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Old 09-26-2019, 09:56 AM   #1
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Waterproofing under decking/under porch roof


Looking for some advice or different ideas for how to proceed.

Photo of the rear entrance/covered porch. The garage is an addition, it has a flat roof. There is an entry way from inside garage to the basement, below the rear porch/kitchen door. The porch is above this garage/basement entryway - the goal is to keep this area inside the garage dry.



I removed the decking of the rear porch yesterday to see how it was sealed currently. From the joists up: 1/2" plywood sheeting =>roll roofing => 2x4 deck boards tightly fit together and painted. Deck boards were nailed through the roll roofing to the plywood/joists. Much of the wood (decking/plywood/joists) was rotten. The nails had practically rusted away.











The threshold, decking, and garage roof are all different levels. The footprint of the porch is supported by block walls on three sides (not against the house).


Two ideas I've had thus far

-Build a raised frame on top of the block walls, install Q-Deck, and pour a cement pad. The surface of the pad will be just below the threshold of the kitchen door. The pad would have to be on a raised frame about 6 inches above the block wall. Support posts for the porch roof would be placed on the outer two corners of the cement pad.

- Frame it, sheath it with a slight slope for drainage, install EPDM, then install a floating deck on top of the EPDM. I don't know how I would tie in the posts to support the roof on a floating deck with EPDM beneath.
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Old 09-26-2019, 10:18 AM   #2
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Re: Waterproofing under decking/under porch roof


That would work you would run the roof up and around the post for about 6 inches and then wrap the post with wood to keep it dry.

The top of the deck should be 6" below the bottom of the threshold or as much as you can get.

We would do that with a roof grade solid vinyl sheeting. Or do a membrane on top of the wood structure and put 3" slab af concrete up there.

What ever you do the membrane you use should run up the wall behind the siding and house wrap and under the threshold. Pulling the door is not always practical but you want to leave the membrane so it can be done at a later date, if a problem shows up at the door.
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Old 09-26-2019, 10:55 AM   #3
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Re: Waterproofing under decking/under porch roof


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That would work you would run the roof up and around the post for about 6 inches and then wrap the post with wood to keep it dry.

The top of the deck should be 6" below the bottom of the threshold or as much as you can get.

We would do that with a roof grade solid vinyl sheeting. Or do a membrane on top of the wood structure and put 3" slab af concrete up there.

What ever you do the membrane you use should run up the wall behind the siding and house wrap and under the threshold. Pulling the door is not always practical but you want to leave the membrane so it can be done at a later date, if a problem shows up at the door.
Thanks for advice. You would recommend against the cement pad option?

One obstacle with a floating deck 6" below threshold of door is that the surface of the deck would be even (or nearly even) with the garage roof. I think that I would like to avoid that. However, I do have plans to eventually turn the garage roof into a roof top deck.
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Old 09-26-2019, 11:04 AM   #4
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Re: Waterproofing under decking/under porch roof


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Thanks for advice. You would recommend against the cement pad option?

One obstacle with a floating deck 6" below threshold of door is that the surface of the deck would be even (or nearly even) with the garage roof. I think that I would like to avoid that. However, I do have plans to eventually turn the garage roof into a roof top deck.
If you set up a concrete pad, you would build a structure to support it and then remove the structure below, That is hard to make and guarantee water proof. So you build the wood structure to stay and lay down a water proof membrane and do the concrete, So you have the same effect but dry.
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Old 09-26-2019, 11:41 AM   #5
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Re: Waterproofing under decking/under porch roof


@Fisher1871
I have no experience but they pour slabs in steel buildings over a heavy corrugated steel. The longest they go is 10 ft. If you are going longer than that you could likely load it with rebar to be self supporting and you would still have the surface to make waterproof. That might make the whole deck about 4" thick, maybe.

https://www.canam-construction.com/w...gue-canada.pdf
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Old 09-26-2019, 01:14 PM   #6
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Re: Waterproofing under decking/under porch roof


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@Fisher1871
I have no experience but they pour slabs in steel buildings over a heavy corrugated steel. The longest they go is 10 ft. If you are going longer than that you could likely load it with rebar to be self supporting and you would still have the surface to make waterproof. That might make the whole deck about 4" thick, maybe.

https://www.canam-construction.com/w...gue-canada.pdf
I don't have any experience pouring slabs of this type either. Will have to do some research.
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Old 09-26-2019, 01:47 PM   #7
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Re: Waterproofing under decking/under porch roof


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I don't have any experience pouring slabs of this type either. Will have to do some research.
You mentioned the garage deck. I think you want to be a few inches higher or lower so you will be able to do a proper flashing when you do that.
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Old 09-26-2019, 01:52 PM   #8
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Re: Waterproofing under decking/under porch roof


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You mentioned the garage deck. I think you want to be a few inches higher or lower so you will be able to do a proper flashing when you do that.
Definitely. After the porch decking is complete, the porch roof is leveled (it's all wonky right now), and the porch roof support posts are installed, I am tackling the garage roof.

It's currently rolled roofing nailed to 1/2" plywood. Lots of leaks. I'll probably end up removing all the plywood and have to replace some of the joists below. I plan on doing that next week.
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Old 09-26-2019, 02:40 PM   #9
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Re: Waterproofing under decking/under porch roof


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Definitely. After the porch decking is complete, the porch roof is leveled (it's all wonky right now), and the porch roof support posts are installed, I am tackling the garage roof.

It's currently rolled roofing nailed to 1/2" plywood. Lots of leaks. I'll probably end up removing all the plywood and have to replace some of the joists below. I plan on doing that next week.
So with your plan for the roof and deck over the garage why not incorperate the two with the same system
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Old 09-26-2019, 02:48 PM   #10
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Re: Waterproofing under decking/under porch roof


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Here I have tried to draw what we have done with concrete. the blue is the membrane up the wall under the door and out just wrapping the top of the fascia

Then the shapes of the forms so we got a drip edge on the outside of the concrete.
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Waterproofing under decking/under porch roof-fis-1.png  
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Old 09-26-2019, 02:52 PM   #11
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Re: Waterproofing under decking/under porch roof


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@Fisher1871


Here I have tried to draw what we have done with concrete. the blue is the membrane up the wall under the door and out just wrapping the top of the fascia

Then the shapes of the forms so we got a drip edge on the outside of the concrete.
That's exactly what I was envisioning.
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Old 09-26-2019, 02:55 PM   #12
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Re: Waterproofing under decking/under porch roof


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So with your plan for the roof and deck over the garage why not incorperate the two with the same system
The garage roof is whole other can of worms.

The span is very large. I had a new roof/floating deck engineered, but I don't want to commit to that just yet.
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Old 09-26-2019, 02:56 PM   #13
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Re: Waterproofing under decking/under porch roof


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That's exactly what I was envisioning.
How long are your joists now? between the walls.
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Old 09-26-2019, 02:58 PM   #14
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Re: Waterproofing under decking/under porch roof


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The garage roof is whole other can of worms.

The span is very large. I had a new roof/floating deck engineered, but I don't want to commit to that just yet.
How wide is the garage?
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Old 09-26-2019, 03:06 PM   #15
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Re: Waterproofing under decking/under porch roof


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How wide is the garage?
26' x 32'

Currently has three steel I-beams (28' in length), evenly spaced. 2"x8"s perpendicular to the I-beams, on top of the I-beams forming the framing for the roof.
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