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I have a 16'x20' deck roof that is attached to my garage on one side and (3) 4"x4" posts on the other side.

The rise over the 16 feet is about 2 inches which is not enough for the water to get off the roof. It's pooling in certain spots. Probably because the 16 foot run is unsupported in the middle and it sags.

A friend is recommending that I jack up the roof, saw off 4 inches off the bottom of the posts to get the water to run off.

My concern is that this would rip the roof off the garage wall. Is this a practical way of introducing more pitch? Or is it better to just rip down the current structure and redo it with the appropriate pitch?

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Look into supporting the areas where it dips down. Water will run off if there is any kind of slope at all. The extra low areas causing pooling are the problem. Increasing the slope will make it drain quicker, but still might not eliminate the pooling. –  Zach Dec 9 '11 at 19:17
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3 Answers

You've got a flat roof that isn't "true" (straight - you don't want it level). Your pitch isn't the problem so much as the dips allowing water to pool.

Ideally the best fix here is:

  1. Remove the existing surface material (tar and rubber)
  2. Remove the existing sheeting
  3. Shore up the roof structure with extra beams, and shim it as needed to make it true and consistent from high point to low point.
  4. Install a double layer of sheeting with perfectly flat plywood
  5. Resurface the roof with ice shield, tar and rubber
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Seth,

I put up a 16x40 patio cover roof with some help in just a few weekends. Since your roof is smaller, and you already have posts up, I'd expect it to go pretty quickly at your site. Beacuse of this, I would just rebuild/replace the roof. You can use proper sized joists (rafters?) and beams and get rid of the sag completely.

If you are looking for a quicker fix, and you have the height at the posts, you could try cutting the posts off. If you cut them off I'd suggest cutting them off at the bottom if you can. The post usually take some abuse at the bottom so cutting off a few inches probably could not hurt. If you go this route you really need to look at the connection to the garage and post to see if they can handle being twisted/move into the new position. A lot of post replacement work involves jacking a porch roof, so some amount of movement must be tolerable. Make sure you have a strong post to lift the roof. Work safely, you don't want the roof falling on your head.

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Instead of doing a full tear-off and resheet, I would recommend having a PVC-based single-ply roof installed over the existing roof. With a PVC-based single-ply roof, you won't have to worry about ponding water issues unless they are severe enough to compromise the structure of the roof. Single-ply roofs are very similar to pool liners in design. Whereas a pool liner is designed to keep water in, the single-ply roofs are designed to keep water out. I think this is by far the most cost effective solution and will gain you a superior roof as well.

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