Screw size for fixing post to concrete terrace

[RKF]

I'm constructing a wooden fence to attach to the side of a concrete terrace but am uncertain as to the size screws / bolts required. The posts are 9cm by 9cm, and 2.4 metres in length. I was planning to cut them down to 1.5 metres, giving 1.1 metres of post from 'floor / terrace' level up, with 40cm then available to drill through for fixings to the wall. The fence itself will then have two horizontal bars on it, hence not too much wind resistance.

Questions:

1. I've identified 165mm long screws which with them being countersunk, would give me approx. 7cm of screw actually in the concrete. I was planning on using three screws per post.
2. Is 40cm enough or will the post be more rigid if I allowed say 1m for screw fixings, hence being able to space the 3 screws out more?

Appreciate any help on this.

 

[Deluks]

The twin rails idea sounds good against wind resistance but would it stop a small child or pet from falling through it? Maybe use metal decking panels or hit 'n' miss boarding?

The problem with these things is that you usually don't know whats behind the face of what you're drilling in to.

I'd be veering towards a metal L shaped bracket screwed into the floor and bolted through the post. Long, like 250mm long metal plate into the ground and maybe 75mm going up the post.

The longer the section of post against the wall, the more fixings you can fix it with and also reduces the 'leverage' factor from the top part of the post. As you are cutting a 2.4 down to 1.5 then you will have loads of waste. might as well use the complete 2.4.

You haven't mentioned how high up this terrace is though

 

[RKF]

Thanks for the input...I'd assumed the 'L' bracked idea would be weaker?

The terrace in question is built on a slope. It actually houses a store area underneath it which is closed off. At the highest, the drop from the terrace is c. 2.2m reducing to 1.8m along the lower wall. As I get to the other side of the terrace, I'll be digging big holes to embed the posts in the ground - the terrace is c. 20cm above ground level at that point. And obviously between these two sides, I have a drop which decreases as I come up the slope.

As I have some screws, but no 'L' bracket, sounds like I should start with the full 2.4 metre length and see how strong the fixing it. Do you think my length of screw is good enough (165mm, hence 7cm in wall)?