Joist spanning layouts

[Arthur 51]

Hi all. When choosing joists to span a gap I've only considered laying out the joist in 1 direction as set of parallel 'lines'. But if I wanted to minmise using masonry joists and then just fixing the 2 end joists into the walls and then suspending the rest of the joists between these 'end' joists...does this require a whole new set of calculations for the joists?

eg instead of

parallel lines. All joists wall to wall. waaaaaaaaaall | | | | | | | | | | | | | | | | | | | | | | | | waaaaaaaaaall

..this waaaaaaaaaaaaall |----------------------| |----------------------| |----------------------| |----------------------| |----------------------| waaaaaaaaaaaaall

Thanks for any advice.

Arthur

[The Natural Philosopher]

Not quite sure what you are saying here.. The joist calculations depend on the span they are spanning..not on how they are fixed to the wall. If they end up longer, they need to be bigger. I would certainly use a long bit of timber to rest em on screwed into the masonry. Especially if it formed teh top part of dry-lined stid frame undernaath.

Joist hangars are a pain..

[dom]

Then your outer 2 joists/beams are likely to be much more substantial (e.g. very roughly, something like 100*225mm for a 3.5m by 3.5m room)

- and must have stress calculations approved by the BCO.

What's the problem with running all joists wall to wall? If new masonry, incorporating built-in joist hangers is little extra effort. If existing masonry, using masonry fixings for surface mounting hangers is no greater task than all the joist to beam hangers.

[The Natural Philosopher]

Rest the joist on top of the beam, and knock in noggins between em.

No need for hangars at all.

[dom]

Yup - if there's sufficient headroom. It might also be necessary to consider the point loading of the beams on the masonry.

[Arthur 51]

I should mention that if I use the alternative solution waaaaaaaaaaaaall |----------------------| |----------------------| |----------------------| |----------------------| |----------------------| waaaaaaaaaaaaall

Then the (as they appear on this diagram) horizontals will all cross the top of a wall dividing a front bedroom from the back, So either side of this wall (dwarf wall?) the span will be about 2 metres.

Arthur

[dom]

AIUI the OP's proposal is to use a pair of outer timber beams between the masonry walls and then attach all the joists to the beams - i.e. the floor loading is carried entirely by this pair of beams.

Setting the joist to bear on timber beams is easier yes, but using surface mounted joist hangers IME is no great task.

[dom]

Is this going to be a new or replacement floor in the building? Is the wall dividing the bedrooms suitable to be loadbearing?

[Arthur 51]

I don't mind using timber joist hangers as in ..it seems like a lot less messing about than masonry hangers. And there isn't much headroom to play with so sitting joists on top with noggins is a bad choice for me.

Arthur

[Arthur 51]

Its a single brick wall. that is currently supporting the ceiling joists.

Arthur

[Arthur 51]

Sorry. I missed your first question. Its a loft conversion project.

Arthur

[Roger]

The message from " [e-mail]" contains these words:

That's the way I see it as well and in normal circumstances I don't that is an acceptable layout unless the outer beams are much much sturdier than the transverse beams. Just think of the disparity between the typical say 8" square main beams at say 5' centres (or less) and the 4" by 3" transverse beams between them. A layout often found in many old cottages (and which is more springy than a modern floor).

With this layout (assuming 2 equally spaced main beams and an evenly loaded floor) each of the main beams will carry one third of the gross floor load. With the OPs proposal outlined above the 2 outer beams will each carry half of the gross floor load. Contrast that half with the one tenth gross floor load per joist that a 12' wide room with longitudinal joists at 16" centres would have.

[Arthur 51]

On one side of the loft it might be an advantage to fix a support timber to the wall as it will cross the arch formed by the 2 chimneys merging. My guess is it would need to be 1.5" thick but what depth?. What should I use to screw it to the wall? frame fixings?

Arthur

[The Natural Philosopher]

I'd use 6x2

Long screws..probably coach type screws or something you can get a socket on. And either make plugs out of car body filler, or use expanding bolts.

[Arthur2]

Will 12" centres be good?

Arthur

[The Natural Philosopher]

Bloody good.

[Arthur2]

Black humour or an enthusiastic reception of my suggestion? :)

Arthur

[John Rumm]

Whatever you do you will require a set of calculations. If you want to experiment with the effects of different layouts then it might be worth downloading the demo version of superbeam

If the latter has a bisecting wall which is load bearing, then this would probably be the simpler. Hard to say without dimensions.

I used the central wall to split the distance and carry the one end of two sets of joists. It made it much simpler that way.

[The Natural Philosopher]

enthusiastic.

[Arthur2]

Sorry for doubting you, Nat.

Arthur