Nominal v CLS v Regularised timber joist dimensions

[chris_d]

Dear members,

My overly cautious structural engineer has 'helpfully' specified 225 x 63 mm C24 stress graded timber joists for the upper storey floor of my extension. At one end the joists will sit in masonry hangers and at the other they will be built into a steel beam (actually a universal column for convenience).

I wish to achieve a flush ceiling line to make fixing the plaster board easier. Hence, I intend to notch the bottom of the joists where they sit in the masonry hangers and the steel beam so that the underside of the joists finish slightly proud of the underside of the hangers and beam.

I also need to place concrete padstones to bear the steel beam; the finished depth of the timber joists (as delivered) and the block course datum upon which the hangers bear dictate the proposed vertical position of the beam and padstones. Due to labour, weather and storage constraints, the padstones and steel beam must be in their final position before the timber joists are delivered to site. I therefore require the finished depth of the joists to calculate where to place the padstones ASAP.

I don't exactly purchase timber joists every day but I'm aware that C24 stress graded and 63mm wide joists are not commonly used. The manufacturer of the masonry hangers purposely reduces the depth of the hanger by 10mm from nominal to 'allow for regularised timber and notching for a flush ceiling line'. I've heard of the finishing terms CLS and regularised in the context of joists but I'm not aware of what impact that has on their finished dimensions.

Please could a kind soul educate me with regards to the final delivered depth of nominal, regularised and CLS timber joists?

Thanks,
Chris

 

[chris_d]

Some patient Googling has found this from TRADA:

http://research.ttlchiltern.co.uk/pif294/tdk/geninfo/pdf/2-3_37.pdf

which would suggest that the regularlised/machined target dimensions of a nominal 63x225mm joist would be 63x220mm with a tolerance of either -2/+4mm (Tolerance Class 1) or -1.5/+1.5mm (Tolerance Class 2). Furthermore, CLS/ALS timber is limited to 38mm finished thickness only and would not normally be available in the UK at a depth corresponding to 225mm nominal (184mm and 235mm are the nearest finished dimensions of freely available CLS which come from 200mm and 250mm nominal timber).

However, I'm still very interested to learn what the reality is in the UK! Also, can anyone recommend a trusted supplier in Gloucestershire/Herefordshire?

Thanks,
Chris

 

[merlin]

Hello Chris,
In the real world the regularised joists will sometimes come to site with upto a + - 5mm tolerance! so it is worth sorting out and laying where suitable.
Quite oftern you can get fair old camber on them so particually with the 63mm timbers it will be worth running an electric plane over the bad ones before the floor is laid. Trouble is when you lay the floor it will start rocking and even when you glue and nail the boards down you will feel it under foot plus it might start to squeek later on.
You are right as the notching is concerned, I normally cut out so that 6mm hangs below the beam or hanger. (if you cut much more out the Building Inspector will moan as the depth of joist has been reduced).
As far as the CLS is concerned (I think it means Canadien Lumber Stock) I try to steer away from it as goes all over the place , it is a bit small for door linings and it is like cutting and fixing a piece of bread.
C24 is being used more and more now but it is I think worth it as the finished floor will feel more solid.
As far as a good supplier, maybe Travis Perkins or Arnold Laver as they are the ones i would use in the Reading / Oxford areas.
Hope this helps. Merlin.

 

[Deansocial]

i would just buy 9x3 c24 as its easy to get hold of or should i say more common

 

[Deansocial]

as for c24 being used more that is because often you can get away with using the next size down compared to c16