# Forum Home Renovation Sub Flooring  Joist overhang

## Mayfield0

Hi guys 
I'm building an extension and I just got my plans back from the drafty and was wondering how much overhang you can have on the floor joists?
the proposed ext is 4M so that would be 500mm over hang on each side? what you guys think?

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## johnstonfencing

Hello, 
I'm in no way an expert in this area, but just an observation.
Looking at your attached plan your joists look like they run from existing house out so they would run through onto existing bearer!? And at the other end there doesn't look like there is any need for overhang at all.  
Your bearer spacing a are max 1500 centres so the joist span from the closest new bearer to your existing house is approx 700.
As I said just my observation but I'm sure some more experienced will reply sooner rather than later. 
Cheers

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## OBBob

There's a load bearing wall sitting on those joists I assume ... I would've thought you'd want a bearer pretty much below that wall unless it was particularly designed with that in mind.  
I'm a bit confused what on those sketches is driving your question ... is it the max 1500mm span? Just add another row of stumps to get you to 4m if necessary.

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## r3nov8or

In the bottom diagram section it says F17 120x35 HW joists @ 450 centres. In the AS span tables these can span to 2400 in continuous span (like yours is). So 1500 is a very safe design. The AS allows for cantilever of 25% of the allowable span, if you think you need to, but I don't think you do... 
The top diagram doesn't seem to be to scale, if the joist span is 1500 for a total of 4000 I would have expected the first stump closest to the existing house to be further away. Anyway, and as mentioned above already, the new joist could/should be fixed to the existing bearer, which negates your cantilever/overhang concern entirely, and helps to tie the structures together.

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## OBBob

> The AS allows for cantilever of 25% of the allowable span ...

  
I'll preface this by saying I'm just debating for mu own personal education.  :Smilie:  Isn't that 25% allowance for floor loads only, which I took to mean it couldn't support a roof load bearing wall? Although I'm having trouble visualising that situation.  :Smilie:

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## r3nov8or

> I'll preface this by saying I'm just debating for mu own personal education.  Isn't that 25% allowance for floor loads only, which I took to mean it couldn't support a roof load bearing wall? Although I'm having trouble visualising that situation.

   Yes, I would agree. Maybe an example is similar to this one where the joists aren't attached to the existing building for some reason. e.g. maybe in cases of existing solid brick footings that you don't want to mess with

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## Mayfield0

Thanks for the replies. The part circled is the proposed extension, the existing house has concrete walls.
 I'm just wondering if I place the stumps 1500 apart between the 4M extension that would leave 500mm overhang either side would that be OK or would it be better to do the stumps 1M apart with no overhang or is there another way that would be better? I'm just looking for ideas 
Thanks guys

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## OBBob

Stumps are cheap, I'd just do an extra row if the only other choice is overhang.

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## pharmaboy2

No you can't, your joist ends are holding up a load bearing wall - cantilevers are inappropriate

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## Mayfield0

The picture above doesn't show it but the proposed extension floor is 300mm above the existing dwelling concrete wall, could i dynabolt a bearer to this wall as crudely illustrated below, so as to not have an overhang on the joists and not need 4 rows of stumps?

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## Snipper

Yep.  I would   :Thumbsup:

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## pharmaboy2

> No you can't, your joist ends are holding up a load bearing wall - cantilevers are inappropriate

  Hang on - you're cantilever is to the inside?  So scrub the above advice, it's not load bearing - the roof should be over the existing house.   
Yes, attaching the bearer is a good way of doing it - just be mindful of your termite barriers

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## Mayfield0

Thanks for the replies guys The extension is a decking with a toilet and laundry attached here is another page of the plans

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## r3nov8or

OK, it's a deck now. My guidance on maximum joists spans was for a solid floor, and max joist spans for decks can be less.

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## OBBob

> OK, it's a deck now.

  
LOL ... PG would be most upset, we even got pictures on this one and still ended up down the wrong rabbit hole.  :Smilie:

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## r3nov8or

> LOL ... PG would be most upset, we even got pictures on this one and still ended up down the wrong rabbit hole.

  I spot it now. There is one mention of deck in the first pic, near the bottom  :Smilie:  
Personally I would be happy with a small cantilever of the deck at the existing concrete wall, except where the WC is. Also be careful about your bearer and joist layout to accommodate the plumbing for the toilet, unless you are going with a P-trap out the wall side.

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## Mayfield0

Thanks for the input I really appreciate it

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## Mayfield0

My friend who is a retired builder has looked at the plans but disagrees with the choice for hardwood for the bearers and joists and has suggested cypress pine instead, I suppose i'll have to run that by the surveyor, what do you guys think? Is cypress pine a better choice?

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## r3nov8or

Never seen cypress pine as structural timber in my area. Radiata pine, hardwood and LVL is common

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## phild01

A good reason for cypress is durability and termite resistance.  I have seen it used structurally in the past but not sure how you would go about getting stress graded stuff now.
  Did you ask why he made this suggestion?
Was he thinking timber stumps?  _....I do like cyprus._

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## Marc

Structural 
Only F5

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## phild01

> Structural 
> Only F5

  yep, studs and plates.

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## Mayfield0

> A good reason for cypress is durability and termite resistance.  I have seen it used structurally in the past but not sure how you would go about getting stress graded stuff now.
>   Did you ask why he made this suggestion?
> Was he thinking timber stumps?  _....I do like cyprus._

  He made the suggestion because its cheaper

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## phild01

Bear in mind Cypress is only easy to work when green (unseasoned). Also the sizes might need to be bigger because of the grading.

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## OBBob

> He made the suggestion because its cheaper

  
It's not that big an area ... do a compare between the two options.

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