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Bad Roof Design on Future Greenhouse

 
Matt Todd
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Not strictly a greenhouse/permaculture question... but this is the roof on a porch I want to glaze to become my greenhouse and I don't want it to leak!

When a heavy rain hits the house roof (A) and porch roof (C), it backs up under the flashing (B) and leaks in various places under the porch roof. The porch roof does not have enough pitch to carry away water fast enough. Since I cannot change the pitch of the porch roof, I believe I should somehow seal the gap between the flashing and the porch roof (B and C) to block water entry. I have used a silicone caulk in one spot and it does block the water, but it’s not practical to do the whole porch this way (40+ feet at one inch tall is too much caulk gunning.) What would you do here? Flexible adhesive flashing? Some sort of extruded product? Thanks.  
Porch-Roof.jpg
[Thumbnail for Porch-Roof.jpg]
 
Eliot Mason
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Matt:

The C intersection should be dry, and filling it up with silicone or other sealant violates a cardinal rule of roofing - ALWAYS let the water go.  If some water finds its way through a nail hole or something and into that cavity and it can drain away - or vent away - then you can create real problems!  

So ... why isn't the porch roof draining?  Unless it is completely flat that water should go away, especially as water coming off the pitched roof has energy to help sweep any water off.  I'd look at your gutters and any trim details on the roof edge.
 
Matt Todd
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Eliot Mason wrote:Matt:

The C intersection should be dry, and filling it up with silicone or other sealant violates a cardinal rule of roofing - ALWAYS let the water go.  If some water finds its way through a nail hole or something and into that cavity and it can drain away - or vent away - then you can create real problems!  

So ... why isn't the porch roof draining?  Unless it is completely flat that water should go away, especially as water coming off the pitched roof has energy to help sweep any water off.  I'd look at your gutters and any trim details on the roof edge.



HA, I figured the cardinal rule of roofing would be more like "keep stuff under the roof dry" :)
The porch roof is almost totally flat. I keep the gutters clean. Too much water comes through to be acceptable. I believe that current flashing would work, but the seam ridges keep it suspended when it should actually conform around them. It wasn't there originally. This was a roofers slap dash solution to the leak that was even worse before ("fixing" the porch roof was a condition we made for buying the house, and it was done as cheaply as possible it would seem.) They stopped coming back after I bothered them too many times after every heavy rain.    
 
Eliot Mason
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Matt:

Well, if the water isn't on the roof then the water can't go through the roof, thus keeping the underside dry.

So yeah, if that porch roof is nearly or actually flat then the problems are actually much larger than that one gap.  First, that roof isn't meant to be flat, so water will find its way between seams and around screws and nails.  Second, you've got a potential ice dam at B so water might back up under the roof A.  If your house roof deck is properly covered with ice shield the roof should be ok, but please beware this possibility!

Solutions ...
a) IF that space between C and B actually vents (where the water is currently going, yes?), then sealing it isn't the worst idea - but you may find the water finds another way to annoy you.  To reduce the amount of sealant needed consider a foam backing rod (literally to take up space and give the sealant something to hold on to) or a self expanding foam sealant (one can would probably do the trick.  You might consider something other than standard silicone - black roofing goo probably does better in UV light.

b) consider the porch roof a substrate, not a roof.  Make a proper flat(ish) roof - Seal the top gap as above and then cover the whole thing with roofing silicone or equivalent?  That will increase reflectivity and reduce heat absorption... maybe not a good thing... and seal all those roofing seams, nails, etc.

c) repitch it.  I know you said you can't do that, but really it might be the better solution.

d) wait, you want a greenhouse with a solid roof?  How about removing the steel panels and going with something clear or translucent and fixing the pitch at the same time?  Probably not a task for November, but something to keep in mind.

e) divert the roof water laterally.  I'm thinking of gluing slightly angled boards on the main roof to act as little diversion dams, shunting the water to the side so that it ends up somewhere other than on the porch roof.  NOT a good idea for snow loads, but I suspect that shouldn't be an issue in MO.  Less water on the porch roof should be less of a challenge for the pitch.
 
William Bronson
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What if you cut a hole and installed a downspout right where the leak happens?
Direct the water into a barrel , with overflows going to other barrels or the landscape.
 
Nancy Reading
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Or - an internal gutter to catch the water which can then be used to water the plants in your green house.
 
William Bronson
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Nancy has the better idea by far.
 
Bill Haynes
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The most likely culprit is an ice dam building up at the eaves.
Because airflow is greater at the tips (not blocked by the bulk of the building!) Ice is a frequent problem, when it forms the water then backs up and can pull nails and generally wreck a roof.
This is why old timers did not make flat roofs.
If you install heat trace in a wave pattern on the roof it will ensure a path is created for water drainage. Heat trace is wonderful stuff, its a thermoplastic that conducts more electricity as it gets colder. It won't run for free...or even cheaply, (no its not prohibitive, it will run a few dollars per week of operation and as weather warms it stops conducting electricity) but it will save a poorly thought out roof.
 
John F Dean
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Let's go back to the original post. Why can't you change the pitch of the porch roof?
 
Matt Todd
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John F Dean wrote:Let's go back to the original post. Why can't you change the pitch of the porch roof?



I mean... I could. But that would mean disassembling and rebuilding the 54 foot length of it, which is way more work that I want to do or pay for. Especially since it only leaks in 1 or 2 places. Granted, I have fantasized about just chopping a few inches off the uprights above the porch rail and re-securing them :)

Currently what I'm looking at trying is foam backer rods. Closed cell foam sticks to tuck into that gap and see if it works before attempting any more permanent solutions.
 
John F Dean
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I don't know what your roof plans are for your new green house. I assumed you would be removing the metal roof  and installing translucent panels.  If that is the case, then you only need to add longer rafters over the present ones to increase the pitch of the roof
 
Eliot Mason
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John F Dean wrote: you only need to add longer rafters over the present ones to increase the pitch of the roof



Wait, what?  I fear John slipped and hit the keyboard wrong... adding longer rafters would indeed increase the drop, but the pitch would remain the same: Adding a rectangle just maintains the angle ....

I agree that pulling it down is the better long term solution ... but maybe not a job for November.

Let us know how the backer foam works and if it solves the problem.
 
Bill Haynes
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He meant installing longer rafters via starting higher on the original roof.... in effect turning the existing rafters into the lower chord of a truss......
Or at least that's what my weak old mind sees!
 
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