The Brew Cabin

Hi all! Ok, several weeks since my last post and I'm still dealing with the subbase ;) Can't afford to pay someone so will just keep plodding on whenever I get time!

I've disposed of tons of gravel/old slabs and hired a pneumatic drill to remove some concrete foundations that were higher than the intended patio height. I'm currently digging trenches for a linear drain and connection to gully.

My question relates to levelling the site ready for subbase compaction and then bedding etc. My patio width is 7m and will drain away from the house into a linear drain. I'm unsure what ratio to use for the slope. A minimum of 1:100 would give a fall of 70mm... 1:80 would give a fall of 88mm... and 1:40 would give a fall of 175mm. Is a 1:80 fall ok for riven flags on a patio 7m x 7m?

Althought only a light foot traffic patio, the ground is clay so I'm putting in a small amount of DTP1 or 50mm crusher run (50mm depth), just to be safe. Whats the best way to mark out how far to excavate the subbase? I've worked out the following :

depth of dig = 50mm subbase + 40mm bedding + 40mm flag

which equal 130mm...

I am planning to mark a line on the house (below dpc by more than 2 bricks) which will represent the top of the new flags. I then plan to insert a set of pegs against the house and tie string to them at the level of this line. At the other side of the patio (7m away) I will also peg out and attach the string and will use my spirit level to get a level string line. Then I can lower this side of the string by the decided amount (e.g. 88mm for 1:80 fall). This should give me a final line for the top of the patio, right? I can then continue excavating until I reach 130mm below this string line across the site. I can continue to use these string lines to ensure correct subbase depth (top of compacted base should be 80mm below line) and correct bedding etc. Any problems with this approach?

Thanks in advance ;)

P.S. Bumped into a long lost relative the other day and over a beer discussed what we were up to. I mentioned I'm spending every spare minute doing my patio and he was amazed as he is doing exactly the same, including laying land drainage in his garden. He said he'd found this brilliant website called McCormack or sommick and asked if I'd heard of it! We had a few beers discussing what a brilliant resource this site is (sad I know!) and how helpful Tony is. What a small world.

Is a 1:80 fall ok for riven flags on a patio 7m x 7m?

Yes, but it's right at the lower end of what I would deem acceptable for riven flags. 1:60 is better.

The problem with riven flags is that the uneven surface has a tendency to hold water long after the rain has stopped. If you lay to falls that will properly drain the paving, say a 1:30, then you feel as though you're falling over when you satnd on it, and the patio table has a slope that encourages the peas, tomatoes and wine-bottle corks to make a dash for freedom. So it becomes a matter of finding a happy medium between minimising water ponding while still achieving a serviceable patio. 1:60 is about it, I reckon, so that's....err...dum-de-dum-de-dum...err... 117mm of fall over your 7 metres.


Whats the best way to mark out how far to excavate the subbase??

You've got the right idea - just dig off enough of the clay to form a reasonable base, and then regulate it with the granular material so that, when you come to the Laying Course (the bedding - I'm trying to get used to the official Brussels terminology in the new BS7533:10) you achieve a bed of uniform thickness, and you're not having to vary the bed thickness to accommodate variation in the level of the sub-base material.

Your plan with the string and the pegs is fine, but watch out for tripping over the string lines. It never seems to matter how many times you trip, you always forget the damned things are there!


It's good to hear that your extended family is benefitting from website, but it's not gfair you two sitting there supping ale while I'm stuck in the bloody house tapping away on a keyboard! I'll have a pint of Boddy's, please! :)

Thanks Tony! Let's hope the weather continues into the weekend cos I don't really fancy digging tons of soil/clay in the pouring rain!

If you are ever in the North East, I will gladly buy you as much Boddies as you can drink! When my little project is finished (errr... probably October at this rate!) I'll be sure to send you a little something for all your help ;)

Good luck - keep us informed about your progress. :)