The Brew Cabin

Posted: **Thu Sep 09, 2004 11:28 pm**

First, I'd like to say what an excellent source of information the pavingexpert is! The extensive advice and information has given me the confidence to proceed with landscaping my back garden - terracing, gravelled and block paved areas. (As well as plants!) However I do have some points that I'm not too sure about.
First is a question about angles when using paving flags as a retaining wall.
My back garden slopes towards my house. The slope being about 1400mm in 9000mm. I intend to convert this slope into three levels top, middle and bottom.
I thought I'd use Marshalls 'Croft' walling to form the two retaining walls for the terraces - 6 courses of 100mm in each wall, 600mm total. These seem easy to construct, I can add curves for interest and I think they'll look less severe than a brick wall (which I couldn't build my self anyway!).
If I terrace my part of the slope then the top terrace, being horizontal (well almost), will be higher than the slope itself towards the front. I will then cut into the slope to form the middle terrace so that the back of this will be below the level of the slope - then above at the front, and the same for the bottom terrace.
Now, my neighbour's house joins on to mine - at the same level and also faces up the slope. We have a fence that runs straight up the slope along our boundary. I have therefore to deal with the differences in height between his sloping garden and my terraces where the two meet. I thought I'd use paving flags on end to do this as described on your excellent site. Now, the diagrams for flag retainers show the flag leaning into the bank to be retained at a suggested angle of 5 to 15 degrees. However, in my case the bank is first on one side (when my terrace is higher than the slope) then on the other (when the slope is higher than my terrace). This suggests that the flags should lean one way then the next.
My question: Would it be reasonable to lay all the flags vertically in a 350 mm wide trench ,with 100mm concrete underneath the flag and with concrete to both sides of the trench to say 100mm from the surface? This way the flags would sit in a straght line, close to the fence (150 mm of concrete each side and there would hopefully be enough mass of concrete to retain the soil on either side. I plan to use a mix of 600x600 and 900x600 flags to achieve this. The trench will be at least 300mm deep - cut in steps to follow the levels of the terraces/ size of the slabs. so there would be at least 200mm of concrete above the base of each slab on both sides - usually much more.

Now two small questions!
1) Where can I get the 'line pins' that you use for setting out? They look a lot like short lengths of the steel I've seen used for reinforcing concrete! I can only find wooden pegs in places like Travis Perkins and I cannot get these pegs to go where I want - they wander about as I drive them in and they seem too thick and imprecise to be used to set out edges for block paving.
2) I'm planning to use a mix of 6 ballast to 1 of cement for laying out edging blocks for block paved areas, and also for the paving flag retaining wall above. How will this concrete be affected by rain? Can I work in rain? Will it matter if the concrete is rained on after I've filled trenches with it or laid out edging? Should I cover the concrete with a plastic tarpaulin, or just wait until the weather forcast predicts good weather?

Regards,

Peter Bell.

Posted: **Fri Sep 10, 2004 11:49 am**

Would it be reasonable to lay all the flags vertically in a 350 mm wide trench ,with 100mm concrete underneath the flag and with concrete to both sides of the trench to say 100mm from the surface?

Yes - as long as there is a 'toe' of concrete at the 'front' of the flag to prevent it falling forward, then they can be laid plumb.

Where can I get the 'line pins' that you use for setting out?

You can buy genuine line pins at places like Greenham Trading, Parkers and other Contractors' Tool Merchants, for around a quid a-piece. However, if you have access to a local reinforcing steel stockholder, you can get a 6m length of 12mm bar and have it chopped into shorter lengths. You can get 7 lengths at 850mm for each 6m bar, and they work out at around 60p each.

For most residential paving work, 600mm long pins are adequate, so you could get 10 pins from each bar!

How will this concrete be affected by rain?

I smell a Weather FAQ in the offing. This question has come up quite often of late, so maybe it needs a page of explanation.

Basically, as long as it's not tipping down, the concrete will be fine. There's more chance of you coming to harm in the rain, than there is of the concrete suffering to any significant degree.

Concrete (or any other cement-bound material) will still set in the wet - in fact, concrete sets underwater. Cement does not rely on a 'drying out' process to harden: it's a hydration process, which involves chemically bonding water into the concrete, not driving it off by evaporation. The only hazard when working with concrete in the wet is that it can become messy.

So, for any areas of concrete work that will be visible wjhen the work is complete, it's a good idea to sheet over if there's any risk of rain in the first 24 hours, but for foundation, edging beds, haunching, etc, it really doesn't matter.

The Brew Cabin

Flag retaining wall and other questions.