DPM Sub Level?

[Antony11]

Hello,

I am currently in the process of making a trapezoid shaped shed at the side of my house, its roughly 6m in length and 1.2m at smallest end and 2.3m at the bigger end. I have almost finished digging 25cm below ground, as I am going for a thicker slab and hardcore base, due to the ground below appearing to be 100% thick clay.

After researching it looks like a DPM might be a good idea but have read that it should be installed ground level and not sub level due to water ingress into the slab. So am thinking would it be acceptable to fold the DPM up the side of slab till it reaches ground level and use some kind of sealant/glue or fixing to keep it tight against the slab?... Or not... If not any other ideas on installing a DPM sub level?

Thanks,

Antony

 

[JohnD]

It is quite normal, when pouring a concrete floor inside a building, to lay the DPM under it, making a tray up the sides and lapping it into the DPC in the brick walls. The walls are built up to DPC level before the floor is poured. This will be around 250mm above ground level.

In a house, there will usually be a ground slab and some foamed insulation boards under the floor, with insulation round the edges to insulate the floor from the cold walls.

Someone will find an illustration of a typical construction.

You need to ensure there is nothing under the DPM that could possibly puncture it, such as builders rubble. If you have no ground slab you could use a bed of sand.

In your case, low concrete walls on a strip or a ground slab will save you the effort of building formwork for your floor, and will prevent damage to the DPM. The floor will hold the DPM tight against the walls.

 

[Antony11]

Hello,

Thanks for your reply John... I am not butting the slab right up against the house wall as am concerned for movement in the clay soil, as it looks like pure green/grey/ orange clay which is a bugger really as I wanted to use the house brick wall as an internal shed wall and attach the roof and front and back wall to it. But for peace of mind I am not

But good to hear folding the edges is an option, I will fold the edges up and maybe attach them under the stud walls I am going to make. To keep them tightly fixed and giving the slab a good seal.

Thanks for the advice about the sand, I was going to make a 25mm sand blinding layer before installing the DPM. Would go for 50mm but I know ants love a good sand excavation!

I attached a picture of the area I am working in, in case anymore feedback for foundation tips for this kind if area/ shape.
Antony

 

[noseall]

Can't really see the point of a DPM in that location, lol.

 

[noseall]

anymore feedback for foundation tips for this kind if area/ shape.

Stick to a flat roof.

 

[^woody^]

It can go under or over the slab. It just depends how you plan to deal with the perimeter or finish the top of it.

 

[Antony11]

Stick to a flat roof.

Thanks and good tip. Yeah I am. I have already seen the issue with a slanted roof/ lean to roof and how it starts to go like a helix shape!

 

[Antony11]

Can't really see the point of a DPM in that location, lol.

Howcomes? It still gets a lot of rain falling there...

 

[Antony11]

It can go under or over the slab. It just depends how you plan to deal with the perimeter or finish the top of it.

I was hoping to have the concrete floor as the finish so hopefully under it

 

[noseall]

Howcomes? It still gets a lot of rain falling there...

As woody says, it depends how you finish the area around the shed. It may get saturated in water from above, so protecting it from beneath would be pointless.

 

[Antony11]

As woody says, it depends how you finish the area around the shed. It may get saturated in water from above, so protecting it from beneath would be pointless.

I am planning to have the roof extending beyond the shed base on all sides, so the base getting wet from above shouldn't be an issue