Deck ledger placement with DPM

[Dylan T]

Hi all -

I'm building a deck and and placing a ledger against two external walls of the house. Something I'm unsure about is the placement of the bolts (just planning to use knock-in throughbolts) in relation to the DPM. Now I always thought you'd want to go below the DPM as then any water penetration can't rise above the membrane, but according to the attached picture, you want to go above. So it's prob a stupid question but can someone be kind enough to explain why? The part of the house I'd be going off is a new extension, so to find the DPM i guess its a case of working out a rough height of concrete floor, then 50mm kingspan and then i'm assuming below that is roughly where the DPM will be. I'm assuming it comes out to the external skin and is then trimmed skimmed over on the outside during render.

I've just seen so many videos where people are drilling into the walls at all sorts of heights and doesn't seem to be a consideration for most.

This picture also shows a 10mm gap is required, for drainage i assume. Is it common to use something like plastic shims or plate washers?

Thanks

 

[^woody^]

Surely the height for fixing is determined by the deck surface level?

 

[Dylan T]

Surely the height for fixing is determined by the deck surface level?

Yes but I suppose by using a thicker joist I could offset where the holes would be. I hust had read that you should never drill above the dpm, but with a gap is it okay to do so?

 

[^woody^]

I hust had read that you should never drill above the dpm

Where did you read that nonsense?

 

[Dylan T]

Where did you read that nonsense?

thanks for the reply. First my builder told me as he knew I was planning a deck, but it’s also what I’ve read most online. This website states it’s the most common to do that but says you can go above as long as you don’t hit the dpm. So maybe it’s more just avoid avoiding of piercing it than any water ingress.
I’ve got some Timelapse photos somewhere so I guess it’s a case of trying to work out how thick the slab they poured is and working it all out

 

[Notch7]

This picture also shows a 10mm gap is required, for drainage i assume. Is it common to use something like plastic shims or plate washers?

rain hitting the house brickwork will run down and will hit the ledger board if its bolted to the wall -if you use some plastic spacers like glazing shims , set vertically then rainwater will just run down past


the position of the ledger board is decided by where you want the deck height

there is no impact on the DPC or damp arrangement -just make sure you dont drill right on the DPC mortar line

 

[Dylan T]

rain hitting the house brickwork will run down and will hit the ledger board if its bolted to the wall -if you use some plastic spacers like glazing shims , set vertically then rainwater will just run down past


the position of the ledger board is decided by where you want the deck height

there is no impact on the DPC or damp arrangement -just make sure you dont drill right on the DPC mortar line

thanks a bunch. Dpm will be a bit of a guessing game but I’ll just take my time. One quick q; for spacers do you recommend plastic shims over metal washers?