Replacing a sill plate under an exterior door.

I am finishing the basement in the house we recently moved to and believed that I was in the home stretch - until...
I removed the last of the framing and insulation and found a section of severely rotted sill plate and rim(skirt?? ) board underneath our side door (which is in the laundry room). The damage is countained to a section of sill plate 5 feet long immediately under the door.
The foundation wall is out of the ground by about 20 inches on this side of our house. Thus my surprise at finding the water damage.
I am at a loss as to why we have this water damage. The only thought I have is that snow (we live in Canada) has drifted on the stairs and has rested against the brick and the transition from the foundation wall and the brick. I am guessing that this snow would melt and work its way into the house.
Would this type of action cause a rotted sill plate over time. Our house is about 25 years old and I understand that this side door was not used by the previoius owner... and presumably they did not clear the snow from the steps. The lock was seized when we moved in... the owners said they hadn't opend the door in over 5 years.

Any opinions you can provide would be helpful. I am planning to replace the damaged sill plate and ridge board but want to fix the cause first.

Thanks so much for your help!

Sill plate refers to the 2X that sits on the foundation and is butted to the rim joist. Floor joists are nailed to the rim joist. So the door sits on the sill plate or a threshold plate? Sill should be pressure treated but if it sits in water it will also decompose. A ridge board is found in the roof to which the roof rafters are nailed. I'll assume you mean rim joist. Obviously you have a water issue. Clearing snow, what's that? I live in GA and we may get 2-3" a year, will keep water away from the foundation. The big issue is tearing out a rim joist and sill plate. Not for the faint of heart. You need to install some cribbing and jack up the house to take the pressure off the sill and rim joist and then cut them out and replace.

Thanks for the clarification re terms:... to confirm... a section of sill plate is rotten under the door and the rim joist is rotten in the same area. A closer examination seems to be indicating that the door's aluminum threshold was poorly installed and may be the larger culprit here. I do think snow up against the brick didn't help either.

I have one additional question. I have read somewhere that it is best to tackle the rim joist from the outside. This would be difficult as the house is brick clad. The only good thing here is that there are only two courses of brick under the door before getting to the foundation wall.
Given this, would it be best to take out the door, take out the two rows of brick under the door,replace the sill from below and the rim from the outside and then put in new brick, and new door and do it right this time?
I would do this if it meant the rim joist would be more structurally sound. But is this really necessary for a rim joist section that is about 3 and a half feet?

Otherwise, I think my plan is going to be: take out the door, clean up all the problems directly under the door, install a new door (seal it tight) and replace the sill and rim joist from the inside. I will also sister the floor joists that rest on the sill in this area as there is a little bit of water damage on the end of two joists.

As for the snow, I would be happy to send you some this winter
Thank you very much again for taking the time to answer

Bill: Yes, the correct way is to do the work from the outside but with brick veneer, you would have to support the exterior wall too. I'd remove the door and buy a replacement threshold. If you have indoor access, I'd try to build some cribbing, 4X6 or build up some 2X8 and try to get the pressure off the rotted sill plate, Couple of 5 ton screw jacks should do. Use a Sawz All to cut the sill in pieces and remove it. You will ruin a lot of blades cutting the nails. Wedge in a piece of replacement sill plate, SISTER the rim joist and floor joist with pressure treated wood. It's almost impossible to get a rim joist out from the inside and our code here requires the floor joists to be face nailed to the rim joist. Lots of hard work. The last one I did was 10 ft of sill, rim joist, wall sheathing, 30'of drip edge and 90ft. Of hardie siding. All done from the outside. Took almost 3 days with 2 people. Original plan was to just replace the drip and siding but what a mess in the wall when we opened it up. Good luck and let us know how it turns out. PS Thanks for your kind offer BUT KEEP THE SNOW, I had 30+ years of it in Philadelphia. Brian

Good comments by ma0641. I would add that if the door is on a gable wall or doesn't have much overhang protection, the rot could be from wind blown rain and poor flashing.

Brian,
Thanks for the feedback again. Sorry for the silence... we went camping for the end of summer and just gettign back. I am planning to do the work at the end of September or early October. I am going to support it with some solid framing until I get to it. In the meantime, I am building a detailed plan so that I have it all on paper for guidance.
I will certaily give an update when I get it done.
You are a gentleman for taking the time
Thanks again!

Thanks Springtimehomes!
The wall is indeed on the gable and has no overhang. I have been also been thinking of adding a simple yet tasteful overhang as well... of course after fixing it from the inside and replacing the threshold and door. I gave myself a Christmas deadline to ge the basement finished and I was feeling mighty good until I ran into this mess.
Thanks again... your input is very much appreciated,