600mm or 400mm Centres?

[RussH]

We have a timber frame design which is 600mm centres for studs, joists (posi) and rafters.

It will have a standing seam roofing and Cedral type cladding. 150mm of PIR.

 

What are people's experiences or thoughts on 600mm centres? Will the plasterboard flex? Will the structure feel "flimsy/flexible"?

The cost uplift to 400mm isn't horrendous but I guess the downside is additional cold bridging.

[joe90]

I prefer 400mm centres because I hate the thought of flex however i tend to over engineer’s things, my floors were engineered at 600mm centres but installed at 400mm.

[saveasteading]

Roof battens will be thicker for 600mm. Perhaps the roof sheathing too, simply for temporary support of the roofers.  Plasterboard is OK over 600mm too. 

Agree that cold bridging might be significant, so consider the additional insulation.

 

I would have expected changing to 400 would be fairly expensive, esp on labour.

 

Sturdiness and flex are calculated in the design, so don't worry.

[nod]

I’ve 600 mil centres on the last two 

Seam ok 

If you go 400 you will increase labour and material by 25% ish 
Though the three lots of loose rafters I’ve done at 400

[RussH]

Thanks for the reassuring words! I've had a few timber frame quotes and they're all 600mm centres. 

[Mr Punter]

600 for external walls means more space for insulation.

[SimonD]

1 hour ago, RussH said:

I've had a few timber frame quotes and they're all 600mm centres. 

 

Yes, why use 400mm when 600mm will do. Usually specified by structural design. Whole timber frame in 600c/c here but intermediate floor was 400c/c due to span and a little bit o space saving.

[ChrisJ]

If your floor system is 600mm c-c then wall framing can match and you can align everything - even/equal and proper distribution of loading - essentially you are able to point load the entire structure top to bottom - the better building practice 

 

plus anything your chasing through the walls such has MVHR and plumbing have clear run top to bottom - no zigzags to contend with 

[Iceverge]

If bothered you can counter batten your service cavity internally at 400cc. 

 

You tube has shown 1200cc rafter spacing so I wouldn't worry about it falling down. 

[RussH]

Thanks for the replies. I hadn't really considered this until someone told me that it would feel flimsy. The fact that he's never built a timber frame house was a bit of a clue to his expertise I guess😏😂

[Dave Jones]

save yourself a lot of hassle, have 400 GAPS not centres so when you rip the PIR you have zero waste. You may need an extra truss and spend 10 mins making the plasterboard work but will save hours and £££ on the hateful task of insulation ripping.

[Iceverge]

Use 150mm rafters at 600mm CC with full fill batt insulation. Batten and counter batten above a taped/sealed breather membrane to give the required ventilation gap. 

 

Airtighess membrane below then 50mm counterbatten at 400cc to suit plasterboard with a mineral wool insulated service cavity.

 

U value 0.21. Same as your PIR. Make the rafters thicker for better performance. 

 

No cutting at all and almost zero waste.

 

Better for noise, fire, summer heat protection, thermal bypass, airtighess and windtightness, thermal drift and offgassing. 

 

 

[Nickfromwales]

IIRC the wool plus over-joist ( wall ) detail MBC quote is better performing than the beefier cellulose filled 300mm PH offering. Defo go wool inbetween and full sheets of rigid over joists, and then membrane / service battens / plasterboard.

NEVER would I install floor posi's at 600mm o/c. Like a chuffing trampoline.

[saveasteading]

3 hours ago, Nickfromwales said:

floor posi's at 600mm o/c. Like a chuffing trampoline.

Something wrong then. Joists are designed to not collapse, then checked for deflection at 1/360 maximum.

So in a 3.6m wide room they should not move more than 10mm with full furniture load and people jumping.

 

I wonder, do people jump to test posijoists, but don't worry about timber joists? Then when the furniture is placed, a lot of the bounce is taken away and nobody questions it again.

The first time I used galv steel as a floor, I, of course, jumped on it hard and it concerned me how it shuddered. In life it was fine.

 

If it still bounces after loading then it needs a discussion with the supplier.

[Nickfromwales]

1 hour ago, saveasteading said:

Something wrong then. Joists are designed to not collapse, then checked for deflection at 1/360 maximum.

So in a 3.6m wide room they should not move more than 10mm with full furniture load and people jumping.

 

I wonder, do people jump to test posijoists, but don't worry about timber joists? Then when the furniture is placed, a lot of the bounce is taken away and nobody questions it again.

The first time I used galv steel as a floor, I, of course, jumped on it hard and it concerned me how it shuddered. In life it was fine.

 

If it still bounces after loading then it needs a discussion with the supplier.

The suppliers design with a too-fine element of bean-counting. I guarantee you that the floors / spans / deflection they allow when trying to win business is NOT what they'd specify for their own houses!

Every job ( new build ) that I've worked in that had joists "to spec" @600mm were bouncy. One member on here had his done "to spec" and when he walked through the bedroom the fitted wardrobe doors moved / bumped into each other.

Nope. Standard designs / calcs for these are carp, just like British Building Regs....."What's the crappiest thing we can churn out legally?"

[joe90]

Yup @Nickfromwales, minimum standards.

[jayc89]

I used 600s when battening out my external walls. 400s for all internal stud work. 

[Nickfromwales]

Just now, jayc89 said:

I used 600s when battening out my external walls. 400s for all internal stud work. 

400 is mandatory for bathrooms. Floors should have the same minimum standard. 600 o/c is crap for a floor, unless the joists are oversized.

[ProDave]

I trusted this to the SE.  He specified JJI's on the GF at 400 centres where there were no services down there, and Posi joists upstairs at 600 centres.

 

All rock solid no bounce. except for one detail.  Posi'swere specced for the longest span and then continued over rooms with shorter span.  Except in one room where the span was interrupted by the stairs so no need to continue through so one room as the over sized joists on one half and smaller joists on the other.  I wish I had noticed that and specced them the same.

[Dave Jones]

On 06/05/2023 at 10:20, saveasteading said:

Roof battens will be thicker for 600mm. Perhaps the roof sheathing too, simply for temporary support of the roofers.  Plasterboard is OK over 600mm too. 

Agree that cold bridging might be significant, so consider the additional insulation.

 

I would have expected changing to 400 would be fairly expensive, esp on labour.

 

Sturdiness and flex are calculated in the design, so don't worry.

 

not that ive seen. Standard 2x1 .

[MG-Wales]

Structural engineer (and self builder) here: 38x140 C16 studs @ 600mm centres are pretty much the standard for external walls for most building I engineer. I would only expect closer centres if the walls are > 3.2m tall (uninterrupted by floors or ceilings) or in high wind areas, etc..

It's a bit more complicated with 38x89 C16 studs as these require closer centres (e.g. 400mm, or even 300mm) more often. This usually applies to internal walls, however there are a number of companies which use this size for external walls. In any case, I wouldn't be too worried about the performance of domestic height walls as the limiting factor is not usually the deflection but rather the compression of the timber rail under the load from the stud (crushing force). I would generally trust the engineer's design on these.

 

Regarding floors, I second what most people say in the other comments. Typically floors are designed at 600mm or 400mm centres with the most economical design which works within the maximum deflections allowed. Based on designing i-joists (I don't design metal web joists which are always done by a specialist designer) there are usually 2 almost equivalent solutions: with a wider flange joist at 600mm centres and a narrower one at 400mm centres, these would give similar deflection and very little difference in cost. If not happy with the centres specified, I would try to engage with the timber frame company, ask about the deflections allowed and how much would it cost to reduce these. Often to up-spec a floor is a good investment in the overall quality and feel of the building. Keep in mind that (for a series of factors) in the UK we have some of the worst performing floors in Europe, I would always try to not have to deal with squeaky or bouncy floors in the future.