The Brew Cabin

Posted: **Mon Mar 22, 2010 8:12 pm**

Hello All

I've come back to this site after using the info&advice a couple of years ago to do a small block paving project. That project was only 1.5m wide so falls were not as critical as this time around.

Having read the drainage pages, Tony makes reference to a "combined fall", stating that a combined fall of 1:60 is usually adequate for a patio.

Could someone verify that my calcs for "combined fall" are correct please.

I am intending to lay a 22sqm patio of riven concrete slabs (from EH Smith), which i intend to drain using linear channels at the bottom left hand corner. But I would like some reassurance as to what falls are required.

patio example
eg. 6m length, 3m width,
which is roughly 7m diagonally from high point to low point of paving

if i set endfall and crossfall to 1:60 i get

600/60 = 10, 300/60 = 5
i then combine the two falls to get 15 (which is the diff between high and low points of paving)
and apply it to the 7m diagonal length which gives
700/15 = 46.67

so i'd have a Combined Fall of 1:46.67 ?? steep ??

is this excessive for a patio??

i'm thinking that I should set the end & cross falls to 1:80,
which using the same calc as above would give me a combined fall of 1:62 as 700 / (7.5 + 3.75)

i am keen that it should all drain away nicely, but i don't want to create a hill in my garden.

thanks in advance for your consideration/replies

Posted: **Mon Mar 22, 2010 8:52 pm**

you need to read this as your are getting to coplicated

MSH

Posted: **Mon Mar 22, 2010 10:02 pm**

thanks MSH

i have read that page, hence my question and calcs.

using my example length of 600cm, and width of 300cm, and applying a 1:60 fall to the length and width seperately, this gives a combined fall of 1:46 (because the length fall is 10cm and the width fall is 5cm, totalling 15cm, so 700 diagonal length is divided by total fall of 15cm)

blimey, i'm trying not to type it all out again, but i am...

which i think may be too steep.

so if i apply a 1:80 fall in the same manner as above to the length and width separately, i get a combined fall of 1:62

which fall should i use for riven concrete slabs, without creating a 'steep' patio? i'm favouring the second option, but would like to check before proceeding.

cheers

Posted: **Tue Mar 23, 2010 7:23 pm**

oh... no further responses

i thought this was a simple enough question

obviously not

can anyone offer any assistance with my query?

???

Posted: **Tue Mar 23, 2010 7:39 pm**

Darren... is there a particular reason your are working run off into one corner or is that the existing lay of the area

Posted: **Tue Mar 23, 2010 9:28 pm**

hello IP

thanks for asking

the garden is north facing and shady, so i am planning to install a patio that makes best use of the garden light. so it'll be positioned about 70cm away from the house, with the low "house" end of the patio at 15cm below the DPC.

the ground level gets higher as it moves away from the house, so i am making the fall back towards the house to save on the amount of excavation that i have to do.

a picture is worth a 1000 words etc.

hope this works

RWP is bottom left
neighbours house 2m to left and RWP pipe run between the two houses, presumably to Storm sewer (foul water goes out in a different direction, different pipework)
conservatory to the right

so, the rain can only really go to one place as I see it...

garden is higher at far end, and flower bed is higher than the patio on the left hand side, and conservatory on the right, so i am aiming to connect the linear channel (which can be seen along bottom edge and extending up the left hand edge) to the RWP pipework

hence my thinking of combined fall to take the water towards the bottom left corner

i understand falls. End Fall and Cross Fall.

my question is about Combined Fall.

i'm thinking i'll use 1:80 for End and Cross Falls, to give what I believe is a Combined Fall of 1:62

cheers

Posted: **Wed Mar 24, 2010 4:14 pm**

Posted: **Wed Mar 24, 2010 5:09 pm**

bump

you would normally lay a patio level to the brickwork across the back of the house and falling towards the garden area at 1 :60, the drainage channel can be bedded to a line what is just out of level (turn of the bubble as they say) so the water will flow towards the outlet on the drainage channel

so when you talking about cross falls its what looks right to the eye what is more important too as you don't want it to be too steep

a photo showing the position of the patio would be good

Posted: **Wed Mar 24, 2010 8:18 pm**

hi mickg
thanks for asking

too dark to do photo now, but i'll try to get one done 2moro

the garden is higher at the 'far' end of the garden, hence why i am considering falling the patio towards the RWP for drainage by using a combined fall.

but after reading what you've said, and what the other responses have said, it appears that most of you don't lay patios with a combined fall? ie most patios fall in one direction only?

(i suspect this is why i can't really find a discussion on Combined Fall amongst the Drainage pages.... Tony only makes a passing reference to it, i guess it's more of a commercial consideration?)

if this is the preferred option (and creates a less steep patio because there is no combined fall 'effect') then i can either end-fall it towards the house (end-fall away from house is too much digging), or cross-fall it to an edge.

my query would then be : if i cross-fall at 1:60, how do you get the drainage to fall back to RWP if there is no end-fall? or do you put in a minimal end-fall of say 1:100 so the the drainage channel has a fall?
OR, i could use shallower cross-fall (eg 1:80) and end-fall (eg 1:80) to create a shallower Combined Fall of somewhere near 1:60.....

oh questions questions...

Posted: **Wed Mar 24, 2010 8:38 pm**

rushd wrote:"do you put in a minimal end-fall of say 1:100 so the the drainage channel has a fall?"

use the option you have suggested above

Posted: **Fri Mar 26, 2010 3:34 pm**

hello again

i have taken some pictures so you can see what i am trying to make best use of.

i know the piccies are large, but hey it's a wide-angle camera!

i am planning to fall the patio towards the RWP with end-fall and cross-fall that combine to give me a ratio of 1:60.

if you look to the 'far' end of the garden you can see the height difference created by my rough excavations to date. so draining into the garden is not an option. and with a garden full of pipes and the close proximity of neighbours' houses i couldn't consider a soakaway (it's clayey anyway).

Hence why i am looking to put in drainage channel to encompass the RWP corner at the house-end and connect into the existing RWP setup.

you can also see the metal manhole cover and the 2 plastic ICs.

I am not going to contemplate altering the metal manhole cover (we'll have to make a 'feature' of it), but i am wondering if i can lower/hide the plastic ICs. I have had the lid off them, they are both foul water from utility/loo/kitchen etc, and seem to be in the region of 50cm deep (total guess)?

could any of the PROs give me an idea of what they may do with this plot please. Hopefully you'd agree with my thinking, but if i'm on the verge of c@cking up i'd like to know!

i need to get my plans straight as i have a skip coming this weekend and i want to be rid of the excavation stage.... time/weather/sickness permitting!

many thanks

Posted: **Fri Mar 26, 2010 5:46 pm**

Darren

Just wanted to ask why your going for a cross fall. Why haven't you considered just creating a fall towards the building and running a channel along the end but keeping the channel away from the wall....

Posted: **Fri Mar 26, 2010 6:33 pm**

Hi IP

I intend to create a fall to the house (unless someone can explain another option), so that the water ends up in the proposed drainage channel. but if the channel is level the water cant move, so you need cross-fall for the drainage channel to move the water that the "towards-building-fall" is going to put in the channel.

so what you're proposing is the same thing, unless you put your drainage channel in level? can that work? (trying to keep costs to a minimum so i'm assuming that drainage with built-in falls that can be laid level are alot more expensive)

cheers

Posted: **Fri Mar 26, 2010 7:03 pm**

water finds its own level so if the channel is only just out of level and falling towards the corner where the downspout is it will still flow

even if it was bedded level you would only see a couple of millimetres of water in the bottom of the channel anyway as the plastic drainage channels don't have water tight seals between each other and any surplus water lying in the channel would soak a way into the ground

what you must make sure of is that you don't have any back fall on the drainage channels :;):

Posted: **Fri Mar 26, 2010 7:09 pm**

Looking at the picture it doesn't look a great distance from the patio doors side over to the rwp. What is the width over to the rwp?
You may pay more for channels built-in but you wouldn't need as many compared to what you proposed.

The Brew Cabin

Clarification on "combined fall" - Planning for patio