On Sunday, 27 September 2020 at 17:08:38 UTC+1, John Rumm wrote:
On 27/09/2020 08:51, Martin Brown wrote:
On 26/09/2020 19:10, John Rumm wrote:
On 26/09/2020 18:11, Peter wrote:
Bay window has rot in the bottom of the 6 boards which form the
boxes. Going to cut away and piece in new timber lower 6-12" or so
each one. Could cut 90deg and put a couple of dowels in, or scarf to
increase the connecting end grain area. Or something else?

I quite like doing a scarf joint since you can then use screws into
the face to bring the joint together. You can use a two part resin
filler in the joint as well to form the glue and exclude any voids in
the process, and fill the screw head holes (if countersunk a little).

+1

But I prefer the space filling high strength high tack PU glues for
sealing the joint since the foam is somewhat more flexible to allow for
the wood dimensions varying with humidity. Two part resin filler looks
neater at first but invariably cracks after a year or two.

Two part is fine over the screw heads but I have always found that it
cracks sooner or later if used in a joint.
In reality any glue can fail if there is differential movement between
the parts. The foam of a PU glue does not have much strength though.
Something like gripfil may retain some flexibility for longer.
Ultimately trying to use a similar wood that will expand and contract at
a similar rate, and also painting everything really well to keep the
water out for as long as possible. (and ensuring things like drips,
seals, and flashings are in place and working)
--
Cheers,

John.

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Thanks everyone, lots of useful information there. If I screw a guide board at about 45deg to the existing, cut through with a handsaw/multitool then use the cut-away piece as template for new that should do it? Thinking Gripfill as glue, maybe drill shallow holes in each cut end to form sort of chemical dowels. I do have a circular saw and today bought my first multitool - for the bits of sill I need to cut out and will be needed for the boards forming a box corner - but generally feel more comfortable with hand tools. Please let me know if I've made any silly mistakes with the plan!
Cheers,
Peter