That's a lot of Joists....like A LOT!

[SuperJohnG]

My SIPS kit is going up. I hadn't bothered too much about joists before other than to check they were sufficient to stop any bounce. 

 

But now as the kit is going up I'm noticing there seems to be A LOT. They have doubled up joists and 400mm centres. so roughly 200mm spacing between the them, they are 254mm Joists. 

 

The designer said that was what the programme spat out...which is fine. It would seem the floor should be solid which was my main query initially which it definitely will be I assume, but I hadn't thought of any downsides. But now seeing it, looks like it will be a PITA to get insulation between the joists, then I'll have big areas with no insulation due to the joists.

 

So a few questions:

 

1) Does this seem like a lot of Joists? (Note  no load bearing walls on any joists)

 

2) How best to insulate between them? and will it be effective considering I cannot do where the joists are.  I have rooms above the garage (which are outside the thermal envelope) so that would be my concern there. I have my master bedroom above the lounge and whilst heat loss isn't a concern there, noise transmission would be. I was planning on using resilient bars for the PB there anyways.

 

3) I assume now running services will be a pisser also?

 

  

[Thorfun]

what's the length of that span? 

 

we have 6.5m spans in our master bedroom and I requested a maximum deflection of 8mm and they came up with 300mm spacing and 304x147mm TR26 pozi-joists

 

they're not doubled up either. obviously it's a bit late for you now they're in but doubling them up like that does seem rather excessive.

 

I am also a bit concerned about the 300mm spacing which will leave about 153mm between edges of joists but the timber frame company said running pipes and utilities etc shouldn't be a problem and the floor will be rock solid. I guess I'll find out in a few months when the TF is up and I have to install the MVHR system!

 

good luck and let us know how you get on.

[AliG]

For above the garage, if you have the height it will probably be better to put PIR under the joists, it will be almost impossible to properly insulate the floor otherwise.

 

We once had a kitchen above the garage and it was always colder than the rest of the house so it is important. I would watch particularly for cables and pipes causing draughts in between and make sure it is well sealed.

 

For the master bedroom, resilient bars will probably be enough. You could put some acoustic rockwool in there, but according to the tables it only adds 2dB to noise reduction anyway. The bars and sounbloc plasterboard would make a bigger difference. Again though a lot of the sound transmissiom will be flanking sound behind walls and it is at least as importnat to seal any paths the sound can travel along.

[SuperJohnG]

  On 25/05/2021 at 13:05, Thorfun said:

what's the length of that span?

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It's 6m span, I had checked the span tables and a 304 would have done it at 400mm centres. seems excessive for sure. I'll let you know how difficult it is to turn an MVHR pipe in there!  

 

  On 25/05/2021 at 13:29, AliG said:

For above the garage, if you have the height it will probably be better to put PIR under the joists, it will be almost impossible to properly insulate the floor otherwise.

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Seems definitely like the logical approach. 

 

For the master I'll try and get some insulation in there, albeit it is 2dB a change by 3dB is actually a halving of the sound power. So it's significant. Soundbloc and resilient bars being the best bet.  I'll need to do a wee bit of research on flanking sound, thanks. 

 

[Thorfun]

just to concur with @AliG our architect has specified insulation below the joists in the garage for the room above as per this drawing:

 

 

definitely the logical approach. ?

[Mr Punter]

I am pleased to see people specifying 8mm max deflection.

 

If you need to install rigid pipes or ducting through these, do it NOW.

 

You could blow in thermal insulation - either fibre or polystyrene.  If you do the latter you will not be able to have PVC cables in that zone.

[Thorfun]

  On 25/05/2021 at 16:50, Mr Punter said:

You could blow in thermal insulation - either fibre or polystyrene.  If you do the latter you will not be able to have PVC cables in that zone.

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that's a great option.

[SuperJohnG]

  On 25/05/2021 at 16:50, Mr Punter said:

 

If you need to install rigid pipes or ducting through these, do it NOW.

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Hadn't thought of that till now. Fortunately I'm using flexible pipe and ducting. But I do also still have access at the stairwell Fortunately as I will need to 4" wastes from upstairs..  but all generally short.. it'll just be like a big horizontal game of connect 4. 

[MortarThePoint]

  On 25/05/2021 at 16:50, Mr Punter said:

I am pleased to see people specifying 8mm max deflection.

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In terms of people walking on a floor and a perception of bounce less deflection is always going to help, but a short span that has 8mm of deflection will bounce much more than a long span with a deflection of 8mm. I presume the deflection figure is based on 1.5kN UDL live load. You'll effectively have a mass/spring/damper arrangement and with a larger span you'll have more mass, spring and damper. That may or may not help from a resonant frequency perspective.

 

Hand waving and most importantly ignoring effects of resonant frequency:

    A 6m beam with 1.5kN/m2 * 0.4m UDL applied has a peak moment of 2700NM [1kN/m2: 1800]

       - Adding a 1kN point load midspan increases that to 4200NM, so 56% more [3300, 83%]

    A 5m beam with 1.5kN/m2 * 0.4m UDL applied has a peak moment of 1875NM [1250]

      - Adding a 1kN point load midspan increases that to 3125NM, so 67% more [2500, 100%]

    On that basis, if both floors are designed to have the same max deflection, a 5m span that has the same deflection specification will feel 20% more bouncy.

 

I suspect it's more important to understand floor dynamics (which I surely don't) than set a specific maximum deflection value. However, setting a maximum deflection figure is probably more important with shorter spans.

Edited May 26, 2021 by MortarThePoint

[MortarThePoint]

  On 25/05/2021 at 11:55, SuperJohnG said:

1) Does this seem like a lot of Joists? (Note  no load bearing walls on any joists)

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If you'd doubled up again you wouldn't need any OSB3 ?

 

It'll be nice a firm though

[Mr Punter]

  On 26/05/2021 at 08:56, MortarThePoint said:

 

In terms of people walking on a floor and a perception of bounce less deflection is always going to help, but a short span that has 8mm of deflection will bounce much more than a long span with a deflection of 8mm. I presume the deflection figure is based on 1.5kN UDL live load. You'll effectively have a mass/spring/damper arrangement and with a larger span you'll have more mass, spring and damper. That may or may not help from a resonant frequency perspective.

 

Hand waving and most importantly ignoring effects of resonant frequency:

    A 6m beam with 1.5kN/m2 * 0.4m UDL applied has a peak moment of 2700NM [1kN/m2: 1800]

       - Adding a 1kN point load midspan increases that to 4200NM, so 56% more [3300, 83%]

    A 5m beam with 1.5kN/m2 * 0.4m UDL applied has a peak moment of 1875NM [1250]

      - Adding a 1kN point load midspan increases that to 3125NM, so 67% more [2500, 100%]

    On that basis, if both floors are designed to have the same max deflection, a 5m span that has the same deflection specification will feel 20% more bouncy.

 

I suspect it's more important to understand floor dynamics (which I surely don't) than set a specific maximum deflection value. However, setting a maximum deflection figure is probably more important with shorter spans.

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That is why I like to specify a maximum deflection of 8mm or 0.002 x span (whichever is least).

[MortarThePoint]

  On 26/05/2021 at 09:41, Mr Punter said:

That is why I like to specify a maximum deflection of 8mm or 0.002 x span (whichever is least).

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But I expect the 8mm specification is excessive for large spans as I have calculated. It's probably vital for a feeling of quality at lower spans, e.g. <=4m

[LA3222]

I'd second the getting pipes in now. I know you said you're using flexible pipes for MVHR etc but what about sink wastes etc, toilet.

[Thorfun]

  On 26/05/2021 at 09:49, LA3222 said:

I'd second the getting pipes in now. I know you said you're using flexible pipes for MVHR etc but what about sink wastes etc, toilet.

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it's a great idea but is anyone really that organised that they know exactly where all the plumbing waste is going to be BEFORE the first floor walls are up? pretty sure I won't be. 

[joe90]

  On 26/05/2021 at 10:11, Thorfun said:

but is anyone really that organised

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I was but only because I am OCD at planning stuff, and I gave myself some leeway in case of minor errors. Most of my drain runs were in line with joists so no problem with fall and access. On the sound insulation front, it’s the only thing I would change if possible (to late now), I wish I had double boarded or/and sound bars fir the ceilings, despite 100mm of insulation between joists I am disappointed with the sound travel between floors (and I was careful to fill any gaps around the floors).

[Mr Punter]

  On 26/05/2021 at 09:43, MortarThePoint said:

 

But I expect the 8mm specification is excessive for large spans as I have calculated. It's probably vital for a feeling of quality at lower spans, e.g. <=4m

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Which is why it also needs to be less than 0.002 x span, or are you saying 8mm is too strict for longer spans?

[MortarThePoint]

  On 26/05/2021 at 10:24, Mr Punter said:

Which is why it also needs to be less than 0.002 x span, or are you saying 8mm is too strict for longer spans?

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Yes, I think 8mm is probably too strict for longer spans. I know the normal guidance is 12mm (and 0.003x span) and for longer spans it's probably less important to tighten that figure.

Edited May 26, 2021 by MortarThePoint

[LA3222]

  On 26/05/2021 at 10:11, Thorfun said:

it's a great idea but is anyone really that organised that they know exactly where all the plumbing waste is going to be BEFORE the first floor walls are up? pretty sure I won't be. 

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I would recommend it, I had a real issue getting waste pipes in because I didn't think to beforehand . 

[joe90]

I have no idea what my deflection calcs were for my floors but 12mm sounds an awful lot, my builder agreed with me on solid feeling floors, calcs done with 600mm spacing but installed at 400mm spacing and it feels very solid.

[MortarThePoint]

I'll hold my hand up as a bit of a hypocrite here as I got my garage RIR trusses designed based on 2.5kN/m2, but they have a clear centre of 4.5m and overall span of 6.5m

 

I am surprised you have ended up with so may joists as those attic trusses are 222mm on 600mm centre. Shows the benefit of those vertical members in an attic truss

Edited May 26, 2021 by MortarThePoint

[MortarThePoint]

  On 26/05/2021 at 10:30, joe90 said:

12mm sounds an awful lot

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It's a lot of a short span, but on a long one its pretty small. Bear in mind its not going to be deflecting up and down on a daily basis but is going to get loaded out and deflect. I think you'd fall over if a floor deflected by 12mm whilst you walked on it.

 

@SuperJohnG could probably hold River Dance lessons on that floor.

Edited May 26, 2021 by MortarThePoint

[Pocster]

  On 26/05/2021 at 10:11, Thorfun said:

it's a great idea but is anyone really that organised that they know exactly where all the plumbing waste is going to be BEFORE the first floor walls are up? pretty sure I won't be. 

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Nobody knows where anything is going until you do it ??

It all started off with 1 red vent pipe - looked very neat . Then everything mated and multiplied like a virus 

 

 

Edited May 26, 2021 by pocster

[SuperJohnG]

  On 26/05/2021 at 09:01, MortarThePoint said:

 

It'll be nice a firm though

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  On 26/05/2021 at 10:38, MortarThePoint said:

could probably hold River Dance lessons on that floor

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I can confirm it is totally solid, I'm pretty sure the cast of River dance plus their cousins and It'd still not be at 8mm deflection. 

 

  On 26/05/2021 at 10:11, Thorfun said:

it's a great idea but is anyone really that organised that they know exactly where all the plumbing waste is going to be BEFORE the first floor walls are up? pretty sure I won't be. 

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I know where most of mine will roughly be but not perfectly planned. My bathrooms are directly above the downstairs bathroom and the main soil stack so hoping for minimal runs transverse through the joists. 

[Mr Punter]

At least if you are the one who plasterboards the ceilings you will not struggle to find a joist.

[George]

There are a lot of joists because 254mm deep joists is shallow for a 6m span. That's a span/depth* ratio of ~24 - achievable with a steel beam, but quite an ask for timber.

 

This goes back to the architect setting too small a floor structural zone and on the SE/floor designer for not splitting the spans with a steel beam (although that would make service routing harder).

 

 

*edit for clarity - span/depth ratios aren't for design purposes - they just gives an indication of how big a beam/joist should be

Edited May 28, 2021 by George