Recess tray - How important to be 3-6mm below level?

[tom-mk]

Hi,

We are in the process of building a Nidaplast based gravel drive. With the exception of adding and compacting more hard core and sand inside the edging, we've just about finished all our ground work, including fitting a new recess drain cover.

However we have set the edges of the recess drain cover to be level with the finished level. I have just read in the article Fitting Recess Trays that the edges should be 3-6mm below the level of the finished surface. What's the reason behind doing this? Is this something specific to block paving as opposed to using the recess with gravel? And how critical is it that the recess should be between 3-6mm lower than the final level? If it's really critical there is always the opportunity to take it out and reinstall it (before putting in the final layer of hardcore).

Thanks a lot.

Tom

[Rich H]

Hi Tom. You're right in your assumption that it's to do with block paving. BP is 'flexible' paving while any drain covers are fixed solid. The flexible paving can settle a little over time leaving the drain covers 'proud'.

With a gravel drive, you might want to consider this as well, especially given the amount you've dug out! It looks as though your chamber is built up with bricks so it's [relatively] easy to set the cover at whatever height you need.

[Tony McC]

Rich is essentially spot on: all flexible paving will settle slightly over time, and if you were to pave flush with the drainage fittings there's a risk that, in time, the paving could be lower than the hoppers, which means puddles.

However, the 6mm below guidance is also used with bitmac, concrete, resin and other paving types. It's just good practive to make sure the drain access, be they hoppers, linear channels or anything else, are definitely lower than the paving.

[Dave_L]

But [generally with tarmac in this instance] all covers (manhole covers, stock cock covers etc etc and not drainage) are better slightlyhigher than lower than finished level - nothing more unsightly than a low manhole cover in a nicely surfaced drive - i.e a puddle magnet!

[lutonlagerlout]

i have to say this is the one thing i really dislike about BP being *flexible*
after a few years the covers always seem to protrude a little and to coin a phrase it looks "cat malogan"
even keeping the blocks inside the cover 6 mm down doesnt help that much because the surround is rigid too
LLL

[tom-mk]

Thanks for all the advice. I measured the level of the manhole cover against the brick edging that we've put down and it varies but basically it's level or up to 2mm lower than the brick edging we've put in.

Based on this I've decided not to rip the manhole out and start again as I think it's possible I could destroy a lot of what we've spent days putting in and making it look at lot worse and I'm not sure the extra millimetre would vastly improve the finished product. It's just one of those lessons learned for next time (should we ever lay another drive - not sure my other half would be too keen on the idea ) along with putting the drain cover in after the edging to make measuring the levels easier.

As we are putting in a gravel based drive we are in the lucky position (I guess) of being able to top up levels when and when necessary through the gravel bedding down and natural wastage.

Thanks again for all the comments and advice.