Green Building Forum - Timber sizes and service voids

Timber sizes and service voids

Phil & Colette — Sat, 11 Feb 2023 12:38:16 +0000

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

Timber sizes and service voids

Peter_in_Hungary — Sat, 11 Feb 2023 12:49:31 +0000

If 50mm is specified on a drawing or other documentation then it's 50mm.

For 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.

Timber sizes and service voids

fostertom — Sat, 11 Feb 2023 14:01:55 +0000

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

You may hope!

Timber sizes and service voids

djh — Sat, 11 Feb 2023 16:59:36 +0000

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

I don't think it's that simple, at least in England. Dimensions can be nominal. Nobody knows exactly how thick the internal plaster is going to be so room sizes are an estimate. Similarly a stud wall is made of studs and plasterboard and will be whatever dimensions the studs are, plus two layers of plasterboard and skim. Nobody buys the studs too big and planes them to the exact size needed to match the drawings. But they do plane door openings to match, for example.

As you also point out, what matters is the functionality. To fit a downlight that needs 49 mm depth you need 49 mm depth. That can be clear depth or dug out of whatever is behind the gap. And that may involve contortions to maintain airtightness or any other complication.

Timber sizes and service voids

fostertom — Sat, 11 Feb 2023 17:22:51 +0000

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.

Timber sizes and service voids

djh — Sat, 11 Feb 2023 20:47:44 +0000

Sorry Tom, what plays hell with wall thickness?

Timber sizes and service voids

fostertom — Sat, 11 Feb 2023 21:17:50 +0000

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.

Timber sizes and service voids

Artiglio — Sat, 11 Feb 2023 22:31:18 +0000

Iâ€™ve never seen BC turn up with a tape measure.

Timber sizes and service voids

Peter_in_Hungary — Sun, 12 Feb 2023 07:50:40 +0000

Posted By: ArtiglioIâ€™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.

Timber sizes and service voids

Cliff Pope — Sun, 12 Feb 2023 09:52:04 +0000

The origin of the anomally in sizing is, I was told, that 50 X 50 is the actual sawn dimension but planing the wood smooth inevitably trims off a mm or two.

Timber sizes and service voids

fostertom — Sun, 12 Feb 2023 10:20:36 +0000

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.

Timber sizes and service voids

djh — Sun, 12 Feb 2023 13:24:56 +0000

There's a fairly good explanation of timber sizes at https://www.buildingmaterials.co.uk/nuts-and-bolts/guides/a-comprehensive-guide-to-timber-sizes

Tom, I now see your original comment was specifically referring to corner windows, and understand your concerns. We didn't have any of those, so just sat all our windows at exactly the right inset to match the window sills, then adjusted the window boards inside to cope with wall thickness differences (mostly just by allowing them to stick out more or less, rather than scribing them to exact sizes :) There was enough scribing everywhere else to make the internal stuff meet the plaster on the external walls!

Timber sizes and service voids

fostertom — Sun, 12 Feb 2023 13:56:22 +0000

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.

Timber sizes and service voids

Phil & Colette — Mon, 13 Feb 2023 10:31:03 +0000

Thanks all for the comments.

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? I imagine most builders will just order some 2x2

The lights will be Collingwood H2 Pro, with a total depth of 49mm, but the ceiling will have 12.5 plasterboard ans skim.
There is an Intello membrane above the lights, then sheepswool.
Its an 1850s cottage and ceiling height is a bit of an issue in places, plus original joists so watching weight (although did have 1" of lath and plaster before.)

Phil

Timber sizes and service voids

Peter_in_Hungary — Mon, 13 Feb 2023 14:22:14 +0000

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.

Timber sizes and service voids

revor — Mon, 13 Feb 2023 17:10:42 +0000

Posted By: Phil & ColetteThe lights will be Collingwood H2 Pro,

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?

Timber sizes and service voids

Peter_in_Hungary — Mon, 13 Feb 2023 18:40:20 +0000

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?

Timber sizes and service voids

djh — Mon, 13 Feb 2023 20:27:27 +0000

Posted By: Peter_in_Hungarywill the Intello membrane withstand the heat of the lights over the long term considering one side will have insulation?

LEDs don't get very warm. Intello will stand up to 80Â°C, which is a lot hotter than my LEDs get.

Timber sizes and service voids

Phil & Colette — Tue, 14 Feb 2023 11:27:13 +0000

Posted By: revor
Posted By: Phil & ColetteThe lights will be Collingwood H2 Pro,

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?

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

Timber sizes and service voids

revor — Sat, 18 Mar 2023 16:04:40 +0000

Posted By: revor
Posted By: Phil & ColetteThe lights will be Collingwood H2 Pro,

Good choice have fitted them and H4 throughout our house.

It turns out that I was correct in the right choice of LED spots as I had a problem with one this week. An upstairs leak due to a cracked solvent waste fitting found its way via a resilient bar to a light fitting and damaged it. Being 5 years old and newer ones now available I contacted Collingwood on Thursday to see if I could get a replacement that might match and today (Saturday) got a replacement of exactly the spec of the originals, FOC. Exceptional customer service.

Timber sizes and service voids

cjard — Sun, 19 Mar 2023 08:46:37 +0000

"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

Timber sizes and service voids

number_thirty_three — Wed, 22 Mar 2023 21:56:44 +0000

Posted By: Phil & ColetteI 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

I can massively relate to this - and what I think is even more hilarious is that imperial sizes actually are on the dot - and a 4 inch imperial is actually 100mm! But a metric 100mm isn't.

Anyway, may I attach a follow on question to this:

When you're planning to full-fill a stud wall that is 100mm thick with 100mm of PIR insulation, but actually the nominal size is 95mm, what do you do? My friend had this problem and spent ages shaving 5mm off the back of the PIR. Surely there's a better way?

Timber sizes and service voids

Dominic Cooney — Thu, 23 Mar 2023 07:42:59 +0000

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?)

Timber sizes and service voids

WillInAberdeen — Thu, 23 Mar 2023 08:55:33 +0000

If (ahem) you don't discover your studs are 95mm until you've already bought and cut the first piece of 100mm PIR, you might find a scrap sheet of 6mm MDF lying around that you could cut spacers to pack out the studs thick enough.

Obviously that only happens to disorganised people who don't measure in advance...

Timber sizes and service voids

fostertom — Thu, 23 Mar 2023 09:21:42 +0000

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.

Timber sizes and service voids

Dominic Cooney — Thu, 23 Mar 2023 11:19:50 +0000

Posted By: WillInAberdeenyou don't discover your studs are 95mm until you've already bought and cut the first piece of 100mm PIR

had the same thing on a roof, with "100mm" rafters that turned out to be more like 95mm, with 100mm PIR between and 100 over, it stuck out the underside and I had to be very careful with the plasterboard screws not to pull in past the paper.

The next one I did, I used 90mm PIR between to avoid the problem