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DPC above wall plate on parapet

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Hi Folks,


I am building a single storey with a pitched roof into a valley at the mo, just working on the parapet and a friend has mentioned I need a line of DPC 6" above the wall plate. the parapet is going 2 blocks high, so that would mean I would have a line of DPC on the last block up. see drawing . I've done one wall, bedded the DPC in but have concerns over binding. Also I'm going to do the valley in fibre glass to come up and seal over the top block. Would that suffice instead of DPC?


Is there a standard guide for width of valley, I have a 100mm cavity but thinking of increasing valley size.



Any advice welcome

Screenshot 2021-10-06 at 12.43.55.png

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the rational given was to stop moisture coming down through the top thro the block. As I said, I'm not too comfortable with blocks sitting on plastic up there, and I reckon the fibre glass will do the same.  No cavity tray, just DPC at ground level, 155mm up from deck. Prob a bit late for that now.  Its 100mm cavity with 80mm PIR. I've not closed with a block, so sealing with insulation and a bit of DPC.  I've 6x 2 rafters coming down onto wall plate, so will be creating a valley with OSB and fibre glass.  

Screenshot 2021-10-06 at 14.43.37.png

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The problem you may have is that your final roof covering may need the block to be primed prior to sticking to it which will be hard if its dpc. 


Personally I would use rubber on top, assuming its a warm roof as you show zero ventilation on the drawing should it be a cold roof.

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