How to cut architraves lengthways?

[MrBalham]

Hello, I need to cut 2m architraves lengthways after replacing a window, down the red lines as shown.

I have a jigsaw and mitre saw but can't see how to use either of these to get a perfectly straight cut.

How would a professional do this please?!

 

[freddiemercurystwin]

A table saw or similar, not a jigsaw unless you're prepared to plane the saw cut afterwards and not even possible with a mitre saw.

 

[foxhole]

A professional would replace the architrave with the correct size .

 

[MrBalham]

Ah..but I can't find a company to cut new matching architraves to the required size..as that would be ideal

 

[blup]

Place on top of a board and screw through both from underneath from underneath without drilling or screwing through to the outer face which will be on display. Fix another board of the same thickness next to the architrave, and clamp a straight edge to this the required length from the cut line. Cut with circular saw.

Blup