screwing down plywood over chimney hearth

[DrGonzo]

I'm currently screwing in my 12mm plywood into the floorboards.
There is one area by the chimney breast where the hearth probably used to be 100 years ago. It's concrete so I can't screw into it. Do I just secure screws around it into the wood? It'll leave a rectangular area where there aren't screws every 20cm

 

[gcol]

Ever heard of a masonry bit?

 

[DrGonzo]

do you have to drill a hole first or can you screw it straight through the wood and into the concrete?

 

[gcol]

Yes you drill the hole, knock a wall plug into the hole, then using the screw, drive the wall plug through the wood and into the concrete, then tighten the screw.

 

[DrGonzo]

okay, but at some point here the drill hole in the plywood is gonna be bigger than the screw isn't it...on the account of the wall plug usually being bigger than the screws.
I can see the screw going straight through the wood and into the concrete not actually securing the plywood at all ?

 

[gcol]

Yes the hole will be bigger than the screw thread but not bigger than the head of the screw.

 

[wap]

cant you use self leveler on that bit?

 

[gcol]

Can you explain your meaning wap? The concrete raised section is higher than the floorboards. How will self leveller reduce the concrete section so it is level? Or are you suggesting he brings up the level of the whole of the remaining floor?

EDIT: sorry wap, you threw me off at a tangent then.

 

[DrGonzo]

no, the concrete isn't raised higher than the floorboards, it just seems to be there instead of floorboards but where the original hearth must have been. It's the bit that would have gone under the hearth.

Anyways, it's not working very well. I can drill through but it doesn't seem to be securing very well.
Any other way to do it, like nail through the plywood into the masonry just for that small section? I think that's how the original chipboard was secured.

 

[gcol]

Ah sorry chaps I think I'm mixing up a few posts.

Are you using the right size drill for your plugs? Is the concrete crumbly?

 

[DrGonzo]

screws are 4 x 30mm.been drilling with a 6mm drill bit for wall plugs that say to use a 6mm bit and suitable for 3.5-5mm screws.
Don't think the concrete was crumbly but it's possible I guess although the old chipboard was nailed securely enough.

 

[gcol]

Are the plugs spinning or something?

 

[DrGonzo]

nah, doesn't look like it. It's just not pulling the plywood down and securing it. It remains loose and slightly raised from the floor whereas elsewhere it is secured tightly.

 

[gcol]

You have knocked the plug well into the concrete haven't you? Cheap plugs? Hole deep enough? Concrete dust piled up beneath the ply?

If you have no joy, you can stick that area down with flexi tile adhesive.

 

[DrGonzo]

Actually, it was probably sticking out a bit.
They are cheap plugs yeah and they're 40mm long rather than 30mm.
Any recommended makes and should they be 30mm or less even?