joists are part of a double fan roof truss spanning a total of around 8m, span at the centre of the trusses is about 3m. The 4.6m measurement is for my perpendicular joists.

Your suggestions all make sense although your assumptions regarding the spacing between the existing joists (trusses) is incorrect. These average about 600mm centres although are actually anything between 550 and 650, I guess the builder had an off day.

Secondly what size of joist would be recommended I am considering either 38x75mm or 38x100mm but not sure if this will be strong enough (I guess this partly depends on the distance between joists)?

Your joists are perpendicular to the load bearing joists. As they will be spanning a maximum of 400mm at 600mm centres, 38x75 will be fine...although it leaves less room for insulation.

With the larger gap between the centres of the existing joists would you still be happy with 38x75 or do you think I should go for something deeper?

 

[RonnyRaygun]

With the larger gap between the centres of the existing joists would you still be happy with 38x75 or do you think I should go for something deeper?

No, at 600mm it will still be fine to use 75mm deep timber. You'll need to put some noggins between the joists aswell, to prevent them racking.

 

[slangivar]

With the larger gap between the centres of the existing joists would you still be happy with 38x75 or do you think I should go for something deeper?

No, at 600mm it will still be fine to use 75mm deep timber. You'll need to put some noggins between the joists aswell, to prevent them racking.

Great thanks, I'll make sure I add some in. I assume you mean on the new joists and I'm guessing one should be sufficient for each length of 2.4m joist. They'll be partly supported by the beams of the double fan construction too.