Fixing Wood to Concrete - Advice Needed

[Tierney]

Hello all,

I will be building a fence in a school playground. I have been asked not to sink the posts into the tarmac and to use brackets and plug and screw. I'm concerned (as are the teachers) that the brackets may create a tripping hazard, so I was thinking that using studding or wood to metal dowel screws would provide some extra strength. It may be that I can keep the brackets at a minimum or remove them all together.

The problem with studding is that I can only think of a t-prong nut to attach to the wood; which, won't be great. The problem I have with using a wood to metal dowl screw is that I can only find 90mm long (45 in wood and 45 in the anchor bolt) dowel screws.

The fence is only 3 foot high and there are some right angles which will give some help to stop it tipping.

Any ideas or sources of fixings?

David

p.s. I volunteered to do this work, so don't want to dig my heels in about well digging in the posts!

 

[eribaMotters]

If you fix timber directly to the concrete ground my concern is rot setting in. If you have no other option open to you, such as the most sensible one of dwarf posts concreted in, I would "chem-fix" lengths of studding into the ground and then drop the bottom horizontal member of the fence over these and fix down. The chem-fix is usually available in a large mastic type tube. It is a two part adhesive, the two both present in the one tube and mixing in a special nozzle as you gun it into the hole. You need to drill the holes over-size eg 12mm for 10mm studding. Clean the dust out of the hole, squirt in the chem-fix, and then push studding in, turning as you do so. You should get at least 8 or so fixings done with one tube into holes 150mm deep.

Colin

 

[No skills]

As above, if they baulk at you drilling holes in the ground remind them that the (stainless steel) studding can be cut off flush with the tarmac easily and will not leave any trip hazzard/sharp edges if done properly. Some dpc under the wood touching the ground will ease the rot issue.

 

[Stormer1940]

Chem Fix or epoxy resin with threaded rod and a washer and nut.
Make sure the under side off the timber is treated well (Even paint and once dry smear clear silicone on the bottom)

Similar suggestion to the one above but thats the way I would go.

P.s I say the fence being at 3 ft high is more a tripping hazard :lol:

 

[Lons]

Hi David

Have I interpreted this correctly in that you are fixing directly to a tarmac surface (or is it an overlay of tarmac on say a concrete base)?

If it is a typical tarmac structure then you will have little idea of the materials below the surface. A standard type would be say a compacted roadstone base topped with a sub base and then the finishing rolled coat. These sub bases have no strength and drilling will just push stones aside unless you hit a large stone. You would need long studs to give any chance of a secure fix IMO.

If over a concrete base then a different story of course and drilling will give secure fixings.

I have used these http://www.screwfix.com/p/bolt-down-pos ... of-2/99237 many times.

Bob

 

[deserter]

The meta post system has a metal post holder like the spiked ones but without the spike and has a flange with holes for 4 bolts around it maybe these would prevent the trip hazard.

 

[jasonB]

Well the first thing I would do is make sure my PLI is up to date as if any little darling should hurt themselves on the fence the no win no fee types will be looking for all they can get. Be it a splinter, stuck finger or tripping over. Seriously you need to make sure you are covered even if doing it for free not only if getting paid.

If you are using vertical posts then I would drill up the end with a big say 20mm auger for about 300mm, drill the concrete the same, resin bond a 2ft length of 18mm stainless bar into the ground and then bond the post onto teh dowel. Put a large galv washer onto the dowel before the post to just lift it ogg the ground.

J

 

[beech1948]

JasonB got it right.
Public Liability Insurance is a must have to do this job. Any child with even a minor scrape can come and sue you. Thats you and not the school. The only way around this is if the local authority signs a document which absolves you of all blame whatever happens and then assumes such liability themselves. Good luck getting that signed.

If you are fixing into tarmac and sub base then just don't bother. The only approach which will work will be to dig a hole and concrete the posts in.

If fixing to concrete then any of the methods above should be OK but I prefer the chem-fix approach with stainless fixings; the bigger the better min 14 mm.

Great idea to put one or more galvanised washers below the post to keep it off the ground.

Al

 

[Tierney]

Thanks for all the advice, I like the idea of the post holder with the flange and sinking 4 studs about 250mm in and held with chem fix/resin; but, will speak to the guy at the school to see what he knows about the sub-base. The run isn't that long 1.5m then a right angle then 4m then another right angle and another 1.5m. If I cross brace the posts and rails it should give it some extra support.

I'm now suitably freaked out about PLI, so need to look into that also!

DT

 

[dickm]

Agree about the essential insurance aspect.
Don't know what they are called (could find out if it helps), but the front of our house has an overhang supported on laminated posts, which sit on top of sort of squared-off Y shape metal fixings. These have a cylindrical leg about 15mm diameter, and keep the posts about 75mm off the ground. But I guess they are really only intended to withstand vertical loading and don't have much resistance to sideways forces.

 

[Tierney]

Agree about the essential insurance aspect.
Don't know what they are called (could find out if it helps), but the front of our house has an overhang supported on laminated posts, which sit on top of sort of squared-off Y shape metal fixings. These have a cylindrical leg about 15mm diameter, and keep the posts about 75mm off the ground. But I guess they are really only intended to withstand vertical loading and don't have much resistance to sideways forces.

Thanks, I think I know the fixings you are talking about, but, agree with you that they are probably more for vertical loading. I'll have a look on the internet myself.

DT

 

[Hitch]

Fit a few rails and bits at schools-

Chemfix is excellent....usually the way to go on anything critical, or an if all else fails method... but as mentioned, the tarmac is probably ontop of rolled scalpings or similar. Which the chemfix wont be brilliant in.

The only way id consider it would be cut and dig out a nice neat square of tarmac, cast the posts in postmix, leaving it shy of the surface by a couple of inches, then use some patching tarmac to make good.

I fear you'll have a lot of recalls if you fix into the tarmac/scalps. Suprising how much force half a dozen kids trying to climb on a bit of fence can exert.

 

[Cheshirechappie]

Might be worth googling 'Hilti Resin Anchor'. Hilti do a range of products for just such jobs, but exactly what the sub-base of the tarmac is may limit your range of viable options.

 

[Tierney]

Thanks again for all the advice. I am waiting for the Rawlplug technical department to call me back about their product which they advertise as being suitable for fixing to tarmac.

DT.

 

[Eric The Viking]

I would ask the local authority (or whoever's in charge) for a method statement, specifying exactly how they want it done. That way, you're off the hook if anything happens subsequently.

It sounds like a really risky undertaking, technically and legally!

E.