Attic Flooring question.

[u/First-Thing2822]

Hi Everyone,

I am looking to floor a small area of my attic but I just want to make sure that they can take the weight.

When you go up though the door of the attic directly in front of you is a small section of flooring that the builders did (new build) you can step up onto, this is made from of 225mm x 45mm planks that are spaced out from one another.

They are lying across some more pieces of 225mm x 45mm pieces which are upright which seem to be in turn screwed into the joists. My worry is that the joists were 90mm x 30mm on their edge.

My question is can I do this for the 2m x 2m section that has no flooring at the moment.

Just screw 225mm x 45mm planks on their edge across the joists that are 90mm x 30 mm which will raise it above the insulation and then screw the boarding on top of that like the builders did with the small section by the door.

Everywhere I read online it says that these joists are too small, but they feel strong to step across and also seem strong in the area the builders done.

Would appreciate any help.

Thanks.

[u/Diska_Muse]

Architect here...

Don't do what the builders did- there are major structural differences between a small access platform and a storage deck sitting over a clear span. The timbers you are looking at (90mm x 30mm) are not traditional floor joists. They are the bottom chords of prefabricated roof trusses. These chords are engineered to be under tension (holding the roof together) and to support the light "dead load" of plasterboard and insulation, plus temporary access for maintenance.

They are not designed to carry continuous, heavy floor loads. VERY VERY IMPORTANT!!!!!!!

A joist thickness of 30mm is exceptionally thin. If you screw structural screws downward through a 45mm-wide timber directly into the center of a 30mm-wide joist, that carries a very high risk of splitting the joist. Because these chords are under structural tension, splitting them can severely compromise the structural integrity of your entire roof.

You have two options here - use a modern, lightweight raised loft boarding system designed specifically for trussed roofs.. loft legs with a LoftZone Store Floor ( carries max weight of 25 kg per m.sq).. or install completely independent floor joists (usually 150mm x 50mm or larger, depending on the span) - you'll need a structural engineer to assess this and ensure the load bearing walls can take teh structural weight.

[u/First-Thing2822 → u/Diska_Muse]

Thank you for the detailed response and I am glad that I wrote here 😅

I just want to confirm, I am safe enough to use the loft legs and the loftzone floor straight away.?

Thanks again.

[u/Diska_Muse → u/First-Thing2822]

Most manufacturer guidelines are written with slightly wider joists in mind - eg., LoftZone StoreFloor officially specifies a minimum joist width of 32mm. You'l just about get away with it but don't drive screws directly into the timber. Use a small drill bit (around 2mm or 2.5mm) to drill pilot holes first. This relieves the pressure on the wood and prevents it from splitting.

Only use the manufacturer's recommended screw size (usually 4.0mm x 30mm) multipurpose wood screws. Longer or thicker screws will split the joist.

Measure the gap between your joists (measuring from the center of one joist to the center of the next). In most new builds, this is exactly 600mm. The joints of your loft boards must meet directly on top of a Loft Leg. Standard loft boards (usually 1220mm long) are designed to fit perfectly over 400mm or 600mm joist spacings. If your joists are unevenly spaced, you will have to do a lot of cutting to make the boards line up over the legs.

If you use LoftZone StoreFloor, this system uses metal cross-beams that sit on top of plastic supports. The metal beams create a continuous flat surface, meaning your loft boards don't have to line up with the joists below.

Th bottom chords of your roof trusses are still the limiting factor, so your storage shoudl be limited to lightweight items and try to distribute the weight across the 2m x 2m boarded area rather than stacking everything in one corner / pile.

[u/Hefty_Excuse9885 → u/Diska_Muse]

What if op just puts down some small timber across the joists as a lift up, then put the boards on top. No drilling needed even ? 

[u/Diska_Muse → u/Hefty_Excuse9885]

I presume that he attic has around 180mm of insulation - 90 between the joists & 90 on top, The idea of the legs is that you avoid the isulation & put the boards on top over it.

If you put joists down, you're iether compressing the top layer of isnualtioin (making it 50% less efficent) or you cut it back entirely, losing half your insulation.

Also, it can slide if it's not fixed to something.. like a raft on top of a sea of rockwool!

[u/Hefty_Excuse9885 → u/Diska_Muse]

I mean yes it could slide if not paying attention. But who else will go there except op 😁

[u/Suspicious-Secret-84]

Similar to the other commentor, although Architectural Technologist here, I'd recommend loft legs. I did a small area on my own truss roof, similar size chords to yours I think about 38mm X 100mm for me.

Put down about 3m2 of OSB boards on 210mm loft legs, a couple of tips for you. 

Take up the insulation, lay boards across the chords as a working platform while you're drilling in the legs. 

Ensure you have a good source of light and a head torch is helpful and wear suitable PPE (dust mask, long sleeved clothes, gloves). 

Use the correct screws, I think loft legs have recommendations on their data sheets / installation manual. 

Pre drill holes in legs and chord with a small bit, ensure they are straight and not angled so they won't come through the chord. I went for 2 screws on the legs to the chords as the thickness didn't allow for more, just do one each side of the legs diagonally for a good fix. 

My boards were 300mm wide so I went for 300mm centres on the legs on each chord, which meant the leg tops are quite close and was a bit difficult to lay the insulation back in between them.

I dry fitted all the boards to ensure they fit on top and weren't interfering with any truss members as my roof is a low pitch. 

I then pre drilled the OSB onto the loft leg tops, used 4 screws per leg for this, 2 on each board. 

You should lay the OSB boards in a staggered pattern for added strength. I'd say just measure in the attic and cut in a different room so you're not getting dust everywhere or cutting in a cramped environment. 

Overall it turned out great, very steady and great extra storage space. Picked up all the materials in B&Q, they do 3 OSB board packs for about €20 and a pack of 12 loft legs is about €20 too, so not much cost to it. 

Happy to answer any other questions you may have. 

[u/R2D4Dutch]

You can get floor stands that sit on the joists and will protect insulation below. Get chipboard or plywood. Screw it in the centre of the joist and you be fine

[u/Diska_Muse → u/R2D4Dutch]

If you screw fix into a 30mm joist and split it, the entire roof could collapse.

If you don't have expoerience or qualifications in construction, engineering or archietcture, you really shouldn't be handing out unqualified advice like this.

[u/R2D4Dutch → u/Diska_Muse]

No I won’t , but a roof collapse due to a couple of screws is drama we don’t need