Leaky oak window lintel

[Verdigris]

This is an upper floor window of our Cotswold stone house. It’s at the west-facing gable end.

When there is driving rain from the west it gets in above the window frame. Above the window is an oak lintel. It's pretty ancient and there are gaps where it meets the masonry, I think this is where the rain is getting in.
A neighbour found that linseed oil putty worked with his leaky door lintel. But looking into it further, I think the appropriate materials in this case might be oakum and/or burnt sand mastic:
https://www.greenbuilding.co.uk/GBF...1/burnt-sand-mastic-application-advice-please

I'd be glad to have your views.

As a separate question, is burnt sand mastic a suitable cosmetic treatment for oak beams with shakes?

Verdigris

 

[Cubist]

Best tell your neighbour that glaziers linseed putty will only provide a very temporary weather seal. It will dry and begin to break up in less than twelve months with hair-line cracks forming to begin with and then fall out as it separates from the surrounding materials. Burnt sand mastic will behave in the same way and can also fail much more rapidly than you might expect - particularly if you do not prepare the mix properly. Oakum should only be used where the gaps may be wider than a 2-3mm, its quite chunky and can must be 'rammed' into place. The Stockholm Tar oakum is soaked in will also leach into surrounding materials and paints to leave unpleasant and difficult to cover stains - it can also leach into stones.

For temporary and reasonably long lasting repairs Plumbers Putty (sold in most hardware stores as Plumber Mait) - which never sets and therefore retains its flexibility - can be used but this must be firmly pressed into place around the seams between the lintel, the frame and the stone/brickwork. It's messy to use and is best applied with a small spatula.

A permanent solution can be achieved using Caulking Cotton pressed into place between all the seams around the timbers and just below their surface. This can then be finished with either a lime mortar or exterior lime filler (see Ty Mawr's products) before painting - this last action being to prevent birds trying to pull the cotton out for their nests.

 

[Cubist]

Looking again at the lintel itself you might consider packing the various splits, cracks and holes with the caulking cotton and some lime mortar as these too could be routes used by driven rain to penetrate indoors. If you have other lintels like this one you should look closely at them too to see if similar treatment is needed for them too. An occasional coat of thinned linseed oil will also help to preserve them.

 

[Verdigris]

Thanks Cubist, that's very helpful. I'll get some caulking cotton.

Verdigris