Concrete over floorboards

nicks43skr

Hi,

I'm planning to create a concrete hearth to the front of my fireplace, which will continue inside the fireplace allowing me to have a nice fresh upstand to tile.

The constructional hearth is 750mm wide x 330mm deep.

I want to make the concrete hearth extend to the full width of the fireplace which is 1200mm (ie 450mm - 225mm extra to each side of the constructional hearth.

Would it be possible to concrete over the floorboards by 225mm? if so, do I need to apply some PVA etc to make them adhere better?

I'm planning to pour to a depth of 70mm, so effectively each end on top of the floorboards will be 225 x 35 x 70mm so not a huge weight.

Thanks

cyclonebri1

I was trying to think of a better way but to be honest I can't as I don't think the weight will be a problem.

I think you will be ok provided that you include several layers of reinforcement, in this case I'd go for say 1" sq x 16 weldmesh or similar. If you don't it will certainly crack.

I take it this is to conform with safety reqs for a fire to be fitted?.

Forget about the PVA, wood and concrete will expand/contract at different rates so just let them get on with it, ensuring the slab stays in 1 piece with it's anchorage into the hearth proper will do the job.;);)

nicks43skr

Hi Bri,

Thank you.

The weight to each end will be very small - the floorboard joists will sit below also - I think it's only 55cm4 ish that will be sat on wood each end.

Where can you obtain weldmesh from - is this likely to be in a standard DIY store? will this really be necessary at 70mm deep?


It's to comply with regs, but also to offer something decorative to the outside of the fireplace, and also so we can use it initially as an open fire and then when the money tree blossoms, a woodburner can sit in it.

I intend to overtile the concrete with 100 x 100 x 10mm slate tiles or similar.

cyclonebri1

nicks43skr wrote: »

Hi Bri,

Thank you.

The weight to each end will be very small - the floorboard joists will sit below also - I think it's only 55cm4 ish that will be sat on wood each end.

Where can you obtain weldmesh from - is this likely to be in a standard DIY store? will this really be necessary at 70mm deep?


It's to comply with regs, but also to offer something decorative to the outside of the fireplace, and also so we can use it initially as an open fire and then when the money tree blossoms, a woodburner can sit in it.

I intend to overtile the concrete with 100 x 100 x 10mm slate tiles or similar.

I think it's the critical bit of what you are doing, ie, trying to keep the hearth in 1 piece rather than keeping it attached to the floor. Folks will stand on the edges and the floor will flex to some degree. Given you want a lasting job it's the 1/2porth of tar thing.;)

Chicken mesh is the norm for this sort of application, but I think in this case you need something a little stronger. Ordinary concrete rebar mesh would be too coarse for this application.

Weldmesh from good DIY places and garden centres, used for rabbit hutches, aviaries etc,

6" x 6" Ruabon quarry tiles were/are the norm for hearths, well 'ard and last forever but they are out of business but some are still available. Note if you use those you will need RX and RXX tiles too, (round edge and double round edge for corners. Just my taste, but go with something you like.
I wouldn't use slate for a real fire hearth, it scratches up too easily with ash removal etc.

tony6403

Maybe using some Fibremix would be an easier way

cyclonebri1

Yes it would be easier, would it be as good? To be honest I'm not sure.

I did specify added fibres in the last load of ready mix I used, but I also used rebar mesh.

It's a great product, it certainly makes a mix "claggy", but I think it's designed to give strength over wider areas where expansion craps are likely to occur, rather than in this case where the op is effectively creating an over hanging lintel.;)

RDG

Those thin poly fibres do not increase the strength of the concrete. The thicker ones both plastic and steel do improve cracked strength and reduce crack openings, the steel ones perform better than the plastic.

The thin plastic fibres do however reduce the movement of the solid particles within the concrete (as cyclone says more 'claggy') and also reduce bleed. As they reduce bleed the water cement ratio within the concrete is maintained and therefore you get slightly better curing. They also help with impact resistance.

In this case the larger steel fibres could be used to replace the proposed mesh, adding 35kg/m3 would be sufficient to give the strength needed. Im not sure if you can buy them from a builders merchants though as they are a specialist product.

If you go with mesh it needs to be in the top and the bottom to provide sufficient strength to bending.

nicks43skr

cyclonebri1 wrote: »

I think it's the critical bit of what you are doing, ie, trying to keep the hearth in 1 piece rather than keeping it attached to the floor. Folks will stand on the edges and the floor will flex to some degree. Given you want a lasting job it's the 1/2porth of tar thing.;)

Chicken mesh is the norm for this sort of application, but I think in this case you need something a little stronger. Ordinary concrete rebar mesh would be too coarse for this application.

Weldmesh from good DIY places and garden centres, used for rabbit hutches, aviaries etc,

6" x 6" Ruabon quarry tiles were/are the norm for hearths, well 'ard and last forever but they are out of business but some are still available. Note if you use those you will need RX and RXX tiles too, (round edge and double round edge for corners. Just my taste, but go with something you like.
I wouldn't use slate for a real fire hearth, it scratches up too easily with ash removal etc.

That's brilliant, thank you for your help