Falloff - Calculating falloff in a corner situation

[53-1093878931]

I'm aware that I need to have some falloff (1:40 according to Stonemarket) away from the house, but I am laying a square patio in the corner of two walls. Am I right in thinking that I need to have falloff away from both enclosing walls and that I therefore need to calculate the falloff diagonally across the patio away from the corner?

Thanks in advance and for all the other information that I have gleaned from your excellent site which I'm sure will be invaluable over the next week or two!

[alan ditchfield]

No this is not necessary as the fall is to prevent water holding against the wall put the fall towards the most convenient point of exit for the water away from 1 wall and paralell to the other, diagonal falls will only make the paving more difficult and is totally unnecessary secondly a 1:40 fall seems severe a 1:70 is ample even with a weathered flag ie: to the back line on the spirit level, if in doubt have you thought about a gravel strip. Alan @ Prestige.

[53-1093878931]

Thanks for such a quick response. When you say 'gravel strip' do you mean that a strip of gravel between the patio and the house provides sufficient drainage to eleviate the need for a falloff? Because, yes, I do intend to have a narrow (15cm) gravel strip all around as well as a falloff.

The reason I am using a gravel strip is that I am laying some pathwork away from the patio all around the house, and the gravel enables me to be slightly less exact with my measurements. (i.e I can fill small gaps between wall and patio with gravel). Do I still need a falloff?

[alan ditchfield]

yes to both you will still need a fall to prevent pooling on the patio area, a gravel strip just helps prevent any water holding against the brickwork and if used cleverly it may prevent cutting around drainage covers.

[53-1093878931]

Thanks, Alan. I was intending to use gravel around the drain cover but wasn't sure if it was good practice or not, so your post provides some reassurance!

[84-1093879891]

I have to disagree - using falls of only 1:70 is too flat for riven-effect flags. They end up holding on to too much surface water. For this type of paving, I usually recommend a minimum fall of 1:60 but would prefer 1:40.

As for the gravel strip between paving and building, the so-called 'splash strip', I think this is one of those silly ideas that came about in the late 70s and has blighted us ever since, a bit like stone cladding and space hoppers. If flags (or any other pavings) are laid properly, then they will not hold water against a property, and the degree of splashback from solid paving is negligible.

Gravel splash-strips were introduced to prevent problems with ponding caused by bad laying standards and to save builders/developers a few bob when it came to paving around new houses. It allowed the flags to be laid flat, with no discernable fall, and solved the problem of getting some numpty groundworker to cut-in neatly around hoppers and gullies that are placed tight against the brickwork.

Since then, some poorly-educated designers and architects have come to think that a splash strip is essential, and so include it whenver creating a layout. It is not necessary; it's an excuse.

And young kids just love gathering up all that gravel and putting it into the gullies!

[sharonb]

Tony: I am interested to read your reply to this topic. I am coming to the end of an Open Learning Garden Design course with KLC in London and am in the middle of drawing up a design for the final manual. I have designed a patio next to the house and your comments on the necessary fall for uneven paving are very helpful.

I am wondering about drainage for where the corner of the patio is bordered by two raised beds (ie at 90 degree angle to each other. Obviously if the fall goes away from the house then the water will want to collect in this corner. What would you suggest? I had thought about linear drainage. In one of my manuals it says that in Britain it is not allowed to run surface rainwater into the main drainage system without installing a silt trap. Is this right?

I love the designing gardens bit but am quite worried that a designer seems to need to know such a lot about how to build that is not covered at all by the course. I had to smile at your comment on the gravel strip - I have just put one into the design as I thought it was a good idea if the damp proof course is not obvious. I probably won't do it again!

[53-1093878931]

Thanks Tony and Alan - I think I will go with a 1:60 falloff as the lawn rises away from the house rather than falls, and this is going to cause one or two problems because I'm digging lower than adjacent fence bases, so I need the minimum falloff that I can get away with.

As for your comment about how kids love to throw the stones down the gullies - Do they ever, Tony, do they ever!

[84-1093879891]

Sharon, you could use a length or two of linear channel, if you wished, or use a simple gully arrangement.

The assertion that you cannot connect SW to a main sewer without a silt trap is not strictly true, but is good advice, nonetheless. The sewers are more than capable of dealing with a bit of silt, but a silt trap is a good idea because it prevents smells coming back the other way - out of the main sewers.

So, if you elect to use a linear channel, then it should be connected using a p-trap, which can be a bit of a nightmare in some smaller projects, so for small patios and suchlike, I prefer to use a simple "Yard Gully", which incorporates the trap, the grating, the pot and the outlet all in one compact unit.


I think it's great that we're forcing our new designers to think more about the build implications. For too long, there has been a sense of wilfull disinterst in the construction aspects of gzardens and landscapes - designers came up with grandiose schemes that look fantastic on paper but were a nightmare to build, and some were actually impossible. By making trainee designers think about how pavements and drainage work within a landscape, it can only lead to better, more grounded designs (no pun intended), and a healthier respect for each others' skills between the ones with the pencils and the ones with the shovels. :)

[alan ditchfield]

I recommend a fall of 1;70 as apposed to a 1;60 because a proffessional paver would allways get away with this, granted it may be more difficult for the amateur to get away with as it is a small fall, the gravel strip was mentioned because it is a fail safe against bad paving that may hold water against the brickwork within or behind the paving but thanks for the reply. Alan

[sharonb]

Tony: Yes, I agree, it is a good idea to get designers thinking about how to build, but how do we learn? Is there any type of course you could recommend that would cover basics that would give me a good grounding (there's that pun again)? I have to admit to feeling somewhat intimidated at going along to a course that would probably consists of mainly men.

[84-1093879891]

There's no overall course, although the CITB via the National Construction College does run an appallingly poor course on flagging, and one on block paving.

However, in conjunction with a number of other companies, I'm trying to put together a 'proper' training course for contractors and a separate course for architects/designers/interested DIYers etc, that would be more theoretical and less 'hands-on'. The CITB were asked to participate but they reckon their existing courses, which no-one attends, are "satisfactory".

In the meantime, you have this website, and then, next year, there'll be a book, and you can always post here or email if you have a specific question or a problem that seem intractable. :)

[84-1093879891]

Ooops - forgot to mention to Alan - would you use a 1:70 fall for any riven flags, even the ones that are quite heavily riven, such as Heritage or Wetherdale?

I've no problem with a pro pavior laying a lightly riven flagstone, such as Old Lancashire, to 1:70, but some of the others, and some of the natural stone rivens, really do need an extra bit of fall just to get the surface water way in winter.

[sharonb]

Tony: Thanks v much. The book sounds like a good idea and the course sounds like an excellent idea! Good luck with it all. I hope you can let everyone know when the book is out and the course is organised.

[alan ditchfield]

i would use this fall in most cases obviously some flags are extremely riven and may need to be taken on there own merit, we usually try to lay the flags with the low spots outward so the water will escape more easily. this in itself may create a channel for the water to escape.however some flags are that badly riven or warped that they would hold water with a 1;10. but in my experience a 13mm fall over a 3*2 riven is usually plenty.

(Edited by alan ditchfield at 11:22 pm on June 23, 2004)