Rotten Window Sills

[Willa]

Curiosity got the better of me, so I removed one of the dining room storm windows. This was a struggle, as someone had basically glued it in place with caulk, from the inside. While finally pulling it away from the window frame, a long band of rubbery stretched out caulk snapped and hit me in the face, like a vicious elastic band.

The storm window installer had covered the existing sill with a 3/4" false sill. I imagine this had a dual purpose of covering up a damaged sill and making a storm window that wasn't precisely the correct size fit. This false covering was very rotten.

What I uncovered looks pretty bad, though.

I met with an experienced carpenter yesterday, who is putting out inquiries about having replacement sills milled. I'm scared of the cost, but also know that there is nothing that can fix these deeply rotten sills. The bad installation of the storm windows certainly helped the rot to rot faster though, as there was no ventilation possible unless the windows had the screen opened.

Has anyone replaced rotten sills ? What should I know ? I watched the This Old House segment that showed a rotten sill being removed and replaced. This looks pretty straightforward for someone who has done it before. Anything I need to know about how they should be milled ?

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[Gothichome]

Willa, I priced out a new sill many years ago from a mill shop. Big bucks. Most of the cost was set up. All the windows in Gothichome sit in sandstone sills. The wood part that sits on the stone is actually part of the window frame. I see two options, since your sill is wood and flat I think you could get away with removing the rotten sill and sliding a new sill in. The section of the window box above your hand may be removed and replaced. You will need to be creative though in re attaching it to the box. I might suggest some long #6 screws in a toenail fashion. This will draw the new section up to the frame. After that slide your new sill under and in to place.
Option #2 would be filling the large hole with a block whittled to fit the hole. Using modern construction glue just fill the hole and smooth out with wood epoxy and sand paper. The rotted out box section can then be removed and a new section installed, secured once again with #6 screws.

[Willa]

The carpenter was going to price it out based on having several sills milled at once as they are all the same depth/height but different lengths. He told me to put some plastic over the worst ones to at least keep the rain off/out in the interim. The rotted area is wide and deep. Lasting 120 years is more than most people would expect. If they had been properly maintained they could have easily lasted 30+ more years.

[heartwood]

often times the sill is an integral part of the whole window...sill may be rabbeted into the jamb or framing behind stone sill...hard to tell until the existing sill is removed...not sure you need to two step sill...it's possible one was added at some point...hard to tell in the photos...make sure to back prime and paint the entire sill before installation then paint again once installed...leave a space beneath the sill and do NOT caulk...the space will allow the wood to dry out...

good luck...
...jade

[Gothichome]

Jade, I believe Willa's sill is wood. I think her issues started from day one when the home was new. Notice the sill does not have a slope allowing the water to flow off. Not an issue I think when the home was new, the builder would have built the sill on a slope. Over the years with settling of her home the slope disappeared and it may even have a slight slope in the wrong direction.
Willa, I notice the sill has settled on one side as well this has allowed the water to pool where that large hole is. Notice the brickwork has been tinkered with in this area. They really didn't address the problem though, just filled the sagging with mortar and a piece of brick.
In my view all this can be rectified with out breaking the bank.
Just from the pictures my plan would be remove the sill completely, source replacement woodand remove the fudging in of the brickwork.
Since the original slope can not be replaced (maybe it can but got go with the pics) I would build the slope into the new sill. Not forgetting to put a drip edge on the under side. This could be easily done with a belt sander. The drip edge can be routed in or dremmeled if no router available. Dry fit the sill and trim as needed.
Prime well as Jade suggests, reset under the sash with lime based mortar on the level. Then deal with the window frame rot.

[Gothichome]

Willa, make the sill look like this.

[Willa]

This Old House video explains what needs to happen:

https://www.youtube.com/watch?v=x8iu9dLrV-E

From looking at the window sills on the other side of the house, there is a slight slope to them - though not as much as is commonly done. I wonder if the nefarious storm window installers planed the sills to be flatter to facilitate the ease of installation for their storms, never mind how this will affect the house in the long run !

The rot is too wide and too deep to attempt to repair with epoxy.

[Willa]

make sure to back prime and paint the entire sill before installation then paint again once installed...leave a space beneath the sill and do NOT caulk...the space will allow the wood to dry out...

good luck...
...jade

The back priming is something I had not considered, but makes good sense. Thanks for mentioning that.

All the sills have a drip edge at least. One sill is covered up with a sheet of tin ? zinc ? so I know there will be another surprise waiting there for me, underneath.

[Gothichome]

Willa, your TOH vid pretty well describes how I would go about changing the sill. I though would not be so gentle in the removal process.

[Willa]

Great news from the carpenter today. He priced getting the sills milled at Hoffmeyer's Mill - about an hour away - and their price was about 1/2 what I expected. The sills could get milled within the next 30 days. This info was a great relief. He called while I was scraping and I didn't get his message so I was quite surprised when he just showed up.

But, whew, what good news.