Building a pergola

[Deltawhiskey]

I want to build a pergola but I am confused about what type of treated timber to use and especially the timber in the ground.

David

[ESBkevin]

If you can get chesnut then treatment is not needed. All those post and rail fences along motorways are seasoned chesnut and last decades. But as a slow grow hard wood it is far more expensive than fast grown softwood and chemical treatment. I know of some chesnut fences that were alongside my grandfathers fields. 60 years on they are still standing except where mechanically interferred with (pushed over).
Is the below ground bit growing or likely to grow vegetables or edibles? Modern dipped timber is 'safe' (read not as good as it used to be) for growing but there are still doubts.
Old fashioned creosot (controlled to the trade now) will give timber a much longer life, but has all kinds of nasties in it. Similarly drenching the below ground timber in used engine oil before burying it will preserve it at the cost of perhaps poisoning the soil. I'm not sure if that would constitute and environmental spillage in modern law so it's not recomended.
So chesnut is the preferred stuff. If you use modern 'treated' timber, then pack stones of various sizes around tightly in the hole rather than repacking the soil. That way the wood is less likely to sit in a bath for months on end.

[bikermike]

Our pergola has metal legs in the ground that bolt to wood above ground.

It's lasted a good 15 years so far. What's going to finish it off is that the uprights have flat tops that allow rain to pool on them. I've put little hardwood hats on them, but some are already rotted.

[nickdub]

I'd second bikermike - best to use either metal or concrete supports in the ground then bolt to that. As has also been said chestnut is good for the top, or rough cut poles of yew would be good, though v hard to work with.

You can get pointed metal spurs, which you hammer in the ground, they have a socket on the top for a post - saves digging

[SarrissUK]

Whatever you do, don't use the metal post holders with a flat bottom that gets bolted into concrete. They're useless for anything that needs to be taller than two foot.

[Jay-ell]

You can get the tapered metal post holders which you hammer into the ground and can come in different below ground lengths. That way if you need to replace a post years into the future you just hace to take one out the holder and bolt the next one in.

https://www.wickes.co.uk/Wickes-Wedg...FcbjGwodU08MtQ

https://www.wickes.co.uk/Wickes-Wedg...FRET0wod3fIOeQ

I used them in making the fence for the fruiting hedge and for the front fence on the New Territories.

For posts I've used 2.4m (8') 75m x 75m (3"x3") posts. I also used these for the construction of the gallows on the site.

[Deltawhiskey]

Hi Bikermike,
I used the metal spikes for a dividing fence in the garden and I was considering using these for the pergola. However my ground is clay and stones and I had a hell of a job keeping the spikes plumb. The pointy bit goes all over the place after hitting a stone. Any ideas with that problem would be very welcome. I am growing vines up the poles and because of that the metal spike seems to be the way to go.
David