Loft floor for storage

positron · 18-10-2024 9:41am #1

The joists in this 1996 built house seems to be 2x6 some 22 inches or so apart, with rockwool insulation between the joists.

I want to increase the insulation to 300 mm or so, and also put some flooring for storage etc (I already some flooring on one small section, but I need to free up that space for solar install, and that flooring is done right on top of the joists, so hoping to raise it up as well).

Just wondering what is the best approach? 18mm OSB over loft legs, Or should I use 1x6 timber running across the joists? Fixed to joists using some sort of timber to timber tie? Any downsides to this approach over loft legs? cost / weight / air flow etc?

Calahonda52 · 18-10-2024 10:00am

6 by 1 will be very unstable

best go along existing to make sure you get no air gaps between old and new but may not work with your roof trusses

post a picture of the roof trusses and we will take it from there

GreeBo · 18-10-2024 12:09pm

https://www.boards.ie/discussion/comment/122732583#Comment_122732583

6*1 perpendicular to the existing and fixed at every crossing should be pretty stable, especially once the floor is down.

My concern would be more on the overall weight, especially as once you floor it you are more likely to start storing lots of stuff up there.

OP what are the spans for the existing 2x6?
As an example, pine 2x6 @ 24 oc can span ~7.5ft for deck joists, in a ceiling they are likely spanning much more than this, since they are only to support minimal load, unless they have been specifically designed otherwise.

Calahonda52 · 18-10-2024 2:04pm

OP, will leave u in Greebo's capable hands.

Having actually done this work, personally, for several years, in all sorts of attics, I think I know what works and what doesn't work though all stages of the steps that are required for a safe attic during the work, hence my request for picture but Greebo know best so am out of here

DC999 · 18-10-2024 6:09pm

https://www.boards.ie/discussion/comment/122733958#Comment_122733958

Tbh I'd like to hear as I'll be doing that myself too. I'll be doing that in a small section for light but bulky stuff like Christmas decorations. It's really a glorified walkway from the attic door to the water tank and solar inverters on the chimney breast.

I'm well aware it's not good to start to add load to the joists that they aren't xpecting. My parents joists bellied a huge amount due to storing wayyyy to much furniture + stuff in the attic. Took many years. But they ended up having to do a huge repair job.

whizbang · 18-10-2024 10:59pm

6 x 2 are tough enough, but at large spacing between, there is a big risk of them twisting and spreading apart. In a normal floor there would be plenty of cross bracing to prevent this, (noggins) but not in an attic.

To get around this you must lay any additional joists crossways. You can get around this by sheeting over and screwing the sheeting tight to the joists.

My preferred way is to sheet and then lay rigid PIR on top, with an additional sheet protection on top.

6mm ply is ideal, but I used 2nd hand laminate flooring

positron · 19-10-2024 6:34pm

Thank you for the comments and suggestions so far - trying to post some pictures as Calahonda asked and trying to explain what I see as much as I can (which is not much given I don't know the difference between joist/beam/truss/girder/timber etc, so apologies in advance).

So the picture below matches the the design of the roof.

This is a semi-detached house and the picture above would be as it looking thru one side of the house (gabble end wall?). I marked two arrows where I assumed there is a beam of some sort running from one loadbearing wall to the other loadbearing wall, but I am just after finding out that there's no such beam. The 2x6 joists, around 2 foot apart, run approximately four meters from wall to arrow 1, 3 meters to arrow 2, and then another four meters to the other wall at the back of the house. Does this mean the joists are like 11 meters long?!!

Some (not very good) pictures. You can see a two water tanks (which I noticed is sitting on two sets of 9" wide timbers on their side spreading the weight across four joists, raising the tanks 18" over the joists, perhaps to aid water pressure).

And you can also see the side of the joists here with current level of insulation (please excuse the clutter in the foreground) - I am going to roll out some insulation over the existing insulation, across the joists. Not planning to create any loft flooring there.

PS: The clutter in the foreground is sitting on top of some flooring we laid 15+ years ago, it's just OSB gently placed on top of the existing joists (between the two arrows in the first drawing), and the load bearing wall to the left is where the solar inverter and battery etc is going to go. I would like to raise up this floor and add insulation under it as well.

And the other side has some extra insulation already rolled out - but this is where I am hoping create the new loft floor, by adding some height to the joists (loft legs or 1x6 across etc as in the first post), fill in with insulation, floor over it to store some light stuff.

I hope all of this makes some sense.

positron · 19-10-2024 6:50pm

Bonus question… Reading up on "truss roofs", says it's built to certain specification and you should not use it for anything else - not even a different roof tile than the one it's designed for - .. makes me wonder, does it mean we can not add solar panels to truss roof? Many neighbours in the estate already has panels up on their roofs, not sure if they did anything to upgrade their truss roof system?!!

whizbang · 19-10-2024 8:29pm

In the second pic, almost in centre, we can see light from outside/below ? This is where the fibre insulation struggles, when the wind blows through the insulation.

While internal walls may be timbe frame shouldn't be counted as load bearing, in reality there will be a usefull support from these. can you plot these on drawing.

Your current roof tiles probably weigh a few tons. A few hundred kilograms of solar isnt a big addition, particularly as the wind loading wont be increased.

Lumen · 20-10-2024 8:33am

Loft legs are point supports, 6x2 or whatever are beams. That presumably affects what sort of flooring you can put on top.

With 2 inch joists you can butt-joint the boards over the joists (with noggins to support the perpendicular joints) , whereas loft legs will need tongue and groove and for those T&G joints to be perfect (i.e. glued) all round.

To be it seems like much less of a PITA job to use 6x2. I hate T&G.

As to which direction, I don't know that it makes a massive difference. I get Calahonda's point about parallel suffering less from gaps between the layers, but on the other hand perpendicular will have less cold bridging through the joists, is probably more stable and won't suffer from running joints. It's also prob easier to run electrics with perpendicular.

I think I'd do it perpendicular, it just feels right.

positron · 20-10-2024 4:29pm

https://www.boards.ie/discussion/comment/122738602#Comment_122738602

I have that day light coming thru a few more places on the other side as well. Should it be perfectly sealed? I thought it's okay to have air moving freely above the insulation but I could be wrong.

@Lumen, If running say 1x8 beams perpendicular to the joists, and spreading OSB over it, what would you recommend to fix the 1x8 to the joists - Would noggins and the OSB itself (once screwed to these these 1x8s) be enough to hold everything together?

Lumen · 20-10-2024 9:01pm

https://www.boards.ie/discussion/comment/122741545#Comment_122741545

I can't see how 1" would work, that's commonly 22mm wide. You need 3mm gaps and 8mm distance from fixing to edge, so you'd need to skew nail the butt joints to hit the joist. Or leave the butt joints unfixed, which would require wide boards which you won't get up there unless you have a massive hatch.

I don't know, I use 2" timber for this sort of stuff.

GreeBo · 21-10-2024 2:56pm

https://www.boards.ie/discussion/comment/122742739#Comment_122742739

If the OP has a stash of 1" and runs perpendicular then they can create 12" supports that they use to tie the 6x1 to the existing joists. This would give full height support and avoid twisting, but if all the timber is being purchased its likely cheaper to just buy 6x2, but this would double the additional weight.

My concern is the load bearing capacity of the existing joists, doubling the weight (by adding cross joists and flooring) before even storing anything up there would be a significant worry for me. Adding another floor is not adding any strength to the existing structure.

Solar panels should be ok as the rafters are transferring the majority of weight into the walls, but getting professional, structural advice and plans/specs and then maybe doing the actual work yourself would be my advice.

GreeBo · 22-10-2024 11:09am

https://www.boards.ie/discussion/comment/122745281#Comment_122745281

I just spotted that the OP is talking about 1x8 beams now. I wouldn't attempt to do anything with 1x8s other than use them as floorboards.

OP my advice would be to just lay more insulation over the existing and ensure that you dont block the eaves but also that the air flow from the eaves is not getting under the insulation. Once the insulation is above the existing joists you can switch to perpendicular or at least offset the joints to avoid leaving gaps.

Then, for storage, pick a small area where the joists are bearing on a wall below, ideally not a stud wall, but you may not have much choice. Often the area with the highest head room will be near the attic hatch and will be near the hotpress or something, so you will have multiple stud walls. I would floor a portion or this area and minimise what you are planning on storing up there.

Loft floor for storage

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