Joist spacing comparison

[zippidydodah]

Hi all,

For a first story addition floor would it be better to use 19mm PB sheeting on joists at 450mm centres or 22mm PB flooring on joists at 600mm centres? I don't want a springy floor.

Cheers, Andrew

[glock40sw]

Stick with the 450 centres. You can still use the thicker PB as well for peace of mind.

[Burnsy]

Make sure you glue with sikaflex or similar as well as nailing to stop squeaking and other movement noise.

[sports fan]

dont forget solid blocking between joists

[ausdesign]

Either option, but the 'bounce' will depend greater on adequate the joist size.

[DvdHntr]

What Ausdesign said. Bounce is dependant on the stiffness of the joists and bearers. The stiffer (deeper or higher grade) they are the less bounce. The flooring can obviously play a role but 19mm at 450 and 22mm at 600 will be little.

[Tubby2]

Either option, but the 'bounce' will depend greater on adequate the joist size.

...and the weight of the bouncer!

[zippidydodah]

Thanks for the replies.

I'm in the planning stage at the moment for a first floor addition to our existing house. The house is a basic rectangle with verandahs all around and built in the mid 1980s. The extension will dupicate the floor area of the existing house and will have verandahs on the first level across the front of the house and at the sides. The wall to wall dimensions are 8500mm x 18980mm.

The wall frames are 90x45 but I've yet to determine the stud spacing. I plan on reusing the existing roof trusses and roof sheeting on the first level though we'll obviously have to put in a new gyprock ceiling on the ground level.

I'm comparing Hyjoists (engineered timber) with fabricated steel joists (Hopleys Steel Joists). I've yet to have the engineering work done to detmine the joist sizes (or even if the existing walls and footings are adequate for that matter) but after looking at the spans required, with a web thickness of 300mm the steel option is marginally cheaper.

I like that there is lots of space between the chords of the trusses to run services with the steel joists compared to cutting holes in the webs of the timber ones.

Any pros and cons with these products?

Thanks, Andrew

[echnidna]

You don't need to replace the downstairs ceilings etc.
Fit the upper storey floor joists above the ceiling joists and don't physically link them together. This method will also transmit far less noise through the upper floor.

[ausdesign]

I think the problem he'll have with the existing ceiling Bob is in reusing the roof trusses.

[zippidydodah]

I did wonder whether it is possible to preserve the existing ceiling by destroying the trusses and leaving the bottom chord in place. I don't think this would work on its own as the bottom chord would be very weak and has to span in some cases the entire width of the house (8.5m) where there are no internal walls. Could the bottom chord be attached to the new joists and use the new joists as a hanging beam?

The internals of the existing house are going to change some what which will necessitate a resonable amount of plaster work to the ceiling in any case.

I suppose it comes down to which costs less, trusses or plaster? And also the inconvenience of having to move out of the house for a while or not.

(this stuff keeps me up at night and my wife wonders why I seem perpetually preoccupied while kicking these scenarios around in my mind!)

Cheers, Andrew

[ausdesign]

Yes as Bob was working towards Andrew.
You could run the new floor joists beside the existing truss - Floor joist spacing would possibly run at 450 centres & the existing trusses at 900.
Nail laminate the bottom chord to the new joist & then remove the webs & cut the top chord flush [or below] the top of the joist.
It may be an option to run a plate on top of the existing plate to raise the joist [35 or 45mm ] to clear any obstacles such as wiring etc.
This is usually done also in case the bottom chord has not loaded fully and/or still has un upward curve which will mean that the new straight joists will put pressure on the ceiling lining - I know what I mean.