two timber questions

[Fred]

Hi,

Whilst browsing in Wickes, it occurred to me that the rough sawn timber sizes were 22mm, 47mm, etc. I can understand that planing will shave a few millimetres off the wood but why is rough sawn sold in these strange sizes? Why isn't it 25mm or 50mm, multiples of inches rather than just under? (I suppose I should say before someone else does, that I do realise an inch is not exactly 25mm)

Another timber question: the treated timber had that characteristic green tint to it, yet it was covered in patches of grey mould. Surely that's exactly what the treatment is supposed to prevent? What's going on there?

TIA

[The Natural Philosopher]

Dunno. My BM 's rough sawn 2x4 is 50mm x 100mm within tolerance..

the grey is not mould.

[stuart noble]

If it's construction grade material it doesn't get planed, but is sawn to a good enough finish for carcassing etc. Spruce blunts planers like crazy IIRC. Softwood can suffer from blue stain, which goes right through the timber, and is a fungal infection

[andrew]

I think it is just the saw blade width. Assuming a 3mm saw blade, cutting 50mm of material from a larger piece gives you a 47mm finished size. The advantage of doing it that way rather than having the finished sizes as multiples of an inch is that for instance a 12 inch piece of timber can be cut into 6 x 2 inch pieces, 4 x 3inch pieces, 2 x 6 inch pieces etc. To work to finished sizes of inch multiples, 6 x

2inch (50mm) pieces would need 315mm of timber, whereas 2 x 6inch (150mm) would need 303mm. A

[The Natural Philosopher]

that aint it. The greenish tint is copper salts and IIRC so is the grey

- just different copper salts.

Its all changed a bit since people got iffy about arsenic in their decking.

[The Medway Handyman]

Its not mould its excess chemical treatment, most common near knots - which can't absorb it.

[dom]

It's the saw kerf. Think huge sawmill type operations.

[Nightjar]

They are old imperial sizes simply converted to metric:

22mm = 7/8 inch, which would plane down to 3/4 inch. 47mm = 1 7/8 inch, which would plane down to 1 3/4 inch.

Colin Bignell

[fred]

Although I have seen near rotten and well mildewed tiling battens on sale at B&Q where they had been stored in an exposed location. Possibly the worst supplier of treated timber on a planetary scale and of course the didn't give a f'ck.

If the stuff at Wickes is actually wet I would pass it by just in case.

[Rick]

OK a number of points here ..

On softwood the so called standard 4 x 2 sizes etc., these refer to sawn timber size at a stated moisture content.

These were metricated ( as UK has to do what France tells it ) and the standard sizes for thickness are

16, 19, 22, 25, 32, 38, 47, 50, 63, 75, 100, 150, 250 and 300mm

The standard widths are:

75, 100, 115, 125, 138, 150, 175, 200, 225, 250, 275, 300mm

Not all thickness are in all widths ... but that is the standard range.

The BS standard for this requires these sizes to met -1mm to +3mm up to 100mm size -2mm to +4mm above 100mm

So for a 4" x 2" your timber can be in range 99-103 x 49-53 mm

However it could be sold outside of these sizes as long as it does not claim it meets BS EN 1313-1

If you buy is PSE (planed on 2 face) there is further loss in size ... the example of 4" x 2" now becomes 47 x 97 and with allowable deviation can be 96-100 x 46-50 Most wood shops would be aiming for the minimum end of finished sizes to maximize their output. (there is also Class 2 which has tighter tolerance)

PAR would lose even more as it is planed all all 4 faces.

There other type of timber (I built my house out of it) you may come across ... certainly becoming popular in the sheds, CLS (Canadian Lumbar Standard) This has rounded arrises (long edges) and is much easier to handle than rough sawn ... having all 4 faces machined. It is typically much consistent in size.

Typical sizes found in UK are

38 x 63 and 38 x 89

Notionally the 3x2 and 4x2 formats, and used where those would have been used. Extensively used in Timber frame construction as it is all much more consistent in size.

As to the part of your question relating to Green tint .......... This will only be there if the timber is pressure treated .. typically Tantalised treatment.

BTW - once you cut it, any exposed timber has to be treated to maintain it's qualities.

It is difficult to comment on the grey mould you mention without seeing it, this could be standard oxidising of softwood, which turns surface light grey.

Rick

[Fred]

This wasn't a grey tint, it was circular patches of grey that looked like mould on old bread and which wiped away. It wasn't all over, just discrete patches and it was only on some, not all of the timber. I could be wrong, perhaps it was a salt, but I took great care to pick the ones without it, just in case. It just seemed strange this was on treated timber.

[Fred]

Thanks. I did wonder if it was the width of the blade (kerf?) but I couldn't see why they didn't compensate for a 3mm loss by cutting, say, 53mm wide. However your answer explains that: money! By cutting

47mm, they get more from the original piece of wood. I suppose that 3mm is neither here nor there when constructing say a stud wall.

[Fred]

I don't remember it being near knots. Some of the wood was bundles in packs, and the grey deposits only seemed to be on the exposed faces of the pack (I opened the packs to look for good stuff).

[Fred]

Some was damp, and like you said, I left it well alone.

[Fred]

Thanks for a detailed reply.