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Nominal vs. Actual Dimensions

kramit

I've only ever worked with one firm and it's always a trade off or argument between myself - architect - contractor, or myself - contractor and seldom the sub conts. 

I mainly only use AutoCad for designing everything. 

I've looked everywhere in Forums and I haven't found anything regarding this.

What do you all use? What are your thoughts on this? 

 
Sep 22, 14 3:49 am
el jeffe
if a product or material has a commonly accepted nominal dim value, use it.
there is no argument.
Sep 22, 14 8:41 am  · 
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curtkram

when you buy a 2x4 from home depot, say 'i would like a 2x4'

when you draw a 2x4 in autocad, draw it at 1.5x3.5

nominal values are for easier communication, not for design tolerances.

Sep 22, 14 9:21 am  · 
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gruen
When I draw a plan w 8" concrete block, it's drawn at 8". When I draw the detail, it is 7-5/8". Scale matters too.
Sep 22, 14 9:39 am  · 
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Non Sequitur

I draw in mm.

I cast to the depths of the dark abyss all those imperial units. My wood studs are 38x89mm.

Sep 22, 14 10:19 am  · 
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curtkram

i live in the dark abyss.  all your imperial units end up right here.

Sep 22, 14 10:44 am  · 
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My partner always dimensioned to face of stud, I always dimensioned to finish surface.  Our carpenters were very patient.

Listen to el jeffe (Hi jeffe!!). There is no argument.

Sep 22, 14 10:47 am  · 
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Non Sequitur

Face of stud?

Centre-line or nothing for me.

Sep 22, 14 11:13 am  · 
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^ Is that a nominal centerline or an actual centerline?

Sep 22, 14 11:52 am  · 
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SneakyPete

The framers I've talked to like face of stud, as CL of stud gets covered by the stud during layout.

 

The designers I've worked with like face of sheathing (for white box architecture with ID or IA) or face of finish (for finished space), as it's the framer's job to do the math and we, as designers, should worry about the design intent, not framer's wishes.

 

Their opinions. Mine is irrelevant at this juncture, as I'm not in charge. :)

Sep 22, 14 12:09 pm  · 
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mightyaa

Nominal, so my wall is put in 4" instead of 3-1/2"... Then I dimension to the critical side of the stud or opening, but never the wall width (wall types set that assembly up).  It's actually a long discussion about why. 

One reason: It's an 'old school drafting trick'.  Back in the day, you didn't have CAD to auto-dimension.  And you sure weren't going to redraw a entire set of plans because you decided to add a sound channel or something to a wall assembly.  Instead, you change the wall type or window/door schedule (one detail versus redrawing a whole set of plans). 

Another reason: All you really need to dimension is where you want the framer to snap the line that will be used to set the baseplate.  Once the framer is done, everything else hung on that framing is going to be whatever dimension it's going to be regardless of your dimensions. 

And the list goes on.  Basically because you can dimension to fractions doesn't mean you should.  Try to remember the framer isn't working with a micrometer; Don't dimension to tolerances that are unrealistic to expect out of the tools in the field.  .  

Sep 22, 14 1:02 pm  · 
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gwharton

Draw to actual size (not nominal), rounded off to the nearest half inch.

Door openings drawn in plan to nominal width of leaf, dimensioned to centerline unless noted otherwise.

Other dimensions are generally to face of framing/structure for residential, poured concrete, and light-frame construction (U.N.O.), and centerline of wall for commercial (U.N.O.)

Face of finish dimensioning should never be used unless final clearances or tolerances are critical, in which case they should be specifically noted as such in that location only.

Using metric dimensioning in the USA (or any other place where metric is not customarily used in construction) invites contractors to mock you for being a clueless idiot, just like using feet/inches in a metric standard country would.

No dimension should ever be given in increments smaller than 1/2" or 10mm unless tolerances are critical in a detail. If you don't understand why this is a good practice, go spend time working construction until you do understand.

Sep 22, 14 1:06 pm  · 
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SneakyPete

Also check with the firm you are working for, as they will likely have standards that won't make sense to you, but are important to the people signing your checks.

Sep 22, 14 1:07 pm  · 
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gwharton

More:

Slope dimensions are always rise:run except for ramps, which are dimensioned with spot elevations at top, bottom, and inflection points, with a maximum slope in percentage noted between. Never use degrees for dimensioning sloped surfaces unless there's a really compelling reason to do so (and usually that reason isn't a good one even though you think it is, so just don't). Instead, if the slope can't easily be dimensioned in rise-run using normal tolerances (1/2"/10mm), specify it using spot elevations for top and bottom.

Wall height dimension for a wall supporting a sloped roof above is to Top of Plate or bearing surface.

Structural steel is dimensioned to centerline or top of bearing.

Floor elevations are always to top of finish floor, regardless of what's underneath them. Do NOT deviate from this, since it's a major life-safety and liability issue.

Guards and handrails are dimensioned from finish floor to top or center of rail, depending on what sort of system you're using.

If you want something arranged with equal spacing, specify the total dimension of the whole, whatever alignment you want (center, etc.), number of items, and that they are to be equally spaced. Do not dimension each increment repetitively.

Dimension something once, and only once. Do not repeat dimensional information anywhere. Even with modern computer-facilitated coordination and BIM, it WILL get screwed up at some point. Particularly since details are still not fully BIM-generated. For example, if you call out Top of Parapet on your roof plan, don't also note it on sections, wall sections, and details.

Sep 22, 14 1:44 pm  · 
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To be correct about lumber, the actual dimension is the rated dimensions the material is surfaced to. Nominal dimensions is the unsurfaced raw cut dimensions that it is roughly cut to. The actual dimension in reality of any one piece is going to variate by some very small amount and how clean and straight the cuts are made.

When something is rated at an actual dimension of 1-1/2" x 3-1/2" at a certain moisture content level, it is what the measured cut dimension was with the surfacing machine. The rough dimension (prior to surfacin) (the measured "nominal dimension" after the big band saw cuts them from logs into dimensional lumber.

The surfacing is done with another machine that will leave a smooth planed surface with takes about a 1/4" from each side.

Most dimensioned lumber you find at say Home Depot are S4S (Surfaced on 4 sides or both edges and both sides). There exists rarer S2E which is surfaced on 2 edges. Which means both the 1-1/2" wide edges are surfaced but the sides are rough.

However, you must pay attention to moisture level. Some boards that are green are often something like 1-5/8" x 3-5/8". It will shrink as the moisture level drops to moisture equilibrium level of the area.

As long as you understand the expansion and contraction level of the materials used, you should be good.

Sep 22, 14 2:31 pm  · 
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chigurh

I imagine balkins saying all that in the voice of professor Frink from the Simpsons.

http://en.wikipedia.org/wiki/Professor_Frink

Sep 23, 14 11:24 am  · 
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shellarchitect

use lt. ga. metal framing, problem solved

unless there's any masonry, than see above

Sep 23, 14 12:41 pm  · 
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JonathanLivingston

++ chigurh LOL

Sep 23, 14 2:33 pm  · 
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Non Sequitur

dark abyss indeed:

Sep 23, 14 3:10 pm  · 
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JonathanLivingston beat me to it.

http://www.fool.com/investing/general/2014/09/18/lowes-hit-over-the-head-with-a-2x4.aspx
Sep 23, 14 3:52 pm  · 
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There aren't enough telephone poles to hang all the lawyers from.

Well, maybe there are. But there's only one way to find out.

Sep 23, 14 6:29 pm  · 
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go do it

I have never understood the use of nominal measurements in dimensioning. It really make no sense to use a measurement that is not the actual measurement of the material being used. NASA would not do that. Come on y'all some of us are being precise out here.

 

Edit: Never mind about that time NASA and Lockheed Martin lost the Mars Climate Orbiter because of a Metric / Imperial measurement miscommunication.  

Sep 23, 14 10:54 pm  · 
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2x4's were originally 2" by 4". 

Sep 23, 14 11:29 pm  · 
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Yep, when they were rough cut.

Sep 23, 14 11:45 pm  · 
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go do it

^

^

It is like that Geico commercial , everybody knows that. We are not building in 1910 though we are building now and a  2x 4 is... well you know.

I have worked on a lot of old houses and it is always fun to find studs that are a true 2 x 4. Some are so rough cut there is still bark on them. Not really, just kidding. They are heavy though.

Once in a stud cavity of an old house I found a perfectly preserved skeleton of a mouse that got stuck in the wall some how. 

Sep 24, 14 12:34 am  · 
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Carrera

Having been in construction - the studs (metal or wood) are drawn with strings of dimensions showing actual stud dimensions and are not dimensioned to their finished thickness. Imagine yourself laying out partitions with a tape in one hand a chalk line in the other…what are you to do with the calculator you need to subtract the finishes to find the mark on where to place the stud….put it in your mouth? If it’s a wood frame structure the exterior wall dimensions include the thickness of the sheathing – the sheathing is placed on the studs before erection. You always dimension to the rough for layout – on CMU you dimension the CMU not the CMU plus applied finishes. CD’s are for constructing the building and you cannot build the building if you cannot lay-out the building.

Sep 25, 14 9:07 pm  · 
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anonitect

I would rather have to say that a 2x4 is .267 x .114 shaku than convert to the metric system. Americans who want to convert to metric have no poetry in their souls. The more we standardize, the closer we come to the ultimate boredom of the singularity. Coordinated universal time can blow me, too.

Sep 25, 14 11:33 pm  · 
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