Figured i would get some good ideas on how to make some sort of ceiling for a lower level porch beneath my deck which is framed with wolmanized 1×8, looks to be around 10ft. x 18ft. would like to make it water tight or close to it.

There are products specifically made for this type of thing on the market. But you can use corrugated aluminum or fiberglass/plastic panels secured to the underdecking. You’ll want some slope to make the water run off, either into a gutter & downspout or if you’re not worried about directing the water to any place in particular. Just let it run to the edges. They are available at just about any hardware store.

FIRST – if the floor joists are 1×8, you probably have a severly underbuilt deck. The joist material SHOULD BE 2x. BUT – assuming it is 2×8 joist material… Several potential solutions come to mine. First, there **ARE** (I believe) commercial solutions – Google “under deck rain systems”. But my guess (and ONLY a guess) is that this WILL BE pricy. BUT, I suppose you **COULD** find out their general concept and try to replicate it yourself.

Another thought, it might be time to consider upgrading the SURFACE of the deck: Put down some Hardi-Backer, tape and seal ALL the seams, and then put a new tile surface on top. Piazza tile… something like that. HOWEVER, with THIS solution, you should look at the span of the joists – do they run the 10′ or the 18′ direction? The Hardi-backer and tile WILL add a ton of weight to the deck. If you pursue this route, you should get a professional opinion before proceeding. I had to go that path when I did a carport addition – I used a logal engineering firm, and their minimum price covered my work, and I think it was a few hundred dollars. At a minimum, they will want joist material, joist size, joist span, spacing BETWEEN joists (prob 24″?) and weight of materials to be added to the surface – per square foot. They will run calcs and let you know if the structure, AS BUILT, can tolerate the additional weight.

A light weight variation of the above… staple a couple of layers of plastic (the more mils the better) on top of the deck, and install some indoor / outdoor carpet on the deck. If you go this route, i would suggest leaving the underside exposed – do not cover the joists bottoms with a “ceiling” – for two reasons 1) exposed will allow air circulation and help prevent – or at least minimize – rot. (By installing a plastic surface, all air flow is greatly diminished) and 2) it allows for a visual inspection of the joists to see how they are doing – if rot DOES set in, you can see it faster.

Whatever you do, do NOT do this… staple plastic to the bottom of the joists and install a ceiling below that (V-rustic, or whatever). If you do THAT, you WILL cause the deck to fail. By stapling the plastic to the bottom of the joists, you would be creating a situation where the joist will be sitting in water and will rot in no time. It WILL fail.

I found four commercial systems: http://www.dry-b-lo.com, http://www.underdeck.com, http://www.drysnap.com, http://www.lockdry.com. These are around $5 per square foot.

I would make this by screwing a 2×4 to the house about a foot under the deck, another 2×4 to the legs‹put it enough lower that you¹ll have a nice slope to let the water drain‹then laying sheets of metal roofing across these two 2x4s. I¹d screw the metal roofing to the 2×4 out at the legs so that it won¹t come off, and then put a 2×4 across the roofing at the top 2×4 so the wind can¹t pick the metal up.

We used the ‘underdeck’ product. Looks GREAT. It was much nicer than what Lowes’ had on stock. Because we could not find (underdeck) locally, we asked one of the smaller hardware shops if they would consider ordering for us and bring in with one of their shipments… Made it much cheaper than having the
company ship to us.

If, like us, you want to install ceiling fans, make sure they are a good quality AND water sealed. The one that is under the upper deck has leaked – fortunately, it is an outdoor fan (from HOMEDEPOT). After consulting with Underdeck, (we sent them picture)they recommended that we ‘re-seal’ with clear poly. However, it did leak again. It had to be ‘fixed’ from above – nothing to do with the underdeck. A kinda channel was put in around the fan to help the water (from very strong rain only) go around the fan. All is well now. Having the extra space for all seasons is great. Make sure you do follow the installation instructions and build it at an angle. We added gutters around it, as well.