hole in the worktop

[dotto]

Hi all.
A friend has bought a marble washbasin and wants to build its own worktop and cabinet underneath for the storage of towels. My question is when drilling the hole in the worktop to accommodate the basin waste outlet should the hole in the worktop be exactly the same dimension as the outlet so that when i tighten the back nut, from underneath the worktop, it grips on the worktop holding the washbasin in place or should the hole be bigger allowing the back nut to go through the worktop when tightening it up ( but then if somebody pushes the basin it will be easily displaced)? BTW i should mention that he wants the whole washbasin to rest on the worktop surface. Thank you 4 your advice.

 

[dotto]

Hi all.
A friend has bought a marble washbasin and wants to build its own worktop and cabinet underneath for the storage of towels. My question is when drilling the hole in the worktop to accommodate the basin waste outlet should the hole in the worktop be exactly the same dimension as the outlet so that when i tighten the back nut, from underneath the worktop, it grips on the worktop holding the washbasin in place or should the hole be bigger allowing the back nut to go through the worktop when tightening it up ( but then if somebody pushes the basin it will be easily displaced)? BTW i should mention that he wants the whole washbasin to rest on the worktop surface. Thank you 4 your advice.

Any help on this one please?

 

[gavinda]

im no plumber but surely the backnut is there to grip the tap and help make a seal from ' wet top' to ' dry bottom'.

 

[regsmyth]

I would have thought the hole needs to be big enough for the nut to pass through. It's important that the waste/plughole is securely tightened to the basin. If you just tightened it up against the underside of the worktop, you could have a leak and not spot it till the worktop rotted out.
The basin should have it's own way of securing to the worktop. If not, a continuous bead of sealant all the way round will do a pretty good job.

 

[Kes]

We can assume that the washbasin has a flat bottom that fits flush on the surface of the worktop, and the worktop is laminate?

Three or four years ago I was saddled with a circular (semi-spherical) ceramic washbowl sitting on a laminate worktop (not my choice, but there you go). There was no provision for holding the washbasin down, and it had a fairly small contact area, about 6" diameter. I drilled the hole in the laminate just large enough for the waste to go through. I bedded the (solid) waste in the basin with silicone sealant, applied sealant to the base of the basin - I can't remember if I put sealant all the way up to the waste fitting - and then bolted the lot up from the underneath of the laminiate.

It certainly holds the basin well. There is only one joint that really needs to be watertight, and that's the waste fitting to bowl. Of course there's no way of checking that it is watertight, but a good application of sealant is the best you can do. It's all fine so far, with the proviso that the basin gets only occasional use.

Of course if your marble washbasin takes three men to lift onto the worktop then it's weight will hold it fine.

Oh yes, use a brass waste fitting, not a plastic one.

PS In deference to Tickly, the above refers to a solid (non-overflow) waste.

 

[TicklyT]

If there is an internal overflow waterway within the basin, and/or overflow slots cut in the basin waste, then the the joint will be exposed to water unless the waste is sealed to the bottom of the basin.

I would consider a 'stepped' hole, to accommodate the waste backnut sealing the waste to the basin, then fit a second backnut to the waste tail to secure the basin to the counter.

That way you can make up and test the basin waste seal before permanently installing the basin on the counter.

Alternatively, a parallel clearance hole, and fit a packing piece or large washer (I found a 1 1/2" backnut slipped over a 1 1/4" basin waste works), then the second backnut to secure the basin.

 

[dotto]

If there is an internal overflow waterway within the basin, and/or overflow slots cut in the basin waste, then the the joint will be exposed to water unless the waste is sealed to the bottom of the basin.

I would consider a 'stepped' hole, to accommodate the waste backnut sealing the waste to the basin, then fit a second backnut to the waste tail to secure the basin to the counter.

That way you can make up and test the basin waste seal before permanently installing the basin on the counter.

Alternatively, a parallel clearance hole, and fit a packing piece or large washer (I found a 1 1/2" backnut slipped over a 1 1/4" basin waste works), then the second backnut to secure the basin.

Thank you v much 4 your inputs guys. I think i'll go with regsmyth and put some silicone under the base of the basin to stick it down onto the worktop surface.I just popped in B&Q and they have these sort of washbasins, on show, siliconed down. I tried to move one of those and it did not budge, so i think the silicone business should do the job. Thank u everybody.