gapcrack

Hi all, I would like to sort it while the sun is out and it might have dried out this crevice/crack. The cellar is below this area and I have noticed the joist has rotted so I want to prevent further water ingress. I have a tube of CT1 ready to go. Is that what I need or would you recommend...

I know it may have been the worst job a carpenter has made to my house. Got old stair post clad with 10mm timber and gaps was filled with filler. This keep cracking when season change and temperature in the house rise. Would be good to open the gap between the 2 timber and fill it with...