# Forum Home Renovation Structural Renovation  Joists/rafters - 450 or 600 centres

## Cuppa

I am at the stage of wanting to buy the timber to frame my granny flat inside the shed.  Ive looked at span tables & am having trouble deciding whether to use 450mm or 600mm spacing for the joists/rafters. These will form the ceiling for the dwelling & provide a mezzanine floor for general storage. They will be covered with gyprock underneath & yellow tongue type floor sheeting above. I  expect that a mezzanine floor capable of supporting the same as a normal domestic floor would suffice.  
Rafters/joists will be MGP10 240 x 45 spanning (single span) 3900mm (including the width of the supporting walls - 90mm x 2). 
According to the span table http://www.chhwoodproducts.com.au/us...e%20Set(3).pdf _max_ span for this size timber using 600mm spacing is 4300mm with a floor loading of 40kg/M2 & _max_ span @450mm spacing is 4800mm.  (Local supplier does not carry 240x35).  
What I am unsure of is what sort of floor loading the gyprock & yellow tongue will give. I dont have any doubt that 600mm spacing will hold things up, but what Im not sure about is whether it will give sufficient rigidity to prevent the plaster underneath cracking when the mezzanine floor is being walked on. 
I will go with 450mm spacing if I need to (even though I already have a number of packs of insulation batts intended for 600mm centres) but doing so will cost an additional $450 so I would prefer not to be over engineering just to be safe.  
Advice welcome ........... please. 
Thanks, 
Cuppa.

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## METRIX

Simple, Yellow Tongue can only span 450mm, if you want to span 600mm you need to use 22mm Red Tongue or 25mm Blue Tongue

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## Cuppa

Hi Metrix, that’s why I wrote ‘yellow tongue type’. Maybe I should have written particle board flooring?  I understand that yellow tongue is 19mm & 600mm needs 22mm minimum. If I go with 600mm I_ will_ use the thicker flooring. 
However the primary thrust of my query is whether the 600mm spacing will provide sufficient rigidity to prevent the gyprock underneath from moving enough to crack?

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## barney118

I built a deck once with 240 x45 joist spanning 4.1m and found it bouncy, the other end of the deck was 3.6m and not bouncy. Seen as you are designing a floor with additional weight of ceiling I would imagine it too would be bouncy. If I were making this I would use ijoist with 90 mm flanges for better fixing of ceiling and particle board. Download the chh software design IT.   
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## Cuppa

> I built a deck once with 240 x45 joist spanning 4.1m and found it bouncy, the other end of the deck was 3.6m and not bouncy. Seen as you are designing a floor with additional weight of ceiling I would imagine it too would be bouncy. If I were making this I would use ijoist with 90 mm flanges for better fixing of ceiling and particle board. Download the chh software design IT.

  Have just had a look on the DesignIT iPad app.  (Can’t run the full app on my Mac).
Smallest ijoist (Hyjoist) with 90mm flanges is HJ240 90 which it tells me will span 3900, (with 450mm centres, ceiling attached & 40kg/m2 dead load, & 1.5kPa/1.8kN live load) with a Rigidity ratio of 1.9, & a capacity ratio of 3.0
So certainly more than the minimum  AS1684.1 standards  
...BUT I still have absolutely no idea what this really means. It sounds like it’s over specified so shouldn’t be too bouncy - (but then I thought the 240x45 MGP10 was going to be OK. Arghhhhh!)  *What Rigidity Ratio should I be aiming for given that these results are calculated with ‘ceiling fixed to the underside’ included? 
EDIT: * I have now been exploring the ijoist possibilities further (my head is spinning) including looking at prices. It appears that prices are reasonably comparable to MGP10 timber which is a positive. ‘Deeper but narrower’ seems to offer more rigidity than ‘shorter & wider’ for lower cost 
HJ300 63 would result in a rigidity ratio of 2.0 & Capacity ratio of 3.0 (very close to 240 90 for around $2 per metre less).  
As the ‘300’ is closer in price to what I was budgeting for mgp10 240x45 that’s the way I’m leaning if I go with ijoists 
I’m assuming that 63mm might be a reasonable compromise between cost & ease of flooring installation compared to the 90mm? (is the reason 90mm is easier because of the width to kneel on?) 
Again would appreciate your thoughts as to whether I’m on the right track.

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## Pulse

you don't need 90 for fixing, 45 is fine  
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## r3nov8or

You won't be doing much kneeling on the joists, more so on the boards you are fixing. If it's sometimes necessary and can't be done  comfortably just lay down another temporary board/offcut to kneel on

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## Cuppa

Thanks folks. I think I’ll probably go for the 300x63 hyjoists (unless advised otherwise!). It *may* be overkill, but at least I’ll be confident they will do the job. The 300mm depth will also help a bit in regard to an LVL ‘bearer’ I’ll be using - attaching hyjoists to one side with face mounted brackets - (I already have the LVL - bigger than required, but I got it at a good price). It is 2  360x45 which I’ll vertical nail together. With a flat floor above the ceiling this will mean the beam will only project 60mm below the ceiling level which I think is acceptable in a 2.4m ceiling height.

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## Moondog55

Why not F17 LVL in 190 * 45 at 450mm centre AND use the thicker flooring as well?
I priced LVL yesterdays and 190 * 45 at Bunnies was a reasonable cost compared to hardwood or softwood and it comes nice and flat and straight
You are allowed to interpolate the tables and if 170*45 is rated to span 3600 I'd be happy to use 190 * 45 over 3720mm as I don't count the walls as part of the free span according to the book open in front of me
I would block the ends and centres to stop them rotating
Big difference in timber cost between a 190mm deep and a 240 deep
I'd need a drawing to understand the comment about the 'bearer'

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## Cuppa

> Why not F17 LVL in 190 * 45 at 450mm centre AND use the thicker flooring as well?
> I priced LVL yesterdays and 190 * 45 at Bunnies was a reasonable cost compared to hardwood or softwood and it comes nice and flat and straight
> You are allowed to interpolate the tables and if 170*45 is rated to span 3600 I'd be happy to use 190 * 45 over 3720mm as I don't count the walls as part of the free span according to the book open in front of me
> I would block the ends and centres to stop them rotating
> Big difference in timber cost between a 190mm deep and a 240 deep
> I'd need a drawing to understand the comment about the 'bearer'

  Thanks for your thoughts, Ill look into it further. However a quik squiz at the designIT app, shows 190x45  Hyspan + as having specs only just over the minimum requirements regarding Rigidity, which makes me think it could be a little too bouncy for a gyprock ceiling underneath. I could only find 140 x 45 F17 LVL on Bunnies web site, 6m lengths which worked out at $12.32 a metre so I would expect the 190 x 45 to be a special order (in 6m lengths?) and cos at least as much as 300x63 Hyjoists probably more. (I havent priced the Hyjoists locally yet but prices online suggest  $13.34 a metre). Delivery is another issue. Im hoping I can get them, (44 x 3.9m) & a full pack of 90 x 45 x 4.8m delivered by a crane truck so they can be placed inside my shed, without being in the workspace, & without me needing to move the lot by hand. I dont know how your Bunnies operate, but I would be very doubtful if my local one could manage this. With the flooring, having thought about it more, I would prefer to keep to the 19mm yellow tongue rather than going heavier. Im pushing 60 & not very big & will be working alone most of the time (wife gives a hand if I need it at weekends) so ease of lifting the boards up are a necessary part of the equation too (& thus the reason for choosing joists at 450 ather than 600 centres). 
The dotted line in the diagram below is what I have referred to as the bearer. It will sit between two stud walls & will support the end of the joists running across to the kitchen wall (3.9m) & the joists on the other side of it (bedroom side) will run parallel to it. (also 3.9m).

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## phild01

3.9m span:
 MGP10 240x45@600 8mm deflection from dead load dynamic response ratio 0.81
                  MGP10 240x45@450 6mm deflection dynamic response ratio 0.67 
                  Smartlvl 15 240x42@600 5mm deflection dynamic response ratio 0.60
                  Smartlvl 15 240x42@450 4mm deflection dynamic response ratio 0.51
                  Smartlvl 15 240x58@600 4mm deflection dynamic response ratio 0.46 
SmartJoist  300x40 @600 5mm deflection dynamic response ratio 0.54 
I would choose the 240x42 lvl at least, they perform better than MGP for floor joists.

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## Pulse

I'd use 240x45 hyjoists, performance doesn't matter, no dynamic loads, 20mm ceiling deflection you don't notice, our old place had 60mm-80mm before we retrofitted hanging beams.  
hyjoists are light and cheap  
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## Cuppa

Thanks phild01 & Pulse. I’ll check what the price differential between 240x42 lvl / 300x63 (& 45)  & 240x 45 hyjoist is. Pulse I’m thinking if the extra cost for 300 deep hyjoists isn’t too great it might be worth it just in case I want to store anything heavy up on the mezzanine floor in the future I’d be able to do so without concern.

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## Moondog55

Fair enough comments and as a confirmed TA of a similar age I agree with choosing the best value
The deflection may well cause cracking in the plaster if using the marginal size

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## phild01

SmartJoist 240x54 @600 7mm deflection dynamic response ratio 0.69 
Maybe go bigger for less floor bounce. 
What are you using for the beam?

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## METRIX

Also depends if anything else is being put on this beam, ie: Roof load ?, or you said it's inside the shed does this mean the roof is independent ? 
Also how are you dealing with the vapor barrier for the slab, or did your shed slab have one installed when it was poured ?

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## Cuppa

Beam is overkill as I got it (them) cheap. 2  360x45 LVL’s. which I intend nailing together. How I’ll get it up is yet to be worked out! 
Nope, no roof load, independent structure inside shed.
Slab had plastic laid underneath it. When choosing a company to get the shed from I was astonished to be asked if I wanted plastic laid underneath it by one of them. I wouldn’t have dreamt that anyone would lay a shed slab without it!   
Yes I know many of the ‘chairs’ are missing. The concrete trucks were driven onto the slab site (over the reo). The chairs were added progressively during the pour.

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## METRIX

As a shed is classified as a non habitable structure you would treat these as you would a driveway or carport, therefore vapor barrier is not necessary. 
I would always put a VB irrespective, you will lift the LVL easily, two people needed.

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## Moondog55

You can raise the beam the same way I did mine using the old Egyptian technique
Except rather than a block at each end I used my big load straps, I have enough of them to support and raise
It is a slow process tho, raise an end a bit with the big strap, support with the medium, unlatch big strap raise a bit more; go down the other end and repeat until at the right height and using the shed structure to hang it all off
Or you could use a few hundred milk crates lifting one end at a time

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## METRIX

All you need is two people (including you) lift one side into position, have one person stabilize the beam and ensure it does not slip off, the other person lifts the other end, that's it. 
It's only a 360x45 @ 3.9M weight is bugger all.

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## Moondog55

> All you need is two people (including you) lift one side into position, have one person stabilize the beam and ensure it does not slip off, the other person lifts the other end, that's it. 
> It's only a 360x45 @ 3.9M weight is bugger all.

  I agree that if you have two people it's much easier but OP has just himself; as did I on that occasion; some things are not just twice as difficult on your own but much, much harder and I wouldn't call the mass of a 2 X 360*45 LVL negligible myself even if they are lifted separately and nail laminated  after placing

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## Cuppa

Yes, if only one 360x45 I know my wife & I could manage without too much drama.  
My preference is to nail them both together on the ground. It _might_ be possible to get some help with lifting when the time comes, but being fairly new to the area doesn’t help in that respect. If not something along the lines of Moondog’s strap suggestion with my wife & I together should work.

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