The Brew Cabin

Hi,
I am laying block paving for off street parking in front of my house. As the land is clay I decided to put the blocks on a 100mm concrete platform. I have now finished this and am planning to put soft sand (no cement) down to bed the blocks. My question is this:-

Will rainwater drain away through the sand between and under the blocks and run off the concrete platform edge as I planned or will I need a canoe to get between the car and house?

Firstly, I don't understand why you've elected to use a concrete base layer, but that's by the by at this stage. Secondly, you should not lay the blocks on soft sand, but on GRIT sand. It may seem a minor detail to you, but the way these sands behave under loading determines whether your paving lasts longer than one season.


Turning to the drainage, any water penetrating the pavement will become trapped between the concrete base layer and the paving layer, and so there must be some 'escape route'. This may be off the edge of the concrete, as you hope, but that, ultimately, depends on what the ground is like at that point. If it's solid, impermeable clay, then you could be making a problem for yourself: the laying course (the grit sand discussed above) must be able to drain to perform properly.

With concrete (CBM) or bitmac base layers (DBM), it's usual practice to incorporate some form of drainage within or directly adjacent to the base layer to ensure the laying course is adequately drained. This may be a fin drain or a length of land drain, but some provision is made, otherwise the laying course saturates, becomes fluid under load, and the paving starts to move. :(

So; what's the ground like next to the concrete base? Will it provide adequate drainage or do you think you'll need to install a land drain?

Hi Tony,
Cheers for the prompt reply. The concrete base has clay soil topped with tarmac at the entrance to the parking area, a wall with foundations on the right and clay soil with a path made from paving slabs on the left. All these surfaces would inhibit drainage as they are level with the block surface. However at the lowest edge of the concrete base there is a 1 foot gap between it and the front of the house which is an existing concrete path about 8 inches lower than the parking area. This effectively creates a concrete channel which has a surface water drainage grid at one end. This is where I intend to drain the water from under the blocks after it filters it's way through the grit sand (thanks for the tip)
Does this sound like it will work.
Cheers,

P.S. I got the advice (of using a 100mm concrete base as opposed to hardcore on clay soil) from B&Q here:-
http://www.diy.com/bq....8.jhtml
Maybe they are being a little over zealous.

Your drainage plan sounds ok, as long as it's not going to rest against the house or breach DPCs. However, from what you've described, I still can't see why a CBM base is required when a standard granular sub-base would be more than adequate.

As for B&Q, that well known firm of block paving specialists - I'd be much happier if they left the construction advice to professionals. If I promise not to sell cheap doors and light fittings, will they promise to leave off the crap advice?

The trouble with many of these 'suppliers', and some of the manufacturers, is that when they are asked for advice, because their insurers have scared the pants off them with regard to liability, they tend to go for a spec that is way, way, way over the top. They should stick to what they do best - flogging materials - and not try to be all things to all people. :)

Point taken. Next time I know where I will look, unfortunately I only discovered this forum after finding and following their advice.

Great site, excellent resource


(Edited by Boggie at 9:46 am on Oct. 29, 2002)

Hi lads,
I just read the B+Q "advice" as well.
Just what on earth do they think you are going to park on your drive after you've done it their way?
A Volvo FH16?; A chieftain tank? Or a fully loaded Boeing 747 cargo plane?
I lay blocks for a living and just use 4" of good old type one and plain sharp sand, perhaps a bit more stone if the ground seems suspect - and we haven't had any sinkages yet!
I'll go to B+Q and tell them off.

Have you ever attended one of their 'How To' lectures that they hold in the Warehouse Stores some evenings? Ber-luddy hell fire!! I nearly smacked the paving demonstrator for telling me to "leave advice to the professionals, sir", when I asked why he was telling the handful of folk watching his efforts that using a spadeful of cement with the jointing sand would make their block paving more solid.

Eejit!

Now that I would have liked to see!

- Mike -