Loft floor

[PaulCrowther]

Hi everybody
I am currently thinking of installing a loft floor by myself .
It will be 5m clear span with 600mm centres using C24 2500mm x 75mm x 225 mm timber butt jointed together to make 5m joists because its the only way i can get the timber in loft.
I was planning to butt joint using splice plates (550x98x18)
will this work?

 

[JohnD]

No.

 

[PaulCrowther]

Thanks John, any reason why its ok to do this for a joist repair but not to create a long span joist out of two shorter timbers?

 

[JohnD]

you have a long unsupported span, with a simple join in the middle. It will break apart at the join the moment you take the support away.

If there was a load-bearing wall under the centre of the span, you could overlap the joists there, but it will not work with an unsupported span. Even a single timber would sag under its own weight.

 

[tony1851]

Butt-jointing timbers with spliced joints works (within reason) if repairing rotted joists at the ends.

But if you are joining them in the middle, that is where the bending moment (and hence, stress) is at a maximum. A simple spliced joint would almost certainly not be strong enough in that position.[/u]

 

[PaulCrowther]

Any ideas how a 5m span be covered by a joist cut in two? maybe splice plates and then a piece of timber on either side bolted together and maybe the butt joints be created at different points on all the joists .

 

[tony1851]

If you want to join two pieces by splicing additional timber to the sides, this is the scale of connection you would need to ensure that the two-part beam has similar stiffness to one full-length beam.
Even then, you would need to have plenty of bolts to reduce the local stress on the timber, and they would have to be carefully spaced.
Typically, a connection of this type would be SE-designed, but obviously that's an addtional cost.
Why not try and make a mock-up and see how stiff it ends up?
Is it really that difficult to get full lengths up?

Untitled

• tony1851
• 29 Apr 2015
• 1

 

[JohnD]

you say it is a 5 metre clear span. You mean there are no walls underneath? Which way do the existing roof timbers run, and what are their dimensions? Photos would be nice.

Are you going to use it as a room, or to store the Xmas tree and your Video?

 

[PaulCrowther]

I have a photo below showing supporting walls ,i might be able to create the butt joint 500-1000 mm away from the timber end and then bolt them together using rsj splice plates
//media.diynot.com/229000_228879_93873_29217491_thumb.jpg

 

[freddiemercurystwin]

Untitled

• PaulCrowther
• 30 Apr 2015
• 1

 

[freddiemercurystwin]

Moderator 11 you're an embarrassment!

FAIL

 

[PaulCrowther]

I am going to use steel plates 500mm x 200mm x 10mm with 8 holes centrally
for 16mm coach bolts and sandwich the butt joint which will be made 1250mm from one end of the joist.

 

[JohnD]

you described it as a 5m clear (i.e. unsupported) span but it has an intermediate supporting wall.

If you lay the timbers to overlap over this wall then the unsupported length will be shorter, which is very desirable.

Are you thinking of turning the loft into a habitable room, or for light storage of Christmas trees?

 

[PaulCrowther]

Hi
the joists are going to run parallel to the large supporting wall you can see in the centre,there is a small section of wall near the loft hatch but i think judging by the over engineered splice plate i am going to use with 16mm bolts ,i just cant see them failing,the loft is going to be used for all our household storage i.e. bulk food buying, tools, wifes wardrobe etc. we will be going up and down most weeks but not spending time up there, just to collect our stuff etc.

 

[JohnD]

the joists are going to run parallel to the large supporting wall

Why?

Use it as a support for the joists.