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The product covered in this section is 'Internal Door Casings'.  Find out all there is to know.

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Internal Door Casings

Door casings come in standard sizes of varying widths for different wall thickness.  The rebates also correspond to standard door thickness and widths.
When you purchase a door casing, you may be surprised to find that it comes in kit form, made up of three components, two jambs and the head. The head is trenched-out to form a groove that the two jambs slot into.
The internal door casing is a rather flimsy affair. This is due to its small section and for not having the benefit of any jointing method at the corners. That said though, it works well when correctly applied and fitted. 
The first thing on your list is to match the casing's overall size in relation to the thickness of the brickwork, block-work or stud partitioning (including plasterwork or plaster board) that you are fitting it to.  If you are working in older property, you will probably find it difficult to match, however, this does not present any real problems, as there are a number of alternatives open to you:
  • Have a casing made to your exact requirements.
  • Purchase the nearest available size and add to it.
  • If the casing is too wide, have it cut down to size.

Assembly Time
When it comes to assembly, it's simply a matter of connecting the three parts and nailing them together (no adhesive required). The nails, 75mm, are driven from the top of the head through into the two jambs, usually using three nails per side for a standard sized casing.
It's a good idea to prefix two nails in the casing head (preferably around the centre) before you proceed, which will make things easier later.

Lay the three parts down on their edge onto a flat surface, with the rebate facing up (also ensuring that the three rebates are facing inwards). Whilst holding one jamb in position, slot the head on to it, tapping it into place, then, drive the pr-efixed nail in about 25mm, (or sufficient to just hold the head in place). If you are happy that the two parts match perfectly, drive the nail home.  Follow the same procedure with the second jamb. Then insert the remaining nails. (The number of nails to use is relative to the casing size).
I should mention that it is better to ensure that the rebate matches rather than the edges, as the edges can easily be trimmed with a plane later if required.

Parallel Lines
Even though the casing is nailed together, it's still flimsy, out of square and the jambs are not parallel. You are about to change all that! 
Getting the jambs parallel is easily achieved by fixing a cross member (50mm x 10mm and 50mm longer than the overall width of the door casing) 200mm up from the bottom of the casing, and fixing it, using one 35mm oval nail at each end. The width at the bottom should be the same as the top, which is pre-determined for you by the fixed door casing head.  When secure, use a tape measure to check your widths top and bottom, ensuring that they are both the same.  Finally, saw the excess timber off each end.
With this timber in place your door casing is now parallel and ready for fitting

Braced up

If you are new to DIY, you may find it helpful to brace the casing.
This is done by fixing two 400mm long braces onto the corners. These braces should be similar in section to the cross member that you fitted across the bottom of the casing, 50mm x 10mm.
Place a large square, into one corner of your door casing and adjust the casing so that it matches the square.  Place one of the braces onto the corners at about 45 degrees, ensuring that it has an equal overhang of about 25mm and fix it at one end with a 35mm oval nail.  Double check for being square, adjust if necessary and fix the other end.  Follow the same procedure with the other corner.
Now that the braces are in place, fit the second nail into each end as well as the second nail into the cross member and saw off the excess timber.


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