# Forum Home Renovation Decking  Protection for decking joists

## john777

Hi, Can anyone please recommend a long lasting purpose made product that you paint onto the top surface of decking floor joists to protect against water rot?.......eg: membrane paints?I know of the rubber / plastic strips that you can buy for this purpose but do not want to see the overlap rubber edge from below as this a deck on a first floor and the underside of the deck is visible from below. The underside of the deck including floor joists and floorboards are to be painted white. There is a roof covering the verandah but approx. a 500mm strip of the edge of the verandah can get wet during heavy rain storms.thanks

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## Bloss

Just get a roll of plastic DPC (damp proof course eg: Canberra Building Supplies, Building Supplies, Building materials, Building, Free Delivery ) trim to the exact width of joists cheap as chips so double thickness if you wish.

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## john777

> Just get a roll of plastic DPC (damp proof course eg: Canberra Building Supplies, Building Supplies, Building materials, Building, Free Delivery ) trim to the exact width of joists cheap as chips so double thickness if you wish.

  Hi, 
Won't water just travel around to the underside of the plastic by capillary action then dampening the joist and making the drying out process longer?.......this is why the purpose made rubber strips have a 45 degree turned down edge on both sides

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## Bloss

> Hi, 
> Won't water just travel around to the underside of the plastic by capillary action then dampening the joist and making the drying out process longer?.......this is why the purpose made rubber strips have a 45 degree turned down edge on both sides

  Not a problem in my use of this stuff for about 20 years earlier than when special purpose materials were available - and in very wet climates (Nth Coast NSW & Qld). 
But yes I usually overlap the joist edges so there is an angle run off.  
The OP seems to think that it is important not to see a single thin line of black plastic looking up from underneath. IMO it will be all but invisible and who cares anyway - I'd use the regular joist strips, but if he won't then plastic DPC is fine. Even a couple of good coats of exterior primer would extend the life. And OP says it is really only 500mm ends . . . a non-problem really. 
I'd use treated timber in any case and not be too concerned - unless poor timber is used (eg: old oregon decks) the deck joists will last way longer than when the next lot of changes to the house and deck are made - usually around 12-15 years . . .

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## ringtail

Ormonoid bitumen compound would be the go. Thin the first coat down so it penetrates well then put a second coat un thinned. Sit the tin in the sun to make the product easier to brush. Personally I would install additional protection as well. As bloss said it would barely be visible and I think putting form ahead of function will give you very costly headaches in the future

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## intertd6

> Hi, 
> Won't water just travel around to the underside of the plastic by capillary action then dampening the joist and making the drying out process longer?.......this is why the purpose made rubber strips have a 45 degree turned down edge on both sides

  What you have stated is absolutely correct. Which makes it pretty well useless unless the sides are turned down & kept down somehow so water can't track back up under the flashing. Bitumen or paint would be better.
regards inter

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## Bloss

> What you have stated is absolutely correct.
> regards inter

  Ah well thats good then eh . . .

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## selfindulgence1

One other option ......... Water based or oil based bitumen paint painted on your joists.
Plastic flashings are ok but if the water does the capillary thingy.......then moisture under plastic flashing does not dry out properly and timbers start to compost.
Cheers

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## stevoh741

malthoid is the go. The bitumen in the paper impregnatesitself onto the top of the joist and no water gets in there. Cheap too.Overhang barely noticable but if your overly anal see ringtails post above.

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## ringtail

You saying I'm anal about these  :Tongue: things Stevo ? Alright, you got me. Guilty.

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## strangerep

> The underside of the deck including floor joists and floorboards are to be painted white.

  Have you considered zinc flashing? E.g., Flashing Zinc Soft 150mmx6mx0.7mm Zincflash - Bunnings Warehouse
It would probably look ok against white.
Extra care may be needed as you bend it over the joists so you don't accidentally stress or tear it. 
Painting the tops of the joists is unlikely to make much of a difference (and might actually make it worse).
I had alcor flashing on some 30-year old deck joists. The decking boards on top eventually rotted, and the joists without alcor on top also had to be replaced, but the joists that _did_ have alcor protection looked almost like new on top, except for the nail holes.

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## stevoh741

> you saying i'm anal about these things stevo ? Alright, you got me. Guilty.

  :d

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## stevoh741

what the?  Big grin not workin???   :Biggrin:

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## ringtail

Last few days of hols. Kicking back at Straddy.

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## Bloss

> what the?  Big grin not workin???

  Only if you leave off the second colon :d   :Biggrin:

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## stevoh741

> Last few days of hols. Kicking back at Straddy.

  Hope your getting some sun over there ringtail. The sunny coast is the rainy coast at the moment...

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## ringtail

> Hope your getting some sun over there ringtail. The sunny coast is the rainy coast at the moment...

  Ummmmmm, no. 1 day out of 13. However, still beats being in Brisbane where its too wet to work outside anyway.

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## stevoh741

> Ummmmmm, no. 1 day out of 13. However, still beats being in Brisbane where its too wet to work outside anyway.

  too true. Worst day of holidays is far better than the best day at work  :Yippee:

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