I echo what has gone before. I've made and repaired a few garden gates and doors over the years.
Do use air dried timber and if necessary tell the client that its essential that the wood be conditioned and that takes time. Its a fact of life for a good product. It will save you problems later.
Probably a bit late for this, but I've got some lovely barn and garage doors made from Western cedar. It has very high tannin content and lasts for ever and is very light weight. It what greenhouse doors and wood were made from - probably to late for this commission but worth considering. Western cedar can be difficult to source but not always.
Avoid T&G joint for the boards if possible. I've repaired some old Edwardian oak gates/doors and they had lap type joints lasted 60 years. If the client wants that look (ie like a garage door), then allow gaps for movement. But I've found gates tend to rot in the T&G over time. Mortice and tenon for the rails to keep the frame sturdy but allow for the boards to move, ie fix in the centre of each board so that it can move laterally either way.
Finishing oils or not?
I'd talk to the client about finishing oil or NOT. If you apply a really good finish, it will need to be re-coated every year or so to keep the appearance good. In my experience leaving it unfinished looks nicest over time, it goes silvery and stays that way, maybe gets a bit green from algae, but an occasional wash with jays fluid cleans it up - once every 5 years or so. or a very light oiling. Otherwise I've found the finish starts to weather unevenly. - except for the top rail - see bellow. If the client wants to prevent the oak going silver, it will need coating in a UV absorbing finish and that will need rubbing down and re-coating every few years - quite a lot of work, but thats their choice.
Do protect the top rail with a lot of finishing oil, or a felt or metal cover- its not seen so you can do that. Design it so standing water runs off/ is on metal or felt. One advantage of the curved top rail is water will run off and its looks nice.
Thanks for all advice so far everyone.
@TominDales when you say lap joints do you mean a sort of 50/50 overlap on every board, like a shadow gap cladding profile ?
I was considering this idea, thinking I could still add a V to give the same look.
I will need to profile it into the head and stiles as well.
This job Has still not started as I had to build and install some sash windows in the same house first and nothing is ever straightforward of course.
I have a big stack of air dried oak coming this week for the two gates and also a front door and frame.
I have been reading up on different oil finishes. I was going to use Owatrol but they say ideally need to put on a different product leave it a year then apply the oil or use a preconditioner that cannot be used on Oak.
I need to finish the job and leave, can't be doing refinishing in a year.
I can of course advise my client that recoating after 18 months is a maintenance thing and will leave them with the remains of the can of whatever I use.
So what finish does everyone use or recommend for external oak, and half internal half external in the case of the front door?
Osmo uv oil ? I understand it will need a primer of preservative first.
Fiddes exterior wood oil ?
Rubio monocoat ?
Something great I haven't heard of yet ?
The finish desired is fairly natural colour with a satin type finish and no film finish.
Pretty confusing when you read all the small print on stuff.
Ollie