Dealing with sills rotting from underneath?

[MisterBoy]

I have these 1860 bay windows, I was aware of some problems but it's turned out to be more severe... Sill seems to be rotting from bottom up. I started off just trying to fill but in some places it's far too progressed.
Looks like someone already tried my idea of using sealant, either it failed or just trapped the rot inside!

Rebuilding the thing isn't an option right now so... What can I do? The wood is NOT rotted all the way through.

My thoughts were:

1. Rip out the sealant and loose material, and the crumbling wood in the worst areas
2. Spray rot treatment in
3. Slosh wood hardener in to stabilise the soft wood
4. Somehow fill
5. Paint

Is this reasonable? Given the size of the gap in this worst place, what would I fill with?
Is using a flexible sealant a good or bad idea.

Fortunately the very bad part seems limited though I fear rot is lurking elsewhere. Is it worth looking, or will the rot stop as long as it's kept sealed... Can wet rot turn to dry rot or are they different things?

 

[foxhole]

Are the lower sashes sealed shut?

 

[MisterBoy]

Are the lower sashes sealed shut?

Yes, painted shut

 

[foxhole]

As a temp measure you could fill voids and over clad with ply .

 

[MisterBoy]

The voids are quite deep, would you just suggest getting a big tub of regular filler or is there something better?

My primary concern since I'm keen to get it sealed to the elements is if existing rot will progress, or if keeping it dry will mean it goes into some sort of stasis. I don't want to rush to fill it only to realise treating the rotten wood first could've helped.

 

[big-all]

if the rotten wood is dry except from the front i would be tempted to as foxhole said ply plate but leave a 5mm gap along the front for air to get in at the bottom and water to get out as only driven water would then get behind
then iff a problem a 6mm bead off mastic to seal
now the ply will last 1 winter or two at the most but will give you thinking time

 

[MisterBoy]

Would anyone mind explaining in more detail about ply? Are we saying to cut a strip same height as the sill and mount in front to hide the hole, or did I misunderstand?

 

[foxhole]

I would clad front edge and top into window since it’s not in use .

 

[KenGMac]

Suggest you Google or have a look at U-Tube for "half Sill Repairs"

This is a very long established way of removing and replacing the section of a timber sill to an old Sash and Case window with a similarly shaped timber section.

Replacement timber should be treated.

Ken

 

[MisterBoy]

Suggest you Google or have a look at U-Tube for "half Sill Repairs"

This is a very long established way of removing and replacing the section of a timber sill to an old Sash and Case window with a similarly shaped timber section.

Replacement timber should be treated.

Ken

Cheers Ken I'll do that. I was familiar with cutting out the bottom of upright pieces to splice in new, hadn't realised you could do anything with the sill other than take everything apart!