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Do I need a Underlayment over Joists when Replacing Porch Floor?

Posted by chipster_2007 (My Page) on
Mon, Aug 26, 13 at 16:21

I have read/seen where tar paper or ice/water shield was laid over joists when flooring was replaced on a porch floor. Is this beneficial of should I just replace the flooring without any underlayment? Its function would be to protect the joists but I am not so sure. Any helpful advice greatly appreciated.


Follow-Up Postings:

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RE: Do I need a Underlayment over Joists when Replacing Porch Flo

Never did that here in the MidWest on porches or decks. I can see why the idea turned up, but there are a couple of hidden gotchas in it.

First is the extra cost for material and labor.

Second is the probability of moisture(from humidity) being trapped between the material and the 'protected' joist---causing accelerated damage.

And those occur when the material is applies in strips over the joists---if applied in sheets---water collection and quick damage.


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RE: Do I need a Underlayment over Joists when Replacing Porch Flo

It's not required but I''l say that laying a simple piece of 15lb. roofing felt on top of the joists goes a long way at preventing the tops from rotting out. Rain run off and moisture get to this surface through the gaps in the decking and it takes a long time to dry out relative to the rest of the structure. The water retention promotes rot. The membrane will keep the water off of the joists.

Ice/water shield has the extra benefit of sealing around the decking nails/screws however by design it doesn't cure. Therefore I can see it flowing and possibly dripping in hot summer weather. Plus it's more expensive than the felt. I'd recommend the felt. YMMV


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RE: Do I need a Underlayment over Joists when Replacing Porch Flo

Strips of building paper were commonly used before pressure treatment became common. This is less useful with PT lumber but although it won't rot for a long time it can dry out and crack.

Grace makes a product for this purpose called Grace Vycor Deck Protector.

Here is a link that might be useful: Grace Vycor Deck Protector


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