How to fix leaky post and railings on a flat roof

Post and Railings on roofs make up for most leaks

Post Flashing is one of the major issues on residential flat roofs

Most roof leaks are caused by inadequate flashing on a post. This is one of the most common areas where a leak starts. Watch this video to see what to do.

Click here to see “How to find leaks and make Flat Roof Repairs

A Schematic diagram of a post on a roof


When wooden posts are used for railing, it is best to go with pressure treated 4 x 4 posts. These posts should be fastened to the rafters with bolts and nuts. Normally a carpenter is best suited for this job. Roofing contractors normally don’t want to deal with the installation of post and railings. Once the 4 x 4 posts are in place and the plywood deck is installed, then a roofing contractor can install the flashing around the post and install the rest of the roof.

Installing a roof is costly, by not installing the posts correctly will cause leaks later on and that will mean a possible roof replacement. Do not cut corners when it comes to posts and railings, this could be the difference in maintenance-free or a very expensive proposition later on.

A post has 4 sections

  1. Post cap
  2. 4 x 4 Post
  3. Sleeve or jacket
  4. Flashing

Many times a contractor will only install a 4 x 4 post without the sleeve. The flashing is exposed and water can find its way down the 4 x 4 post behind the flashing. Often times a contractor will caulk between the post and flashing. The caulking is never a permanent fix.

Besides post, other areas can also cause leaks. Check this page out to find out where to look and how to make roof repairs

Railing post components illustrated


Once railings have been installed without a sleeve around the post, it becomes much harder to fix the problem. because of caulking and paint that seals the 4 x 4 post, it could be years before the problem is noticed. A post leak can go undetected for many years,  but the beams and plywood around the post will eventually rot.

Several manufacturers make composite sleeves or plastic sleeves that are readily available at Home Depot or Lowe’s. The best part of these sleeves are, they never have to be painted.

The Post covered with sleeve and cap.

Anytime post and railings are installed, a sleeve should always be installed first to make the posts leak proof.

Metal posts and railings have to be dealt with differently with. Some metal railings are welded to the post that makes for a rigid structure. In that case, the metal post will stand loose on top of the roofing membrane and will not need any flashing. There are cases where the metal posts had a flange at the bottom to be able to screw the post to the roof. This is not always very desirable. Special care must be taken to seal the screws that went through the roof.

Railing Post sleeve cover

Roof with Post and Railings

In the image, you see a roof with railings. Over the years they had numerous leaks and they could never pinpoint where it was coming from. This was the most inexpensive type of railing to install, everything from 2 x 4’s. Being the most inexpensive post and railing, it ended up being very costly. The roof had to be replaced, the post had to be removed and new posts and railings had to be installed.


A closer view of the post flashing area.

You will notice that paint and caulking was covering the flashing at the bottom. The caulking and paint had to be reapplied every several years to keep a tight seal around the flashing.

Corner Post railing

Another image of the post flashing

The same thing can be seen here. There is no sleeve around the post to protect the flashing. What kept most of the water from leaking in, was the paint and caulk. Most of the time, a leak does not show up on the inside of the house but rather it gets soaked in the insulation and plywood decking. The plywood will eventually leak.

Post flashing not correct

Copper flashing was visible after removing the paint

The reason why these posts were not leaking very much was that the flashing around the posts was cut into the post. The fact that the copper flashing was bent into the post made a big difference and lasted a long time before it started to leak.

Post flashing exposed after paint removed

Portico roof damaged

look at the roofline of this portico. It has sagged in the middle. This post has been leaking for a long time and caused the beam to rot. Damage like this is very costly and could have been avoided was the post flashing properly done.

Portico Post was leaking causing damage

Damaged Portico roof due to Railing Post leak

Because of a small detail that was overlooked, this entire portico roof has to be rebuilt. The leak around the railing post was not detected until it was too late. It took several years of water leaking into the substructure and the joists to rot.

Portico Roof Damaged due to post leak

Another example of a 4 x 4 post that didn’t have a sleeve

These railing posts can be leaking for many years before actual damage can be detected. These posts were caulked at the flashing and painted black. The paint helped to seal the 4 x 4 post not to absorb moistly.

Railing Post not flashed properly

A 4 x 4 Post without a sleeve and post cap 

The 4 x 4 post had to flash at the bottom that was caulked to seal it from water getting in. Then the rest of the post painted to prevent it from soaking water every time it rained. This post several other posts did finally leaked and caused extensive damage to the structure.

Post Flashing caulked to seal

Structural damage to joist and ceiling due to P4 x 4 post leaking

This is the result of a railing post leaking. This took many years before it was detected, but then it was too late. Repairing the damage was far more costly than what it would have cost if the posts were covered with sleeve and post cap.

Damaged ceiling from Post Railing leak

Step by Step example of how a post and sleeve should work

Copper flashing for a post

Copper flashing that is specially made to fit around a 4 x 4 post

Copper flashing for post railing

4 x 4 Post is the most common

Most of the time a pressure treated 4 x4 posts are used for railing. The copper flashing can be bought or made on site.

4 x 4 post that fit into the flashing

Installing a 4 x 4 post

This is just an example of a 4 x 4 post. When properly installed, the 4 x 4 post must be fastened to the joists.

The 4 x 4 post should be fastened to the joist

Inserting a sleeve over the post

After the copper flashing and 4 x 4 post, but before the railing is installed, the sleeve is inserted over the post.

Adding a sleeve over the post

Post cap installed over the post

Once the flashing, 4 x 4 post, and the sleeve are installed, a post cap must be installed to complete the process.

Final step is a post cap

No copper flashing on post

After we removed the post cap and sleeve, we found that there was no flashing. That is a rare thing to see.

No copper flashing was found on this post
Please write reviews about flat roof repairs at the bottom of the page
Please write reviews about flat roof repairs at the bottom of the page

8 thoughts on “How to fix leaky post and railings on a flat roof”

  1. Hello. I have an upper porch with a metal railing, not the 4×4 wood posts. Water is getting into the downstairs from the metal deck railing post. Repairs with tar or roofing cement has not worked. Do you have a step by step process that would help me seal a metal deck railing at the metal where the metal post meets the deck?

    1. Het Wayne
      I think your answer will be Turbo Poly Seal. If you Google Turbo Poly Seal you will see many videos I have posted on how to use it.
      Send me some pics of the issue you have to the 203 858 0080 and I will have a look at it. Then I can give you a better idea.
      For now, I really believe Turbo Poly Seal will do

  2. Great information. Thank you for sharing. I do not have access to copper post flashing. Will the Trex Rain Escape 4×4 post flashing work in an SBS self adhered roof?

  3. Hi Naas, 
    We have small ceiling leaks directly below the roof. The leaking usually occurred during a heavy rainfalls for a prolonged period of time. After running several water tests we finally identified the spot that caused the leaking.The spot is around the metal railing post on our roof terrace. The roof terrace is Duradeck or TPO(not sure which one) covered by porcelain tiles. Do you think Turbo Poly Seal would work on this problem? I also sent you 3 pics to give you a better idea.Thanks much.

  4. We’re now required to have a railing on our flat roof b/c there’s a door leading out to it. This is both a city and an insurance requirement.
    The roof is 4 years old. It is Certainteed Flintastic SA Low-Slope Roof System. How should the posts be installed so we don’t have any leaks? Many thanks.

    1. Naas Ferreira

      The best way to install posts is to cut a square hole through the roof where the post should be fitted. Then you can flash the bottom of the post with Turbo poly Seal. Then you add a Post Jacket over the post the cover the Turbo Poly Seal flashing.
      If the roof is relatively small, like 15 x 10 feet. Then I would suggest a floating railing system. This is where the railing is secured to the wall on each side and the rest of the posts are sitting on top of the roof secured with metal L brackets.

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