How to prevent external corners on window sills opening up? and could I tape over them?

[mpooley]

Hi I built a cupola for my workshop 14 years ago and it is still in pretty good condition.. but
a few years ago I noticed some rot where the corners of the sill had opened up
I cleaned all of the corners out and filled with an epoxy filler and apart from some cracks round the edge of the filler it doesn't keep bad.
But needs doing every year as they always open up and sometimes they rot a bit more and I have to chop it all out and start again.
I notice this is beginning to happen on my home made greenhouse sills too.

I was thinking of covering the joints with flashing tape I have left over. Just covering the top of the joint leaving air to get to the bottom?

Do you think this would be wise?

is there anything else you could suggest to stop this happening?
Thanks for any advice.

Mike

 

[Bod]

Could you post some pictures of the problem please.

Bod

 

[Adam W.]

I suspect that they are mitered corners and there really isn't much you can do about it as they always seem to open up due to wood movement.

The ones on the workshop lantern are puttied and painted on a regular basis to stop them leaking this time I'm using linseed oil paint to try and give it a bit more protection. I used butt joints on the lantern on mums' extension, which seem to be coping better.

Covering them up with sticky flashband isn't a good idea, although an applied copper or lead flashing might help, although water could would still get in under the flashing.

 

[mpooley]

Yes they are mitred corners. Haven't seen any putty used for years I never even thought about that. I have never actually used putty or seen any for sale. I'll have to have a look.
I thought that If I leave the gap open and cover the whole corner topside with tape , allowing air to get underneath into the gap why do you think that wouldn't work? It's very sticky tape I cant see water getting under it

 

[profchris]

Not to put words in Adam's mouth, but I think he means moisture would still get in. The exposed end grain at the mitres will absorb moisture from the air when it's humid. If you cover the top, that will still happen but you create a still pocket of air inside the joint, which dries out slower. Result, rot.

Putty seals the end grain as well as filling the gap.

 

[RobinBHM]

The way to avoid the problem at the beginning:

1. mitre cills as normal
2. put 3 mm radius on mitre edges
3. apply end grain sealer
4. paint mitre ends

when fitting:
1 use MS polymer on mitre
2. after fitting fill V groove (where 3mm radiuses abut) with MS polymer


your best solution now:

1: chisel a V groove on mitre top face and edge
2. sand to get rid of sharp edge.
3. end grain seal raw timber
4. paint
5. apply MS polymer in V groove.
6 paint over MS polymer if needed.


this is the recommended method by joinery paint manufacturers such as Teknos, Sikkens etc.

You simply can create a sharp mitre and avoid water ingress - mitres always open up, because wood expands and contracts. And if you have a flush joint, when the joint opens, there is effectively a painted cill with a cut straight through the paint film exposing the raw timber below.

 

[Adam W.]

Yes they are mitred corners. Haven't seen any putty used for years I never even thought about that. I have never actually used putty or seen any for sale. I'll have to have a look.
I thought that If I leave the gap open and cover the whole corner topside with tape , allowing air to get underneath into the gap why do you think that wouldn't work? It's very sticky tape I cant see water getting under it

Thermal movement causes the adhesive to eventually fail and dust and water gets in where the flashband puckers up. If you've already got a paint coat or varnish finish on the sill, then failure of this surface compromises the adhesive too.

Because a sill is subject to very harsh environmental conditions, I view it as a case of continuous maintenance rather than a fit and forget. So when I clean the windows, which isn't very often, I also maintain the sill.

 

[Cooper]

Haven't seen any putty used for years I never even thought about that. I have never actually used putty or seen any for sale.

I can't add anything to RobinBHM's excellent reply but your reply reminded me of a little rhyme we used to say to the kids in school.
In glue and dust we place our trust
If that won't do then putty must
If all that fails then bloomin big nails!