Fence post plate concrete footing - how big?

I'm going to be putting up a 6'x3' decorative screen vertically (see www.screenwithenvy.co.uk), using 100mm wooden posts (their posts are far too expensive). I don't fancy using post spikes as they look difficult to get perfectly positioned and vertical - vital for this job, plus it would also mean buying a sledgehammer!

Instead, I was planning to pour a small concrete footing for each post, and use those bolt down post plates. How wide and deep would each footing need to be?

Another option might be to screw the plates to a couple of paving slabs (concealed under the gravel next to the patio). Would this be sufficient, and if so what size slabs would I need? My concern here is the length of the screws, given that the slabs will be barely 2 inches thick.

Why not just postcrete them in?

ktuludays said:

Why not just postcrete them in?

Click to expand...

I didn't want anything permanent. Once the wood starts to rot then it's not easy to replace. I suppose I could postcrete in a spike...

I've some something similar. You can use the bolt-down or cast-in steel sockets. The bolt down ones in particular are much easier to remove when you want to change things round. You need a block of concrete deep enough that it will not be disturbed by garden digging, and I recommend that you use shuttering, above the ground and a few inches under, so that you have a neat-looking concrete foot. By raising it several inches above ground level you reduce the risk that rainsplash or damp soil will accelerate rotting of the post.

HOWEVER

I used this for a carport and shed. The load will mostly be downwards, which the sockets and block can easily accomodate. if you use it for anything like a fence or screen, the loading will be sideways away from the wind, and this will cause the posts to move in their sockets, and the concrete block may even move unless it is much deeper. For example if you had a six-foor fence, you would need to concrete in the post at least two feet deep. However well you tighten the sockets, they will not give such good lateral support.

You could also use concrete spurs, that the wooden posts are bolted against. The wood is raised above ground level to protect it from damp, rot and insects. you can paint the concrete and sockets. I use dark chocolate masonry paint on the concrete, to blend in with the dark fence stain on the wood. The steel sockets are treated with an anti-corrosion primer and oil paint.