Patching and Plastering - patching rotten sill plate

Hi! We are doing a reno of a basement room (soon to be our master bedroom) and found that there was a small (4") section of the sill plate on our cinderblock foundation that had rotted. It appears that the rot has gone no further. We also suspect that the paper (old time vapour barrier) that was put down on the cinder block before the sill plate seems to be missing in this one spot. How do you suggest patching this one? I'm not sure if we could actually cut out the old rot but we are wondering if we could slide a vapour barrier covered piece of galvanized metal in underneath to ensure it doesn't go any further. Any suggestions? Your help would be appreciated!

Have you checked with your local codes about egress requirements from a basement bedroom? Most likely you will have to enlarge the window to provide a safe means of egress, and the existing sill problem will be moot.

Thanks for your suggestion. However, the sill plate isn't a window sill. It is a piece of wood on top of the cinderblock foundation and beneath the rafters (ie the house is built on top of it). The plate is not beneath ground at all but where the top of the foundation is topped by the rafters that begin the wooden structure of the house. We have looked at enlarging the window but we live in a small rural town and codes are not generally valid or adhered to here. We can both easily get out of the window (it is larger than the norm for a basement window) and therefore feel that the safety factor is there.

If you can saw out the rotted sill section, replace it with a like-sized piece of pressure-treated board.
Good Luck!
Mike

I've done this type of repair but it was simplified by the fact that the exterior clapboards were removed for replacement too, which gave me access on both sides of the wall. If you can get a Sawzall in position to cut out the rotted portion of the mud sill, then using a couple of timbers under bottle jacks, raise the floor joists just enough to take most of the pressure off the sill plate at that point. (Don't jack the wall up enough to actually move anything or crack plaster/sheetrock!) Work out the old rotted board. Slip in a bit of flashing, if possible and with a hammer, massage in the new pressure-treated piece. Good luck!

Hi there! Thanks for the ideas. We've done further investigation (which is why it's taken me so long to post. It seems that the wood (which is now about a 4 ft section of the sill plate which supports the first floor joist) is sound in many places and where there is rot, it is not rotten through. It is mainly behind the concrete front steps and therefore is not accessible from outside. The floor joists are 16" centres and with the depth of the concrete block we have only about 6" to get into the cavity, very tight for working in. Right now it seems that it would be impossible to remove the sill plate without removing the siding and concrete front steps. As the sill plate is not rotten per se, just a bit of rot on it, we have come up with another idea. Let me know what you think please!! There is room on the cement block foundation to lay another 2x10 joist about four inches ahead of the original first (outside) joist. We have easy access to the top of the main beam in the house as well as the side wall sill plate on the foundation (which is just fine). What do you think about covering a new 2x10x14' joist in vapour barrier and installing it at the front of the concrete block foundation? It would provide support to the upstairs wall should there come a time in the future where the damaged sill plate actually rots through and affects that first outside floor joist. Suggestions and opinions are valued!!

If I understand what you are proposing, you have a lip on the inside of the cinder block wall which you can wedge a 2 x 10 in position (rather than on top of the old mud sill). If that is the case, I imagine what you propose would work. I wouldn't fool with vapour barrier, but use a pressure treated 2 x 10. The downside to this proposal is that you will be unable to see if there is any further damage or water infiltration behind the 2 x 10. Good luck!

Yes, we can rest the new 2x10 from the side wall sill plate which is in good condition right through on the cinderblock to the main beam. It would touch the damaged front sill plate along one edge which is why we thought perhaps we should vapour barrier in order to create a physical barrier to keep away the rot. I think the pressure treated 2x10 would be the way to go, now that you've mentioned it but do you think we should use vapour barrier as well to cover the part that will touch the damaged sill plate? Appreciate you input!

I'm still inclined to think that the vapor barrier would not be needed if PT wood is used. On the other hand, It wouldn't hurt anything and the expense is minimal.