Sealing bottom-underside of deck?

Questions & AnswersCategory: Wood Deck QuestionsSealing bottom-underside of deck?
Ralph de Gouveia Pinto asked 7 years ago

I’m getting a deck built out of regular wood (not pressure treated) And the person building it says that there is no need to seal the under side of the deck. He says that the sealant will seep through the top and soak till the bottom.

I just wanted to know if there was any truth to this or should I insist that he treats the bottom as well.
Thanks in advance.

3 Answers
Crowder Painting answered.

Truth? No. The stain will not seep all the way through the wood. At best it will soak a 1/4 inch into the wood.

Normally staining the underside of the deck wood and frame isn't needed nor is it a good idea. The wood needs to breath and release any collected moisture and staining all of this will prevent this from happening. This will speed up rot.

Anonymous answered.

okay, I was under the impression that the underside had to be sealed to prevent moisture from getting in..

Crowder Painting answered.

Here's the problem that causes rot. The top of the frame, where the floor boards are screwed, is still raw and it has a hole where the screw is. This is where water gets in. It needs a way out and this is the nearest edge. If this edge is sealed the water is stuck and can't get out quickly.

I have seen deck builders use a rubber membrane to seal the top of the frame. This is the same self adhering rubber membrane used for roofing. I think this is the best idea I have ever seen for deck construction.

I have seen a lot of rot damage on deck frames and all has had the frame sealed. Something to think about.

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