Rim Joist (wall plate) marking out: towards a check list

[ToughButterCup]

As I suspected, this learning curve really is pretty steep. So in an attempt to make sure that I reduce the number of errors in establishing our first floor at the correct level, I thought it might be an idea to start a thread - hopefully I'll be able to summarise it as a check list later.

 

First: the end in mind

Accurately mark out the position of   the top of the wall plate (rim joist) such that our 202 POSIs sit at the correct height.

 

 

Follow this bit of maths through with me please:

Assuming the top of the POSIs is the same level as the  top of the wall plate , it (the rim joist) needs to be 20mm  (the depth of the decking) below FFH; (2350+255); that is (2605-20) = 2585mm above FFL

 

In order to mark it out, since our floor is currently 25mm below FFL (currently we are at sub-floor) we need to add another 25mm = 2610mm above our current floor

 

Naturally, it isn't that simple is it?

Our timber spec is C24, treated, 200 by 50. Well wadya know? Timber of that spec isn't.  It's less than 200 and less than 50 (195 by 45 ish). Which means the bottom of our POSIs are going to be hanging low (to coin a phrase). Our hanger spec is 200 by 50

 

Does it  matter that the bottom of the hangers are going to be below the bottom of the wall plate?

[RichS]

No, as long as they are securely fixed it won't matter a jot. Timber sizes given are always nominal, 195 x 45 is normal for a 200 x 50. Likewise 100 x 50 will be 95 x 45.

[MikeSharp01]

The FFL needs to be checked with the architect as they may have the FFL as the top of floor finishes (wood / carpet/ tiles/ screed) or the top of the sub floor onto which the finishes go. If the former then you need to also add in the thickness of the floor finishes.

[ToughButterCup]

  On 28/09/2017 at 11:48, MikeSharp01 said:

The FFL needs to be checked with the architect as they may have the FFL as the top of floor finishes (wood / carpet/ tiles/ screed) or the top of the sub floor onto which the finishes go. If the former then you need to also add in the thickness of the floor finishes.

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Thats what I need.... GOTTCHAS... Thanks @MikeSharp01.

[Lesgrandepotato]

Are there steels involved? Are you notching under or over the steels?

 

i 'upset' our builders by insisting we did these differently in different bits of the house. End result is right though. 

 

[ToughButterCup]

  On 28/09/2017 at 13:00, Lesgrandepotato said:

Are there steels involved? Are you notching under or over the steels? [...]

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Notching? 

[Construction Channel]

FFL usually does include the finishes, does kind of show it on the little set of lines mixed up in the numbers in the bottom right.

 

TBH the bit you are going to worry about more is the head height in the bathroom so it might be an idea to work down from your top plate. 

 

do you know what the numbers next to WPH and FFH are referring to?

 

 

[ToughButterCup]

WPH =  wall plate height  (yeah, yeah, I know)

FFH = First Floor Height.

 

You are right about headroom. It was forced on us by the planners. Bless them.

[Construction Channel]

  On 28/09/2017 at 16:50, recoveringacademic said:

WPH =  wall plate height  (yeah, yeah, I know)

FFH = First Floor Height.

 

You are right about headroom. It was forced on us by the planners. Bless them.

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 I got that bit, its the 30.255 and 31.80 I was trying to work out what they refer to?

[RichS]

  On 28/09/2017 at 17:00, Construction Channel said:

 

 I got that bit, its the 30.255 and 31.80 I was trying to work out what they refer to?

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Datum levels, AOD

Edited September 28, 2017 by RichS
Clarity

[Construction Channel]

  On 28/09/2017 at 17:31, RichS said:

Datum levels, AOD

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That's just ridiculous. Why put them all on there. Surely one would have been sufficient. 

 

Thanks for the answer though. And I suppose that answers Ian's question. The top of the rim joist should be 30.233 above sea level or whatever your datum is set at. 

[MikeSharp01]

  On 28/09/2017 at 18:19, Construction Channel said:

30.233 above sea level or whatever your datum is set at.

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Should read 30.255 and 30M is a long way down. They must be working from some fixed datum a long way away unless its the bottom of the piles, it was piled I think. The other thing to check is how the current unfinished floor on the ground actually compares to that TOC value if it was a planning condition.

[Construction Channel]

  On 28/09/2017 at 18:30, MikeSharp01 said:

Should read 30.255

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You haven't taken off the chip board ?

[ToughButterCup]

Quoting  from this afternoon's  email exchange with Sam our architect.....

'...The FFH we've shown is to the presumed finish of the flooring. The 255mm for the floor zone is made up of 15mm plasterboard and skim, 202mm posi joists,  38mm in total for sheathing and floor finish (i.e 18mm flooring OSB 3 and 20mm total for the bamboo flooring and underlay).

 

And, while we're at it , what's a TOC value? @MikeSharp01 ?

 

[PeterW]

What's your joist spacing ..???

 

18mm is only good to 400mm centres, 22mm is much stronger and you will notice the difference and it's not much more cost wise either. 

[ToughButterCup]

Ermmmmm.... 600mm.... 

[PeterW]

Right.... so you have to use 22mm..!!

 

Architect should know that ..!

[ToughButterCup]

Oooooo errrrr....   Have to ?  Sure?

[Construction Channel]

I assumed TOC was top of concrete. 

 

Even if if you haven't GOT to. You'll wish you had. 

[ToughButterCup]

  On 28/09/2017 at 19:17, Construction Channel said:

I assumed TOC was top of concrete. 

 

Even if if you haven't GOT to. You'll wish you had. 

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Story of my life....

[PeterW]

The maximum span for 18mm P4/P5 chipboard is 450mm, the maximum for 22mm is 650mm. 

 

Just re-read the spec though and he's specified OSB3... that does go to 600mm span but it's more expensive ..! It also is not as easy to get hold of as T&G boards for flooring....

[MikeSharp01]

@recoveringacademic TOC is top of concrete and the value is the 27.255 height from your drawing. I assume the 27.255 dimension is to the bottom of the piles because if it is from some other survey datum point it may be worth verifying it so you can be sure everything is correct in relation to the datum if it is not the bottom of the piles.

[MikeSharp01]

  On 28/09/2017 at 18:40, Construction Channel said:

You haven't taken off the chip board

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Sorry, just the drawing value, no sums done.

[Construction Channel]

  On 28/09/2017 at 20:26, MikeSharp01 said:

Sorry, just the drawing value, no sums done.

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don't panic, turns out I was wrong anyway, I should have put 30.217 but i didnt realise Ian was planning to use rubbishy 18mm flooring......  

[joe90]

My builder is like me and does not like 600mm joist spacings for first floor, he spec,d 400mm joist spacing with 22mm T&G chipboard. Proper job, no bounce.