Hope someone can clarify this.

I find timber sizes very frustrating as 50 x 50 mm square is "nominal" and actually 47x47 or even 45x45 depending on the finish and processing

So..

If a 50mm service void is specified is that a true 50mm or a nominal 50 mm made by using "50mm" battens? Or is something thicker cut down to that size.

Void will be in ceiling with electrics and slimline downlighters.

Thanks

Phil

Posted By: Peter_in_HungaryIf 50mm is specified on a drawing or other documentation then it's 50mm.

You may hope!

Plays hell with overall wall thickness if made of two even three layers of timber - which in turn means, in corner-window situations, that the windows have to be too close or too far from external wall face, which puts the pre-cut weather-cills well out of place. To mention just one effect.

Variable timber cross-section dimensions

Posted By: Phil & Colette50 x 50 mm square is "nominal" and actually 47x47 or even 45x45 depending

I can choose between that range of UK sizes, or go to US/Canada CLS in which nom 150x50 is 138x38 - but then tho stamped 'CLS' it comes as 147x47! Three layers of timber with that amount of variation can put wall thickness out by 25mm, compared to the carefully optimised (for straightforward 'fits first time' construction) detail.

Posted By: ArtiglioI’ve never seen BC turn up with a tape measure.

Not for service voids but for other bits yes, however it is the follow-on trades that get pissed off when the expected void is too skinny for them to do their work and it is the owner who picks up the bill for increased labour costs because light fittings or whatever won't go in without a bunch of extra fettling.

50x50 is prob the saw-centreline dimension; the blade width takes at least 3mm off that in both directions.

Or in the case of CLS, trims 12mm off both dimensions to leave it PAR (planed all round) with rounded arrises, presumably done long enough after sawing (or carbon-heavy forced drying) that the as-sawn develops any banana shape it's going to, which is planed away, in the hope that it then stays straight.

The UK alternative is, starting with that 47x47 sawn dimension (or let's say 97x47 for '4x2'), it's 'regularised' i.e. planed on the long dimension to get rid of banana shape, so it ends something like 93x47, hopefully creating a flat plane to studwork, for plasterboarding, but still allowing the other face of studs to wander harmlessly.

Corner windows were just an example - causes many other misalignments e.g. how fascia elements (if forced further inboard up the slope) no longer level up with top-of-wall elements like window head.

Posted By: Phil & ColetteI imagine most builders will just order some 2x2

Yes and put up what they get.

Posted By: Peter_in_HungaryFor electrics (wiring etc.) it probably doesn't make much difference (unless you are putting in 50mm trunking). However if the downlighters are 49mm then anything less than 50mm will be problematical.

So if the lights are 49mm and are fitted to the surface of the plasterboard then the available space for the lights will be (using nominal 50mm wood) 47mm + 12,5mm + skim = 60mm so no problems.

Oh and over here no one skims plasterboard, it's all taped and filled joints due to the cost difference.

Posted By: Phil & ColetteThere is an Intello membrane above the lights, then sheepswool.

Just a thought - will the Intello membrane withstand the heat of the lights over the long term considering one side will have insulation?

<blockquote><cite>Posted By: revor</cite><blockquote><cite>Posted By: Phil & Colette</cite>The lights will be Collingwood H2 Pro,</blockquote>

Good choice have fitted them and H4 throughout our house. Are the battens being fixed to the original joists. Is the Intello membrane supporting the insulation?</blockquote>

Yes, fixing perpendicular to original joist to suport support intello and insulation. 400mm centres

"I guess my question was what do people actually do? Is every 50mm void really 47mm or is there a supply somewhere of true 50mm timber?"

It's really 47mm, and no one with a clipboard will get a calibrated rule out and put you in front of a firing squad for deviating from the plan by 3mm

Extra batten, or use 90mm PIR, or use larger timber for stud work and leave a service void, or let it stick out of the non-plasterboard side (where is the stud wall? If internal, why insulate with PIR?)

It's wierd that carpentry, which at best is the most precision-minded of the building trades, has to work with such sloppy sizing of its materials, while masonry, which as a trade just bodges in gaps by eye, is supplied with extremely precise and dimensional-coordinated bricks and blocks to work with.