Drainage for brick shed/summerhouse?

[Dee871464]

Hi everyone. Please forgive my poor use of building terms! We’re self building a garden office/shed. Roughly 18ft by 7ft. Block and brick. Tiled roof. Solar panels. Far more knowledgable friends are doing each part with us. I wanted some advice in 2 areas please.

1. we’re digging down quite a bit for the footings and floor-level is about half a meter below ‘ground’. Our ground is heavy clay. It’s puddling a little after rain last night (see pics). What would you recommend for drainage? My father in law is helping and is Canadian, they have their own ways of doing things and he’s suggesting a sump pump and perforated pipe around the perimeter. Possible with the perforated pipes laid among the hardcore. Can we lay it in the hardcore without wrecking its integrity for the footings? Is there a more simple way of sorting the drainage so the footings don’t sit in water? I’m not talking about roof drainage.
2. Separately, we need to support the fence line with a retaining wall. Thinking brick, gravel with little pipes to let the water escape.  Any smarter suggestions??

 

Pic is of the rough footings. We’re neatening them up. 

 

Many thanks!

[Nickfromwales]

On 04/05/2024 at 19:20, Dee871464 said:

we’re digging down quite a bit for the footings and floor-level is about half a meter below ‘ground’. Our ground is heavy clay.

Are you gluttons for punishment? Why on earth (or clay lol) are you going down so low with an outbuilding??? Is this to get the ridge height down to appease permitted development or planning rues? 

Sounds like you are making ridiculously expensive and unnecessary rods for your backs here, sorry. Having a garden room that needs a sump pump is insane (and my ICF guy is Canadian so I know they do these types of things without batting an eyelid) so I think you need to have a serious sense-check here before going any further. 

[Dee871464]

Thanks for your reply Nick. 
yes we’re going down an extra half metre (slightly more) so we’re under 2.5m above ‘ground’ adjacent. 
 

I am sure you’re right a sump pump is an unnecessary expense and complication. But my father in law isn’t familiar with British building. Would you have a suggestion? I’d appreciate it. 

[Andehh]

Could you tank it all, as if you were building a basement? DPC up the walls and stuff? though IANAL

[DevilDamo]

What are the chances the LPA would have refused a 3m high outbuilding? Would it have had any impact on neighbouring properties? Are you within the Green Belt?

Edited May 6 by Nickfromwales
typo

[Nickfromwales]

5 hours ago, Dee871464 said:

Thanks for your reply Nick. 
yes we’re going down an extra half metre (slightly more) so we’re under 2.5m above ‘ground’ adjacent. 
 

I am sure you’re right a sump pump is an unnecessary expense and complication. But my father in law isn’t familiar with British building. Would you have a suggestion? I’d appreciate it. 

Ok, so why such a "tall" building? Is this destined to have an apex roof? Why not go for a flat roof and avoid the height.

I don't have any other suggestion other than "don't continue with this plan, and work a new plan to have this above ground at soleplate level, doing away with a pumped drainage. There are so many red flags here i am losing count!

 

Please give us the whole story so we can get you the best replies.

[Nickfromwales]

FYI, my 6m x 4m man-shed / office has a sloping flat roof to comply. It begins with an internal head height of 2.1m, at the office where I'm sat, and then rises away to the doorway end. Zero of this is below ground, other than the base. Plenty of room, and acknowledges the PD requirement for max height etc 

[Dee871464]

19 hours ago, Nickfromwales said:

Ok, so why such a "tall" building? Is this destined to have an apex roof? Why not go for a flat roof and avoid the height.

I don't have any other suggestion other than "don't continue with this plan, and work a new plan to have this above ground at soleplate level, doing away with a pumped drainage. There are so many red flags here i am losing count!

 

Please give us the whole story so we can get you the best replies.

We are putting East/West solar panels on the roof and want it on apex rather than a flat or pent roof, so we get both aspects and don’t need the concrete ballasts. 
 

If we stick with this plan, do you have any suggestions? A sump pump isn’t that expensive to buy/install/operate. We will do that if we have to. But I just thought we must have other ways of ensuring the footings aren’t sitting in water. 
We will have DPC of course, but that won’t help the footings 

[Nickfromwales]

14 hours ago, Dee871464 said:

We are putting East/West solar panels on the roof and want it on apex rather than a flat or pent roof, so we get both aspects and don’t need the concrete ballasts. 
 

If we stick with this plan, do you have any suggestions? A sump pump isn’t that expensive to buy/install/operate. We will do that if we have to. But I just thought we must have other ways of ensuring the footings aren’t sitting in water. 
We will have DPC of course, but that won’t help the footings 

It’s never one pump. That’s the issue. You’ll need at least 2 pumps and to be routinely testing, cleaning, maintaining them for the rest of your days. The time you relax and think “I’ll get to it next week” is when the building will be flood damaged.
 

Two words spring to mind, the second is “that”.

 

You can install panels as low as 8° so I still think there’s potential to be at ground level with your sole plate. 
 

If you are staying on this path then the above is the best advice; never rely on one pump as it will fail when you don’t want it to. Also, you need to allow for a system that will be able to cope with a 1-in-a-hundred-year storm to be anywhere near even close to considering this solution. You’ll need a sump about 600mm in diameter, with a closure plate to stop an infant / human falling in, and that will need to have the two pumps dangling on stainless chains so they can be lifted out to be checked / inspected / replaced.

 

What a massive pita though. 
 

Just put the solar due south, get the shed out of the ground, and keep it cheap and simple. The cost of all this craziness will mean your PV payback (RoI) will be multiples of decades so just completely dumb maths afaic.
 

You’ll have zero benefit from it too, as the electricity to run the pumps all wet season will consume all that you generate each summer, and more. Factor in replacing the pump(s) and you’re wasting time, effort and at an expense that you absolutely will not recover in your whole lifetime. 
 

“Nuts idea”, sorry! 🫤

 

Ill get my tin hat ready 

[Alan Ambrose]

The best indication would be: how does the rest of your house drain rainwater? Also, you might want to do a little soakaway test to see just how permeable your clay is. If there's any sand in it it might just drain fine anyway.

[Conor]

The cost of drainage / tanking / extra concrete / pumps etc will cost way more than you'll ever get from a handful of solar panels. Make it 2.5m above the highest ground level with a flat roof.

[Temp]

If you want to continue with a floor 0.5m down then you are essentially trying to build a basement. You need to read up on basement design and construction, external tanking systems etc. I think I would lay a slab across the whole site rather than use trench foundations.

 

External tanking is very dependent on good workmanship. It's important to do a good job of the fillet seal where the blocks meet the foundation slab. You can't just slap some liquid sealer over whatever mud has found its way onto the top of the slab.

 

Edited May 7 by Nickfromwales
typo