I consider myself pretty handy, but when it comes to roofing I have very little experience. I have re-shingled a couple of roof's but that's it.

My home is a 1915 victorian home built with a roof sufficiently sloped to not require gutters. At some point someone put a flat roof sun room on the back of the house. The rain pours off the main house on the the flat roof and pools. It is leaking in multiple places including against structural components of the original house. I know I should probably add gutters, but right now I'm more concerned with stopping the pooling water. It seems that the overall pitch will need to be changed. Does anyone have any experience with something like this? I'm not exactly sure where to start. I tried half a dozen contractors and nobody even wants to look at it. I'm sure it's less than fun work, but I think I need to do it. I don't mind doing it myself I just need to know how to get started.

  • What are the structural and sheathing materials on the pitched and flat roofs? Can you provide images of the interface between the two? What slope (if any) do you measure on the flat roof? If you need to increase the overall slope of the flat roof you can either raise the end near the pitched roof or lower the end away from the pitched roof. The former would typically be easier (you build a low-pitch roof on top of the flat roof) but this assumes that you can tie into the shingles on the pitched roof. If they are asphalt this is easy. Other materials may prove more complicated.
    – Stanwood
    Oct 29, 2018 at 3:05
  • A picture would be really helpful, your answer may be adding tapered sheathing and reroofing or tearing off the old and rebuilding a new roof, framing and all. Without more info, it could be easy, or difficult
    – Jack
    Oct 29, 2018 at 3:06

4 Answers 4


I see two solutions: 1) add slope to your roof, and/or 2) install a tank liner roofing

1) Adding slope can be done two ways: a) cut tapered wood members and install new roof sheathing and a new roof. This can be done on top of the existing roof sheathing or remove the existing sheathing and sister in sloped joists. Either way it will be difficult. b) install tapered insulation on top of the existing sheathing. Then install a low slope roofing. You can buy rigid board insulation that starts at 0” height and increases 1/4” per foot. It can slope one way or two ways (with a ridge or hip).

Either alternative has consequences. You’ll be increasing the slope so the roof will get higher and could encroach on the upper roof.

2) Tank liner roofing is basically used to hold water. So, the ponding water you’re experiencing is not a problem. However, tank liners are not made to resist ultra violet rays from the sun so it will decompose in a few years if not protected. I’d install a painted roof coating that reflects the suns rays. You’ll need to reinstall every few years due to wear.


With the level of detail provided, the question is basically unanswerable.

I suggest you step back and consider what the root of the problem is -- leakage -- and then think about how to deal with that.

A qualified roofer would be able to give you an estimate on sealing up any flat roof. A qualified general contractor could give you an estimate on building a sloped roof and then sealing it. (If you can't get contractors out to look at it, you need to understand why that is. They could simply be busy, or they might think it's a project that's not going to go anywhere due to cost/ feasibility/ etc.)

Once you have numbers from the pros, then you can decide how to proceed.

If you want advice here, you'll need to tell us about location, budget, your skills, and most importantly, provide useful drawings/photos showing both the broad context of the structure and the details (ie, where it's leaking). To prepare for the worst case, you should also have guesses as to how your structure has been damaged by past water incursion.

  • Cost is not an issue. The work needs to be done. I am semi skilled as mentioned above. I’ve done large framing projects and I can follow directions, I just don’t know anything about flat roof construction. I would like to understand why contractors are blowing me off I assume they don’t have time for such a small job. The problem is they are not honest about it. They say they’ll send someone out to look at it and never get back to me. I’m out of town until tonight. I’ll send some photos later.
    – mreff555
    Oct 29, 2018 at 14:27

Pics would really help. That said, one option may be to add a gable over the roof. That's what I did over my very flat front porch. Other roofing materials may help as well - steel roofs can have much lower slopes then typical shingle roofs.

gable porch roof


Flat roofs always leak eventually if they are made of a soft material. The normal solution would be to simply replace the existing rubber roof with a new rubber roof. Making a rubber roof is a messy job and requires some skill. Your first attempt will probably leak, so you probably will need to do it 2 or 3 times before you get it right.

The best kind of flat roof is called a flat seam roof. It requires soldering together panels of copper. It is very time consuming and costly to build, but will last for 200 years if done right. You would need to be trained to learn how to do it.

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