Damp Proofing Screw Pile and post building

[Piers]

I'm trying to understand what I need to do to damp proof a wood-framed building, built on screw pile foundations into a sloping site. It'll look something like this:

 

 

 

The screw piles will extend about 150-200mm above ground level and will end with a u-bracket. Then we're using 100x100mm treated C24 posts to create our level and using 150x50mm C24 timbers for our frame base. On top of this will go our floor joists, also 150x50mm. Basic sketch is:

 

 

My first question is where should the DPC go?

 

The building will have an external, covered deck like this:

 

 

The contractor doing the screws suggested we should have a separate deck structure between the external deck and internal parts and have a DPC between them. The two structures would be screwed together through the DPC but he suggested that it would prevent the decking structure allowing moisture into the building. Like this:

 

 

This makes sense to me but what do I know! 

 

Any views??

[Ed Davies]

My house is a bit like that though on a flat site: post and beam on steel brackets on concrete foundation pads [¹]. Never though there was any need for a DPC. Why would there be? Or, perhaps you could think of the steel as the DPC.

 

[¹] Some of the picture on https://edavies.me.uk/2018/03/some-flooring/ incidently show the construction of this part.

[Russell griffiths]

i would replace the 100x100 timber with steel personaly 

have you seen these. 

[Big Jimbo]

I would go with his decking idea as i think it won't do any harm. Also think about sloping the decking slightly away from the building, as any driving rain coming in under the canopy will be directed away. Woul'nt have thought you would need any above the ground Screws but you could always dip your posts in moulten Bitch if you were worried.

[Ed Davies]

Ah, sorry, misread/misunderstood your original post.

 

Mine will also be similar to yours in that I'll have a separate porch/greenhouse on the western end with its floor level with the house floor. The porch/greenhouse will be enclosed but more subject to possible leaks, spills or condensation than the main house so I'll isolate its floor from the main house structure by continuing the ventilated cavity behind the cladding of the west gable of the house right down to the bottom of the house structure except for a short ”bridge” of decking at the front door into the house.

[Piers]

 

13 hours ago, Big Jimbo said:

Also think about sloping the decking slightly away from the building, as any driving rain coming in under the canopy will be directed away

 

Yes, good idea.