What can i use to fill cut in doorframe

[curtains]

I'm fitting some architrave but someone have cut all the way through the top of the doorframe and left a 3mm gap so what can I use to fill the gap so it's strong enough to nail the architrave to.

[arco_iris]

I don't see the problem.

Architrave is fixed to the door lining and doesn't need fixing at the corner illustrated. In some cases (such as one I've done today) the architrave can be fixed to the wall and the fixing holes filled.

The decorator will caulk the gap and paint over it.

[Grendel]

If this gap is on the end of the upright ( i.e. a horizontal gap) then it's possibly just the joint opening a bit , the head.may have risen a little or has dried and moved opening up the joint. Simpliest way would be to put in some folding wedges directly above to push the joint down tight.

[curtains]

They've cut down through the top of the lining and it's resting on the upright by about 6mm.

[Grendel]

I think I understand. If the head is sitting on the jambs about 6mm each side and you wish to place the architrave around 10mm from the edge then there will be a 4mm hole or gap?
If the frame is being painted then filler will do the job , a two pack car body type of stuff will be fine. If the frame is being stained then that filler could still be ok as it will take stain or alternatively you could cut and glue in a small splice. Either way I wouldn't nail into either a little splice or filler , there's enough land to not need to , just put the pins a little way away.

[big-all]

is the door fitted to the opening??
is it square with the gap or without it

[Grendel]

is the door fitted to the opening??
is it square with the gap or without it

If I'm reading this corrrectly it sounds to me as if someone has cut the ends off the head , cutting on the line of the inside of the trenching and has then tried to remedy this by sitting the head on the jambs by just 6mm either side which I presume must mean the frame is now 12mm narrower than it should be . I think. ??

[big-all]

is the door fitted to the opening??
is it square with the gap or without it

If I'm reading this corrrectly it sounds to me as if someone has cut the ends off the head , cutting on the line of the inside of the trenching and has then tried to remedy this by sitting the head on the jambs by just 6mm either side which I presume must mean the frame is now 12mm narrower than it should be . I think. ??

i am a bit confused
read it one time thought horns removed
read another time as you suggest leg cut to exact length with nothing to go in the slot on the head

[Grendel]

Yes a bit confusing. A case where one picture would be worth a thousand words...

[curtains]

When I take a picture with my phone they come out blurred so I tried to do a diagram with paint. They have only cut one side of the header piece but Grendel has the correct idea. Where the cut is could I use some sort of adhesive as I tried filling the gap with matches but the are not tight enough so I need something that will fill the 3mm gap.

[big-all]

the solution will tend to depend on the access
if you can get a couple off screw in to hold together then a 3.5-4mm slither off wood
iff not somthing like no nails to try and tie them together and prevent movement

[Grendel]

Assuming the lining is otherwise solid and the door is there or being put back rather than replaced then I'd go for a combination of timber with a bit of two pack wudfil to give a smooth finish.
Slightly more radical would be to replace the head as I would think the fixing between head and jamb can't be that good with so little bearing. If the door is going to be replaced it could be worth looking at replacing the frame in it's entirety, it's always going to be narrower than standard and thus is going to mean taking much more than is really necessary off the door.