Is there any way to join/seal large exterior mitre joints such as on a bay window sill so that they don't open up?

Question

As per the title, I work at a conservatory manufacturer and as such we often have large bits of hardwood (up to around 56x220mm section) which need to be joined with a mitre, such that the larger face is sitting on brickwork.

Typically we cut the mitres accurately, machine worktop bolts into the undersides and sometimes dowels or biscuits for locating the pieces together correctly on site. We seal the end-grain of the pieces with a purpose-made end-grain sealer, then paint with water-based microporous paint.

Whenever I go out to site to a job which has been installed for ~6 months or more, the sills and any other large mitred joins have typically opened up. I don't think that the pieces of timber overall have moved, I think it's more to do with across-grain shrinkage of the timber(?). I also see the same on pieces which other people have supplied, installed and finished such as internal window boards/sills.

Is there some way of joining them, some kind of sealant I can use etc. which will stop this from happening?

Note I have seen this question: End grain mitre joints for outside but I believe my question is a bit different because it's specifically dealing with very large section sizes.

Answer 1

Select the wood more carefully
Switch to using quarter-sawn boards. Without changing the species of wood currently used selecting quarter-sawn only for these pieces would go a long way towards minimising the problem because of the reduced shrinkage (typically around half that in plain-sawn wood).

I think this is unlikely to be seen as a cost-effective solution in a modern commercial environment because of the premium placed on QS stock.

Change material type
Stop using natural wood altogether for these elements. This could be much more doable as far as the decision makers in the company are concerned. Either switch to a composite material, something along the lines of DymondWood, or to a stabilised-wood product such as Accoya.

Change coating system
Again I'm sure this won't float as a solution but if you changed your coating system you could stick with your current wood (irrespective of what it is, good or bad) and the issue would vanish overnight.

The problem with this as a fix though is you'd be switching to an epoxy and have to be religious about not leaving any gaps or voids that would allow moisture ingress. In addition to epoxies being very expensive the initial coating would have to be carried out carefully (i.e. it would be slow, and slow costs) but probably most importantly the maintenance schedule wouldn't be tolerable the way it is in a marine environment. Ongoing and regular finish maintenance is an accepted fact of life around boats but I presume it wouldn't be acceptable for conservatories where customers expect minimal maintenance.

Don't use mitres for these joints
This is the simplest and cheapest solution by far but I don't know how acceptable it'll be seen to be.

Wide-board frames/casings were often done using simple butt joints in the past precisely for the reason that mitres open up over time, and the wider the board the greater the gap that opens. By switching to a butt joint the wood can shrink all it likes and all you're left with is a discrepancy at the corner, not an unsightly gap in a mitre.