Fixing Met Posts into Concrete

diy_darren · 17 Jul 2007

I'd like to put a fence up, but on top of a concrete surface. I bought some 4" bolt down metposts and M10 bolts (see photo).

I've never used these expanding bolts before and wanted some clarification before I mess it up! I can see that when the expanding section can no long travel up the thread the tapered end will cause it to expand and wedge in the hole. But, the expanding section will only stop when it rises to the top of the thread, meaning it will expand about 1.5" below the surface. Surely it will be more secure if it expands as far down into the ground as possible - so what am I missing

masona · 17 Jul 2007

No, once the wedge get a grip and tighten in the hole, the bolt will tighten downward, just make sure the wedge holder is just below the surface

diy_darren · 17 Jul 2007

Umm, what's the wedge holder?

How does the wedge get a grip? Doesn't it need some resistance against it (or to hold it) otherwise the expanding section just rides up the thread instead of being pushed apart?

masona · 17 Jul 2007

The cone wedge inside the sleeve, put the rawlbolt in the hole of the base as a test then tighten the bolt, you will see what will happen

noseall · 17 Jul 2007

don't forget!

once you put 'em in, you may not get 'em out .

Trazor · 17 Jul 2007

diy_darren said:
Surely it will be more secure if it expands as far down into the ground as possible - so what am I missing

You are missing nothing, these rawlbolts will pull up the thread as you thought, and will only tighten when they can rise no further.

If this worries you, buy longer rawlbolts.

diy_darren · 17 Jul 2007

When I'm drilling holes for these bolts should they be just big enough to get the expanding section in?

If so, then I can see the hole will provide enough grip to prevent the expanding section turning. As I wind the nut it will pull the wedge upwards and everything makes sense.

Trazor · 17 Jul 2007

Rawlbolts are designed to go just below the surface, and should pull up tight against your base plate or washer.
If you have a gap between the head of the bolt and the expanding section they are not as secure.

In concrete, sleeve anchors are the better option in my opinion.

But which ever option you use, the hole you drill wants to be as tight as possible.

breezer · 17 Jul 2007

I wouldn't fit them there though, in a middle of a carpet is not a good idea

(not enough"meat" in a floorboard)

I always tap them in with a hammer (thats tap, not whak)

Deluks · 18 Jul 2007

Much better (and slightly cheaper) to use hammer in studs (throughbolts/parabolts), or thunderbolts.

Fix each bolt in turn through the metpost, as you drill the holes. Don't mark and drill all in one go cos they sometimes don't all line up properly when you try and put the metpost down. Also start the hole in the concrete with a smaller diameter drill for greater accuracy.

diy_darren · 18 Jul 2007

trazor said:
Rawlbolts are designed to go just below the surface, and should pull up tight against your base plate or washer.

If I'd have known that beforehand I'd have not bothered to buy such long bolts considering I'll have about 3" of thread sitting below ground doing nothing useful!

I looked at throughbolts, but thought they would leave an untidy finish above the bolt, even if I did cut the excess thread off. Thunderbolts look like a good option though.

The concrete currently isn't level so I'm going to have to try and square it off first (just to get the posts on a flat surface). I was concerned that in doing so I may weaken parts of the concrete especially if the rawlbolt is going to expand just below the surface. Thunderbolts shouldn't have this problem though - sound like a better approach?

WabbitPoo · 18 Jul 2007

Never tried to do what you are doing, but how about putting them onto a dollop of strong mortar, tapping it all level with the bolts part way in. When the mortar is good and hard, tighten up. Should end up level.

Anyone any thoughts on this?

diy_darren · 18 Jul 2007

If it helps, this is the piece of ground I'm trying to fence across

http://www.tilion.org.uk/images/ground.jpg (very wide pic, so linked instead)

Left hand side has a post concreted into the ground, right hand side can be attached to the house wall, but I need to fix two metposts into the uneven ground - roughly along the yellow chalk line.

Deluks · 18 Jul 2007

WabbitPoo said:
Never tried to do what you are doing, but how about putting them onto a dollop of strong mortar, tapping it all level with the bolts part way in. When the mortar is good and hard, tighten up. Should end up level.

Anyone any thoughts on this?

Yeh that works, preferably the fence post should be already tapped into place so you are levelling the post itself and not the metpost. Will need a couple lengths of batten screwed to the side of the timber post and a couple of bricks to support it while the mortar goes off.

masona · 18 Jul 2007

Deluks said:
WabbitPoo said:

Never tried to do what you are doing, but how about putting them onto a dollop of strong mortar, tapping it all level with the bolts part way in. When the mortar is good and hard, tighten up. Should end up level.

Anyone any thoughts on this?

Click to expand...

Yeh that works, preferably the fence post should be already tapped into place so you are levelling the post itself and not the metpost. Will need a couple lengths of batten screwed to the side of the timber post and a couple of bricks to support it while the mortar goes off.

It can be done that way but the rawlbolt have to be tighten gently otherwise the concrete underneath will stop the bolt going downward and the other problem is that the concrete take a long time to harden therefore concrete may cracks when tighten

diynot

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Fixing Met Posts into Concrete

breezer

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