Fixing Fence on top of cavity block wall

thejaguar

I'm planning to add a fence panel about 800mm high on top of a 6 foot cavity block wall (approx 2.4m width).

I'm going to fix three posts (probably 3x2) to the wall and fix the fence panel to the posts. I'll aim to hit the end section of the bricks (avoiding the cavity) and I was just going to use these concrete screws without any plugs.

I'm wondering is there anything in particular I need to consider that I haven't thought of.

witnessmenow

We fixed a fence to a block wall recently (not cavity though) and used 3x2 as the posts, we used express nails to fix it to the wall.

We pre drilled the holes in the timber, clamped the timber to wall, drilled the block wall through the timber, lump hammered in the express nail. (I think I drilled and hammered in each one before moving onto the next one so it ensure line up)

(I know nothing about anything though!)

fiacha

I've used those on solid blocks to secure shed door frames / metalwork, and they have been fine. IIRC they need a minumum of 30mm of the thread in the wall. I used a 1/2 size smaller SDS bit than they state on the box. My SDS drill makes a slightly sloppy hole, not sure if this is the same with them all.

Have you thought about the wind load on the fence ? Depending on the prevailing wind direction / site exposure etc you may need to increase the number of posts etc.
How long is the wall ? Is it in good condition and are there sufficent pillars on it ? A bit of an extreme example, but a neighbor had a wall collapse because the fence panels caught the wind and it fell out into the lane. It was a diy wall which didn't have enough pillars to support it over its length and height.

thejaguar

The wall is brand new and I'm pretty sure it can stand up to it. It's only 2.3m wide - I reckon 3 posts will be plenty. The fence will be hit & miss so it'll allow some wind through it.

It can get a bit windy - but at the moment people can just stroll past and look into my back garden because of a difference in the level of the ground inside and outside. I'll take my chances with the fence and hopefully I don't have any major problems!

listermint

thejaguar wrote: »

The wall is brand new and I'm pretty sure it can stand up to it. It's only 2.3m wide - I reckon 3 posts will be plenty. The fence will be hit & miss so it'll allow some wind through it.

It can get a bit windy - but at the moment people can just stroll past and look into my back garden because of a difference in the level of the ground inside and outside. I'll take my chances with the fence and hopefully I don't have any major problems!

A new wall would be less strong than an old one fyi. Concrete and mortar takes months to chemically set and get to full strength.

Don't take chances the fence could literally pull the entire wall down. I'd be looking to ensure the posts come down to ground level and are fixed in post holes or holders. And the fence has sufficient gaps for airflow.

thejaguar

listermint wrote: »

A new wall would be less strong than an old one fyi. Concrete and mortar takes months to chemically set and get to full strength.

Sorry - brand new meaning 2 years old - I just meant it's not old and crumbling!

awec

If you bring the posts right down the wall and into the ground you could then cover the entire wall with your new fencing making it look more consistent?

More expensive, but sturdier and more visually appealing IMO.

thejaguar

The wall is at the side of my house - the fence (and posts) will be on the back. I'm not too worried about the visual on that side. It's currently my bins.

Having said that - coming down to ground level does appeal to me. Obviously more sturdy and might open up some possibilities down the line if I did want to make it more attractive.

Plenty of food for thought.

witnessmenow

thejaguar wrote: »

The wall is at the side of my house - the fence (and posts) will be on the back. I'm not too worried about the visual on that side. It's currently my bins.

Having said that - coming down to ground level does appeal to me. Obviously more sturdy and might open up some possibilities down the line if I did want to make it more attractive.

Plenty of food for thought.

That was one thing I forgot to mention in my post, we brought the posts down to ground level and each post was held in by 3 of the express nails.

Fine Cheers

Don't know if this is of any help.
Currently cladding my wall and have used 4 x 4 posts so as to get past the piers.
I used the masonry bolts (not screws) to fix the posts to cavity block. No prob.
But yours is slightly different obviously going above the wall.
The timber is simple 4 x 1 and 2 x 1 treated, cheap as chips.
Plan to give it a lick of Textrol in the spring.

SEPT 23 1989

DIY KING wrote: »

Don't know if this is of any help.
Currently cladding my wall and have used 4 x 4 posts so as to get past the piers.
I used the masonry bolts (not screws) to fix the posts to cavity block. No prob.
But yours is slightly different obviously going above the wall.
The timber is simple 4 x 1 and 2 x 1 treated, cheap as chips.
Plan to give it a lick of Textrol in the spring.

Nice job

What size bolts are you using?

thejaguar

This is the wall in question. I want to go about as high as the cavity blocks on the taller wall.

Posts fixed to the ground would need to be fitted flush to the wall ideally. It might be overkill for the size of it.

thejaguar

Slightly better pic.

You can see the fence outside. That's about eye level for an average sized person.

Fine Cheers

120mm M10 masonry bolts
https://www.screwfix.ie/p/easyfix-bright-zinc-plated-carbon-steel-concrete-bolts-m10-x-120mm-10-pack/1224p

Calahonda52

Is th wall clad with stone on the outside?
Any planning considerations?

thejaguar

Calahonda52 wrote: »

Is th wall clad with stone on the outside?
Any planning considerations?

It is.
I never considered planning to be honest. Ill have to look into that.

Calahonda52

thejaguar wrote: »

It is.
I never considered planning to be honest. Ill have to look into that.

The stone cladding will add more mass to the wall.

Planning may not be an issue but given the pedestrian proximity, it may, repeat may, be an issue, i dont know

thejaguar

The addition of the fence will definitely bring the wall above 2 metres and out of the category for exemption.

It's a strange one - because it wouldn't be 2 metres above the ground level outside my property - which is the case of the higher wall to the left hand side.

Our problem is privacy - people outside can just look directly into our back garden. I think there are provision in the local area development plan to allow for privacy, but I'm not sure how they would interact with planning.

Fine Cheers

I wouldn't be overly concerned with planning. Other posters correct to point out potential but what's the worst that can happen. Who's going to object or report you ? Is there a property management company involved on the campus / estate ? If you are selling and it becomes an issue then take it down. You see this type of work done all the time. The block work is well finished and pointed, unlike mine. I'd be tempted to go with metal support posts for longevity but a bit more work re drilling for attaching timber. If going with timber posts then slope the tops for water run off and maybe cover then with some dpc or similar. Is your gate timber sheeted and painted black ? If you match that i think it will look well from outside. Have u photo taken from outside ?

thejaguar

DIY KING wrote: »

I wouldn't be overly concerned with planning. Other posters correct to point out potential but what's the worst that can happen. Who's going to object or report you ?

I was thinking along similar lines. Want to look into it either way.

The gate is metal - I'm thinking I'll probably paint the fence black eventually to match in better. First priority is to get something up for privacy.

I never would have though of sloping the tops of the fence posts. I'll have a look at metal too, though that might be stretching my abilities a little.

Lumen

If you want long-term planning-compliant solution, plant something that will grow tall!

FWIW I don't think a foot or so of hit and miss fencing, suitably secured to both sections of wall, will be a problem structually, since that corner is inherently strong. But it's only a guess obvs, and shouldn't be construed as advice.

thejaguar

Planting is tricky - we don't want to plant in front of the wall because it's nicely finished and looks well from the front of the house.

Behind the wall is concrete. So I've nowhere to plant anything substantial. I had considered building a planter of some sort - even went and bought some bamboo to see how it would look. You can find pictures of my (nearly) dead bamboo in the gardening forum. Apparently bamboo doesn't like wind.

So at the moment I'm thinking I'll get a fence up there and then spend some time thinking about whether I should do something else or just stick with the fence.

Bass Reeves

Your problem will be the cavity blocks. If you drill into the hollow sections of these you will get no fixing point. Ideally you should put fixings at joints but when you go to next block below you will hit the center section of that block that is quite thin so again you will not get a strong structural connection.

An 800mm structure may be a bit high to support on a cavity block wall. If the stone capping did not exist I have suggested getting channell iron and putting it in the hollows of the blocks and filling down two block hollows with concrete. IMO the stone is doing nothing visually for the wall anyway