need help in getting to goal for screened porch

springplanterAugust 20, 2014

in a recent addition, we planned a screen porch above a study at ground level. We were to do some of the finishing work ourselves, such as the tile floor planned.

Presently we have a 3/4 OSB floor over 2x 12 joists. We have been told that to support tile and to be able to apply waterproofing, we need to use 1/2 inch cement board over the OSB. If we do that, the tile is too high for the two doors, one french 6 ft and one single 36 " which provide house access.

Question 1 is 1/2 inch cement board critical or could we get by with 1/4"?

Question 2 I assume that to insure waterproofing, we would need to install flashing between OSB and cement board onto outside wall, correct?

We would also like to use cable railing to preserve views, but I understand that perforating the tile compromises the waterproofing. Can't attach outside because of screens. Any suggestions??

Many thanks in advance

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the answer to all your questions:

    Bookmark   August 22, 2014 at 5:51PM
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Use Ditra instead of cement board. Cement board does not provide waterproofing. And Ditra is only 1/8" thick.

    Bookmark   August 24, 2014 at 3:39PM
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thanks for the answers. I spoke with schluter and they say no go. Since it is subject to rain, i have to go with installing drainage membranes and a mortar bed below the ditra.

Back to looking at the possibility of raising the doors i guess

    Bookmark   August 27, 2014 at 8:26AM
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Cement board is far more water-permeable than Ditra. You can use a roll-on product like Redgard on the osb sheathing, then lay the Ditra mat and use Kerdi tape to seal the seams and tile away.

    Bookmark   August 29, 2014 at 2:22PM
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I had a second story porch on my last home. I used the lower area for storage and did not have to worry about drainage, fortunately. I wanted the views to the ground as well, but code requires a 36" railing. I put the window sills at 18" and made some mahogany railing inserts to fit into each window area. The lower piece was set 3 3/4" up from the sill and the 1 1/2" posts were set 4" apart. the top rail was set to comply with the 36" code height requirement. These worked out well, were removable for painting and/or finishing and trimmed the screened area nicely and while the view to the ground was not unobstructed, it was fine. Cable is nice, but climbable. One story up that should be a consideration.

    Bookmark   September 2, 2014 at 9:08AM
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Thanks for ideas. Casey, what about stability for the tile? The OSB doesn't seem to have that tensile strength, ie, when you jump on it it does show some bending??



    Bookmark   September 2, 2014 at 6:05PM
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CArol;, you answer your own question. If the floor flexes, it is not a suitable surface. As well, I will say that you should go thru this section and read some of the posts about waterproofing a ceiling/roof on a porch or deck. It is not an easy matter and must be thoroughly with the right products. OSB and tile are not going to do it and you will have leaks, dry rot, mold and mildew pretty quickly. You need a sealed membrane and something on top of that which can breath and be accessible. I did mine with a rubber roofing material over sleepers and then supported the mahogany 1x4 decking off the sleepers. I made the 1x4 into "pallets" that I could lift off should repairs be needed. In 18 years I did not have a problem, but one day the subsequent owners will. By the time I was done I whole thing was 7" off the top of the ceiling joists. There are a lot of ways to do this, but I think you need to research this carefully.

    Bookmark   September 6, 2014 at 1:04PM
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