How deep do I need to put posts into the ground for a 3 metre screening fence?

[evabee]

 hello, 

Im hoping someone can help me figure this out please. I am wanting to build a screening to block our neighbours, our house sits high and my kitchen window sees straight into their house/granny flat that someone lives in. 

I wanted the screening to be 3m high with a cantilever bit at the top maybe around 1m, and the screeninging to be approx 5m wide. 
I was hoping to take the screening half way down the fence and build a bench seat or planters or both below. 
So my question is, do I need to dig the hole for the posts about 1.5m? My soil is clay, and really hard to go past about 80cm? Is there another way to secure the posts?

And will 100x100 treated pine posts be ok for this kind of structure or do I need thicker? 

thank you

Eva

pictures for example of what I want to build. 

 

[EricL]

Hello @evabee 

Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about installing a screening fence.

Before you begin, I suggest checking the rules and regulations of your local council regarding maximum allowable height for fencing/screening. Your sample pictures are very nice, since the structure you are building involves an extra element it is no longer an ordinary fencing project. I propose engaging the services of a builder or engineer to draw up a plan for your screening. They will be able to calculate how deep the mounting post needs to be and how big the concrete footing will need be made.

Just to give you a general example, in Queensland, fence posts should generally be buried at least one-third of their length into the ground, and ideally, up to 600mm deep, especially in areas prone to cyclones. For residential fences, a minimum of 600mm is recommended. Please keep in mind that situations may warrant that the post be buried deeper in order to withstand extreme weather.

Let me call on our experienced members @Dave-1 and @Nailbag for their recommendations.

If you need further assistance, please let us know.

Eric

[Dave-1]

Good Evening @evabee 

Nice screen idea and if it was a straight screen and no cantilever then I would have said call it at 800mm But with the cantilever it adds a whole new set of weights to a structure, think of it as an unbalenced seesaw and you want someone out on the high arm. An L shaped structure would work as well as the small L part would support the posts wanting to lean. I would still do all the posts at 800mm if you went with an L shaped structure tho.

I  intend to have a screen next to my fence of around 2.4m I think and the holes I am planning will be 600mm deep at least. No cantilever and protected from wind via the fence. I dont want to add weight to the fence tho as it will pull it out of true.

Dave

[Nailbag]

This looks like an awesome project.

Rule of thumb @evabee is 1/3rd the length of the post and between 300mm to 400mm square. In sandy soils, then a greater depth is recommended. 

Use post hole pincers to dig the holes to keep them compact and well formed. You will need this tool in order to remove the soil once the shovel can no longer achieve this. Add a little bit of water to very dry/sandy soil to make it sticky to help pull the soil out at depth.

Nailbag

[evabee]

Thank you for your replies, appreciate it. 

Hi Dave

Could you please elaborate on the L shape structure?  
Do you mean if I did the structure as an L shape and no cantilever and had the supports as per the photo of the black screening, the 800mm hole would suffice? 

Here’s a photo of my 2.1m fence from my kitchen window where I am wanting to put the screening.

thanks again,

Eva

 

[JacobZ]

Hi @evabee,

Allow me to tag @Dave-1 so he is notified of your question. I'm sure he will respond shortly to explain what he meant.

The first two screens look to be made of aluminium, which is lighter than timber, meaning it can work without the diagonal bracing. If you're using timber for the overhang, then you will more than likely need some diagonal bracing to support it, regardless of the depth of the footing.

As the rule of thumb is one-third the overall height of the post in the ground, you should definitely aim for them to be as deep as possible.

Jacob

[Dave-1]

Afternoon @evabee 

To describe the L shaped structure I was thinking of. Will use the wheelbarrow as a point of reference. And we are looking from top down at your yard from above.

From the red wheelbarrow come towards the photograher by a meter (this is the short part of the L)

From the red wheel barrow go along the fence the 3m you want (This is the long side of the L) I would actually go 4m or even a bit more so there is a good overlap as it will make it easier on the eyes for your side.

The Short side of the L (red wheelbarrow towards you by a meter will act as a brace to steady the privacy screen, whichin turn means you could have that cantilever you mentioned. 

If you need a sketch I can do one tomorrow Just let me know.

Dave