Problem Door Frame

[JtB]

Hi all,
I'm new to this forum and I'm looking for a little advise on a problem door frame. I'm a DIY novice so I apologise in advance if my question seems trivial. The plaster round my back kitchen door frame started to crack a couple of years after my house was built. It was a straight vertical crack about 60cm from the door frame. I then decided to try and fix this so I started off by removing all the loose plaster and found that the vertical crack coincided with the edge of a plastic strip which was joined to the door frame. Anyway, I decided to stick some plaster board onto the plastic strip using an adhesive called "No Nails" and plaster over it. It held for a while, then the crack reappeared again. So I removed the plaster board again and looked more closely at the plastic strip, which I could see moving slightly when I closed the door hard. Now there's obviously a little movement in the door frame, however its so little that I don't think I could make it more ridged. I thought possibly that I could fix the problem by ######## the plastic strip to the wall to stop it moving. But as I found out today, there's just a cavity filled with insulating material behind the plastic strip so I've nothing to screw into. Can anyone advise me how to make this plastic strip ridged so that I can plaster on top of it without any cracks reappearing? Any advise would be very much appreciated, thanks.
JtB.

 

[masona]

Is this door frame UPVC or wood ?

 

[JtB]

Masona, the frame is wood, here's a picture:

What I was thinking of doing, is removing more plaster (from the end of the inner wall), then sticking some plywood across the plastic strip and the inner wall and plastering over the plywood. Do you think this will work?
JtB.

 

[masona]

To make sure that the door frame won't move anymore is drill thru' the frame at 45 degree angle into the brickwork on the left with fischer frame fixing screw which you can get it in 8" long and must have a sharp masonary drill bit for a smooth drilling.

I don't understand why you got plastic strip behind plaster as you will always have problem with it.The plywood is a good idea but leave the plastic strip there as well as it might be acting as a dpc membrane.You then could tile it over to match the rest of your tiles or use a pvc L-shape fascia board in white finish bedded on with No-Nail glue.

 

[JtB]

The plastic strip just covers the cavity gap between the outer and inner walls. In the end I got rid of all the plaster that's visible in the photo, stuck a piece of plasterboard across the whole width and plastered over it. Its now held firm since October and there's no signs of renewed cracking.
Thanks,
JtB.