I will soon start installing door frames in my house. In some places, the gap is 20 cm that I need to fill with door frame and trim. How would you best install a door in this door opening? The door opening is thus 20cm thick.
I would choose to set the door in the studs that are in the direction the door is supposed to open, otherwise it will become hejkon-bejkon when you want to open it properly. Then extend from the frame to the other side where trim can be installed.
How do I fill out the rest of the space? What type of trim/molding should I use and how can I mount it to the door frame in the most aesthetically pleasing way?
There are white-painted "salningsbrädor" made of MDF available for purchase, ready with a rounded edge.
Just cut to the correct width and length. Then attach them with clicks of adhesive.
Easy as pie! I think, having not done it myself, but I have it all over my house.
Exactly, or just use regular MDF, it should be thin enough so that a few cm of the frame is still visible. Screw it into the other studs, countersink first and then fill. Then let the trims extend over the edge of the MDF so that a few mm of it is visible (the MDF sheet's corner should be visible), you don't place the trims flush with the reveal edge, that only creates ugly gaps
On the door side where there is no reveal, just let the trims extend onto the frame a bit as usual.
The door is aligned with the wall in the direction it opens. The trims are easiest made from MDF (possibly construction board) as they might need to be cut in various directions depending on how straight the door opening/wall is (things are rarely at right angles). I usually prop underneath in three or four places to make everything straight and level, providing a good base to attach the trim. In my opinion, it's best to fasten the trim with a nail gun. It creates only small fine holes that are easy to fill (screw holes are not as easy to fill as they sink and require more filler).
The cut side of the MDF board is difficult to make look nice. Half of it will be visible after you attach the moldings. A lot of sanding/priming/filling is needed to make it smooth.
