Can you use bitumen paint on H3 Low Level Deck Joist?

[matdiy]

I'm building a low level deck over a concrete slab. There is minimal height so I will be laying a 70 x 45 on its side and using around a 6mm spacer when attaching to the concrete. The deck is under cover though may be hosed from time to time. I was thinking of coating the joists fully in bitumen paint like https://www.bunnings.com.au/gripset-betta-1l-waterproofing-membrane-bitumen-rubber_p0960165 since it won't add too much to my job and may help it last a little longer? I've seen some coat just the bottom and some coat just the top, but I figure I may as well do it all. My main question is if the H3 here is suitable to be coated in straight from store?
https://www.bunnings.com.au/70-x-45mm-outdoor-framing-h3-treated-pine-5-4m_p8032136
It seems dry but I don't know much about it. Thanks.

[EricL]

Hello @matdiy 

Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about your decking project. 

Yes, you can use Gripset Betta 1L Waterproofing Membrane Bitumen Rubber to coat the bottom of the timber. However, because timber is a natural product it absorbs moisture, and sealing the whole timber in bitumen paint becomes counter- productive. I propose painting only the bottom half of the timber frame which will allow the timber to breath. To protect the surface of the timber frame, I recommend using CAgroup 50mm x 20m Bearer Protector G-Tape. This product protects the top of the timber frame by preventing standing water from settling on top of the timber. So even if you hose it down the top of the frame stays nice and dry.

If you plan on painting the bottom half of the decking frame, my best advice is to let the timber dry off first before you paint the surface. Painting the timber while its damp will trap the moisture inside and you will not get a good adhesion.  

If you have any other questions we can help with, please let us know.

Eric

[matdiy]

Thanks @EricL 

It has confused me a little more though. 

"because timber is a natural product it absorbs moisture, and sealing the whole timber in bitumen paint becomes counter- productive."
Wouldn't sealing it all prevent it absorbing moisture? 

"I propose painting only the bottom half of the timber frame which will allow the timber to breath"
My concern is painting the bottom would act like a bucket so the water goes to the bottom but can't escape? I presume it would just evaporate out the sides though very slowly if I am lucky but in that case maybe I am better not painting it at all? 

"If you plan on painting the bottom half of the decking frame, my best advice is to let the timber dry off first before you paint the surface"
https://www.bunnings.com.au/70-x-45mm-outdoor-framing-h3-treated-pine-5-4m_p8032136
Is this product dried already? It's stored indoors at my local bunnings and appears dry but I am unsure. 

[EricL]

Hello @matdiy 

The timber at the store although under cover will sometimes get damp or the surface gets incidental condensation due to high moisture content in the air especially after a strong down pour. This is why I suggest letting the timber air out first to let the surface dry off. You are correct, sealing the entire timber piece will prevent moisture from coming in, timber being a natural product will not release 100 percent of the moisture inside of it, so there will be residual moisture inside and if trapped due to the total seal will start to deteriorate the timber from the inside. It's important to allow the timber to breath or allow a way for the moisture to get out and not totally seal of its surface.

Your deck although undercover is still subject to the suns heat and changing weather conditions. As the day gets hotter moisture evaporates or expands and the timber pieces of your frame expand, as it goes colder it loses moisture and shrinks back to its original size. 

The bitumen paint's purpose is to prevent standing water underneath your deck from soaking into the timber substrate. If there is very little space under your deck, there will be no air flow and moisture gets trapped underneath. In order to provide an extra level of defence for the timber frame bitumen paint is painted on the bottom and sides of the frame. I hope this has clarified any confusion.

If you need further assistance, please let us know.

Eric