Sawn wood sizes

[Rick Hughes]

I thought I had a pretty reasonable grasp of wood sizes ... but latest delivery surprised me.

I wanted a consignment of good old 4x2" sawn ...... so in metric terms this should come in as standard size of 100 x 47

This is not PSE (or PAR) so expected that size to arrive.

What turned up was 96 x 40 ....

Now most of the span tables are based on TRADA standard sizes ... the TRADA standard states for thickness's up to & incl 100mm the permitted deviation is -1mm to +3mm

How do timber yards sell graded sawn wood that is so far off the standard size ?

If you are using C16 or C24 span tables wonder how much you are under spec by the wood being so far off the size ?

[Ian Jackson]

In message , Rick Hughes writes

Is it not about time that all this 'traditional' sizing bollocks was ended, and if you ask for 4x2, you should not expect to receive 4x2? After all, if I want to buy 1kg of sugar, I would not be expected to have to ask for 100kg (being something like the amount of cane it takes to produce 1kg of sugar).

[Tony Bryer]

I'm looking at TRADA Eurocode 5 span tables and the sizes shown for joists include

..

44 x 120 44 x 145 44 x 170 44 x 195 .. 47 x 120 47 x 145 47 x 170 47 x 195 .. 50 x 120

What's now supplied is a little smaller than in times past but that's generally because it's been regularised.

[John Rumm]

Most of them sell "CLS" these days - which while not planed to the standard of PAR/PSE, is somewhat planed to take off the splinters (H&S strikes again) and knock off the sharp arises.

Not sure - you may find you are still in the statistical distribution allowed for in the calcs on the "standard" size. Is Tony B reading? He would know...

[Tony Bryer]

No, you should be doing the calcs on the actual size, 47x194 or whatever. We had to change the standard sizes in SuperBeam when a checking engineer pulled one of our users on this.

When it comes to deflection (the usual determinant of joist size) a 194mm deep member will deflect about 9% more than a 200mm one (prop D^3)

[Brian Gaff]

Also, as a neighbour found out a year or so ago, timber can be very warped and twisted when delivered. In his case the latter!

Brian

[stuart noble]

ISTR it's not planed at all, but rather sawn with a fine blade. It blunts cutters, which is I think why it's never machined into mouldings etc.

[dom]

More common now is "regularised" structural timber, which AIUI is hit-and-m iss planed. First band-sawn from the round, then by planing the corners (ar ises) radiused, and the faces reduced in size to "fit within" the stated di mension, and deflection tested and marked to conform to that dimension.

You see the occasional hollow where the bandsaw blade has deflected within the dimension that it will be planed to, leaving visible saw marks.

[Rick Hughes]

Agreed ... but step back on thought process .... it was 4x2 sawn and you used to end up with 4x2 with tolerance of -1/8 to + 1/4" if I remember correctly.

Metricate that and the size is 100 x 50 They use Regularized sizes so now quote 97 x 44 ....

However what turns up (graded C24) is is even less than that ..96 x 40 that is a lot down on what was ordered, & paid for.

I have a family member in timber business and he advised that there is HUGE money in selling wood shavings for eco fuel power plants .... so with regularized timber now they are taking an extra cut off the wood, and make money that way. North America started this with CLS sizes.

You (or me) are ending up paying for material we don't get ........ if you went in a pub and asked for a pint and it turned up 20% empty ... you would feel hard done by, that is how much wood is being shaved off.

The other Q was on spans ... load tables ..

44 x 97 is a standard size quoted in the tables .... if teh material is supplied undersized exceeding permitted deviation) you 'should' drop down a size when using span ... i.e to 38 x 97

In practise if 97 x 44 was needed for the span, to remain in design you would in fact have to go up a size to 44 x 120 ...

Just a point for discussion

[Rick Hughes]

CLS as for Regularized timber is sawn and has the Arisses machined off (to prevent splinters)

BTW ... fitting plasterboards and like to CLS is more difficult as the width is only 38 ... split that in 2 and you only have 19mm on which to catch edge of board .... compared to the 25mm you used to have with 2" sawn.

[Tony Bryer]

Here they use plasterboard up to 4.8m long which on walls is fixed horizontally so there are no joins over studs. On ceilings the joins are positioned midway between trusses and back blocked - a small piece of plasterboard glued over the join. Done properly with tapered edge boards gives a completely flat ceiling

[dom]

AIUI if it's specified as 97*44 and C16, it meets the deflection test for that size, even if the timber is slightly under that size.

[The Natural Philosopher]

since average structural lumber comes from a variety of tree species grown under a variety of conditions with a variety of seasonal variations in it, not to mention knot holes shakes and random warp cracks, being prissy about deflection tests is for numpties.

I doubt that you could predict with accuracy better than +- 50% what a given size of timber would deflect to in real life.

for serious structural carpentry purposes you just over engineer by a factor of ten and throw way really bad bits, and are VERY careful about notching.

joinery is a different matter.

[Rick Hughes]

I did something similar .... my wall heights are 2.5m .... I used Fermacell throughout, better than plasterboard on all counts. Simply ordered it it that length ...avoided horizontal joint at 2.4m

Also no taper edge needed .... all fitted with pneumatic stapler ... no plastering required - just go over fixing holes & joint ... and paint.

You can screw fix direct to it ... try doing that to plasterboard ! Also unaffected by water

[PeterC]

He's lucky - I had some fence posts that were both. Difficult to deal with at any time, but alone, a bit windy and not much time...

[dom]

Mechanically graded timber is just that - each piece has been tested as it rolls through the factory, and the grade printed on it is the result of the test.

[sm_jamieson]

How much more expensive is it than plasterboard ? Simon.

[Rick Hughes]

yep each piece has printed grade marks ... a pain as it will spoil finish

[Rick Hughes]

about 50% ... when I did the house.

However no plasterer or plaster required, no drying time, very sound deadening, higher fire rating so thinner board on garage ceilings ect.

In Germany it is very widely used.

Edges are glued (gunned glue) and once set is as strong as board ... I have tested and board snaps elsewhere not on glue joint.

The fact that you can glue with full strength also means board can to joined 'off stud' with no restrictions.

Take a look:

It is heavy though, I used a board lifter to do the ceilings ... (I run Uk_SelfBuild forum) we bought a board lifter as team buy ... each person sells on his share once finished so net cost is zero ... how is that for hire rates :-)