I moved into my house 18 months ago and the back of my flat roof extension has drainage issues.

picture of issue

You can see from the picture that the coping stones don't fit correctly on the corner (red circle) which makes that point become the focal point for all the water run off which is damaging the wall (blue box), and the paint is starting to flake inside the house. I think this is compounded by the greenhouse (orange) which is stopping most water from falling off of the stones onto the roof.

I was thinking about using a dab of cement (at the red circle) to change the shape of the incorrect tile but I am unsure where the water will then run off and I am worried I might make the situation worse if the water then sits against the older, more porous brick wall (black circle).

Here is a close up of the problem:

enter image description here

  1. Do you think that would be an issue?
  2. What do you think of my idea?
  3. What would you do to fix the issue?

All suggestions most welcome. Thanks for your help in advance!

  • That cap really should have been mitred, and they shouldn't have left a depression between the crown and the flashing on the glass. But that ship sailed a long time ago. What's on top of the greenhouse? Is it guttered?
    – Comintern
    Jan 21, 2016 at 23:32
  • Unfortunately the greenhouse has no guttering.
    – Joe
    Jan 22, 2016 at 21:57

2 Answers 2


I'm guessing that the root of the issue isn't so much the joint as the drainage profile being wrong. It looks like the greenhouse was a later addition on what used to be an open deck (that would have been sealed and drained either with a floor drain or scuppers. The cement capping on the top of the wall is crowned, but the greenhouse walls are attached below the crown. This basically makes a water trap for anything that runs down the glass, and is exacerbated by the lack of gutters on the greenhouse. The crack between the caps is just the most convenient place for it to have failed, but it has probably been begging to leak since it was installed.

I'd do one of 2 things (in addition to repairing the mortar joints) - either put a gutter at the bottom of the glass with a drip edge running into it (probably cheaper and easier)...

enter image description here

...or have a metal cap made to correct for the improper placement of the greenhouse walls (likely more expensive but would look a lot better):

enter image description here

Neither of these solutions are ideal, but without removing the greenhouse and re-capping the wall your options are a bit limited.

  • These diagrams are better than I could do
    – Ed Beal
    Jan 24, 2016 at 6:55

First you will need to clean both areas; I would want to use a Latex compound so it will last longer. If you fill the low area and seal the crack above the blue box it should start dripping on the edge. Next to the brick I would a triangle shaped fill with the edge being 2-3” to the window at least an inch or 2" tall, 2” would be better. This would let the water run down the wall and divert away from the brick and drip.

  • Hi Ed, Thanks for the reply. I am terribly sorry, but I don't really follow what you mean. Would you mind drawing me a picture of what you mean? or have another go explaining? Thanks so much!
    – Joe
    Jan 22, 2016 at 22:00
  • Are you suggesting I build a slope using a latex based 'cement'?
    – Joe
    Feb 2, 2016 at 8:43

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