What size Stud work for 106mm door casing?

[Whimsical]

Have 106mm door casing and trying to work out the stud wall timber width required (from Jewsons)?
100mm stud work timber: actual size = 89mm
75mm stud work timber: actual size = 63mm

Allowing 25mm for plasterboard (both sides of wall) and 6 mm for skimming (both sides of wall), I cant get to a 106mm casing width.

89+25+6 = 94mm
63+25+6 = 120mm

Am I just being stupid? I'm sure this must be done all the time? 75mm would obviously be the correct thickness but the difference between stated and actual is confusing me!

Many thanks for any advice

 

[PrenticeBoyofDerry]

Plane squared edge timbers are machined so the dimensions are reduced.
You want to get 47 x 75mm kiln dried sawn single stud

 

[Harbourwoodwork]

Build the stud first then worry about the liner

 

[Whimsical]

Great, many thanks folks!

 

[PrenticeBoyofDerry]

The other suggestion I would make is if you are struggling to find 75mm stud and already purchased door liner casing.
Is consider double boarding one side of wall, you will have to offset the door casing though.

63mm(stud)+37.5mm (Pbsx3)+5.5mm(skim)=106mm
So minor adjustments on skim applications, gets you level.

 

[Whimsical]

Did'nt think of that!.. Many thanks will keep that in mind, cheers!