Protecting wooden posts set in concrete

NGEddie · 20 Jul 2020

I'm building a 3mx3m BBQ shelter. It's a large structure with a roof that will catch the wind like a kite, so I've decided the posts should be buried and concreted for strength rather than attached post holders.

I've got 150mmx150mm pressure treated posts that I'm burying 600mm into the ground.

Question - what can I use to add an extra layer of protection on to the wood before I set it in concrete?

The internet seems to recommend copper based preservatives or old fashioned creosote, both of which I can't find.

Thanks!

Stivino · 20 Jul 2020

When setting posts in concrete, the concrete should be around the post but not under it. I.e. the post should go through the concrete, that stops the post from rotting at the bottom.

KateK · 20 Jul 2020

I used these torch-on bitumen sleeves from a local sawmill when I replaced a garden fence in 2017. Not expensive and easy to use and appear to be working so far.

https://www.postsaver.com

NGEddie · 21 Jul 2020

Stivino said:
When setting posts in concrete, the concrete should be around the post but not under it. I.e. the post should go through the concrete, that stops the post from rotting at the bottom.

I dug the holes after some rain at the weekend and did did dig 150mm extra and fill with shingle.

KateK said:
I used these torch-on bitumen sleeves from a local sawmill when I replaced a garden fence in 2017. Not expensive and easy to use and appear to be working so far.

https://www.postsaver.com

I guess these act as a moisture barrier which could help, though moisture always gets in. I wouldn't know how to start with torch on application though.

The 150x150mm poles have cracks in the sides that look very nice, but I'm sure will mean moisture getting in to the core of the wood.
I'd like to treat the wood to avoid rot. Any suggestions for a good product for testing the ends of the poles that will be buried?

scbk · 21 Jul 2020

NGEddie said:
I'm building a 3mx3m BBQ shelter. It's a large structure with a roof that will catch the wind like a kite, so I've decided the posts should be buried and concreted for strength rather than attached post holders.

I've got 150mmx150mm pressure treated posts that I'm burying 600mm into the ground.

Question - what can I use to add an extra layer of protection on to the wood before I set it in concrete?

The internet seems to recommend copper based preservatives or old fashioned creosote, both of which I can't find.

Thanks!

Coal tar creosote is only available to "professional users" in a minimum of 20l. I have it here, but I don't normally bother with it on posts. The timber would have to be bone dry to accept and treatment, left to soak, reapplied, then left to dry before for a few days atleast.

I don't normally put posts in concrete (IMO concrete accelerates rot), rammed earth holds them rock solid. Or if the soil is really bad (soft) add some stone/hardcore.

DIYspanner · 30 Jul 2020

Engine oil on the post where it sits in the concrete and a thick coating of bitumen paint on the post where its surrounded by soil. For extra protection, wrap bitumen tape over the paint.

JohnD · 30 Jul 2020

Engine oil is a toxic pollutant and you are not allowed to put it in the ground or water.

Wood that is in contact with the ground will rot, especially around surface level where the proportion of water and of air is just right.

You can put concrete spurs into the ground, they will not rot, and bolt your wooden posts to those, clear of the ground.

DIYspanner · 30 Jul 2020

JohnD said:
Engine oil is a toxic pollutant and you are not allowed to put it in the ground or water.

Not in the ground, applied to the post and left for a week to be absorbed into the wood. The bitumen applied when the post is installed in the concrete sleeve, seals the top of the concrete, preventing the ingress of water.

JP_ · 30 Jul 2020

Wood effect plastic posts, just for the supports in the ground?

Notch7 · 30 Jul 2020

KateK said:
I used these torch-on bitumen sleeves from a local sawmill when I replaced a garden fence in 2017. Not expensive and easy to use and appear to be working so far.

https://www.postsaver.com

This is the best answer.

If you can't get them buy a roll of bitumin flashing. Get it nice and warm in the sun, then stick it on and use a wallpaper seam roller to get it down tight with no air bubbles. If it helps use a heat gun.

A mate of mine used to do this -his fence posts are still good after 15 years.

DIYspanner · 30 Jul 2020

There was the plastic post liners that got pitched on Dragons Den.

I've also seen sand install posts. The post is set in sand if it needs to be replaced, the sand is hovered out and the post pulled out. New post in and back filled with sand.

JohnD · 30 Jul 2020

DIYspanner said:
I've also seen sand install posts. The post is set in sand if it needs to be replaced, the sand is hovered out and the post pulled out. New post in and back filled with sand.

I got one like that, accidentally. I was trying to drill the old stump to fragments I could pick out; my big drill got jammed, I pulled on it, and the stump came out of its concrete socket.

I surmise that the heat from the drill (it was charred) had dried out the timber enough to shink a bit and loose its grip.

I actually inserted a concrete spur into the hole, and ran sand round it. Now the sand is damp it grips tightly and does not rock.

DIYspanner · 30 Jul 2020

I replaced 15 posts last year. I pulled the old stumps out, tapered the bottom end of the new ones and knocked them in.

There's a variety of ways to get the stumps out. If not rotted and crumbly that will easily hack out, x2 M12x200 coach screw drill for and screw in with chain attached, then pulled with a good strong bar shackled to the chain.

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Protecting wooden posts set in concrete

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