Mini low slope roof

We are doing a remodel and currently have a kitchen with basement that used to be an old coal house. We are going to tear down the kitchen and turn it into an outside deck area but keep the basement. As such, we need to create low slope roof with a rainwater drain in the middle and then the deck will go on top.
the area is about 12' x 12' and is concrete foundation on all sides.

Also there is the hatch to consider. I was just going to use plywood to build a neck type extension and the deck could be a shoebox cover on top but I need to ensure ice doesn't prevent it from being opened in winter
what's the best way to create this slope? Just 4 pieces of plywood angled down at 1/4" per foot?
​​

 

Personally, I would never consider doing that. It sounds like a maintenance headache in the future especially for any area that gets much winter weather.

Having a deck right over a roof won't allow proper air circulation for the wood to dry properly.

A shallow pitch roof under the deck is just asking for debris to collect. Directing all the water to a center drain means all the water and debris then somehow needs to be removed from the center ceiling area of your basement space. Then there is the pseudo access hatch that you want to access during winter.

What about treating the basement roof as a flat roof. Hot mop tar or a membrane system?

 

I was aiming to put an EPDM membrane over plywood or fiberboard.
i was thinking of a little hatch for the drain to access it and clear out occasionally.
Doesn't a flat roof actually have a little slope as well otherwise how does the water go?
Even a flat roof will collect debris, no?

 

It sounds like you have a plan. A hatch to access the drain and remove debris. Why not slope the roof in one direction so it drains off the side so you don't have to worry about a drain in the center?

 

How do I make the slope is my main issue.
do you just put different thicknesses of lumber from one end to the other to make the slope?

 

Many flat roofs are truly flat. I wouldn't recommend it if you can help it. I wouldn't want puddles to collect under my deck for mosquitoes to reproduce.

If your basement has no roof now then you can simply cut a taper into the roof rafters, sheet and apply your membrane. Make sure you size your lumber based on the small end of your cut.

Or, if there is already a roof (what used to be the floor) then you can cut pie shaped pieces of wood (sleepers) to create the slope. Then put sheeting on top of the tapered sleepers and your waterproof membrane.

 

It's a very gentle slope though. 1/4" per foot and only 6 ft to the center.
i think the idea behind the middle drain was to avoid water pooling around the foundation.
How do you cut a wedge/sleeper that accurately?
Presumably they need to go advert month every 16" for support?
There is a floor there at the moment made up of 1" thick subfloor.

 

Sleepers don't need to be cut super accurate so it can be done with a circular saw. I mark both ends of the board then snap a chalk like between the two points.

As for the spacing of the sleepers that is based more on the sheeting you use on top. Each sheeting is rated to span a certain distance. 16" spacing is most common in N. America but by choosing a thicker sheeting you could go to 19.2" or 24" spacing and have fewer sleepers to cut. Since you will be going over existing joists and sheeting I would put the sleepers on top of the joists below so you don't have to worry about the old sheeting sagging.

 

How much air gap is needed between the membrane and the deck wood?
on my plan it looks like it will be minimal at the sides and a few inches, say a 2x4 by the time you get to the drain part. Could possibly use 2x6 depending on patio door height

 

If your deck is that close to the roof below then you aren't going to get much airflow. I would look for lumber treated to a higher standard as much of what is sold in big box home centers is the minimum. Also, 2x4 really aren't joist material and certainly not for spanning 12'. You really need 2x10 for spanning that distance though you could go 2x8 if you have a 12" joist spacing but I don't recommend it as flexing/bouncing may become noticeable.

I'm wondering if you wouldn't be better off doing a poured concrete roof/patio since you have so little height. How high will the deck be above ground level?

 

The area is currently 4' above ground.
i suppose I could make a more solid frame underneath so instead of spanning 12' it only spans a few?

How do you support the concrete if there's only wood floor there currently? Metal?

 

One way of constructing a concrete roof is with a corrugated form on the bottom, usually galvanized steel but there are composite ones available. This is not roofing material. It is a heavy gauge material with deep corrugations specifically intended for jobs like this and it's very strong. The edges of the corrugated sheeting are supported by your masonry walls (your existing wood roof would be removed). Then temporary support may be needed to support the sheeting during construction. Side forms are constructed to retain the wet concrete and rebar reinforcement is placed. Then concrete is poured. After the proper curing time the forms and temporary supports can be removed. The corrugated steel sheeting is left in place. And you have a solid, reinforced concrete roof for the basement and patio.

 

I'm having trouble trying to plan out the deck substructure. If I have a sloped plywood that is 1.5" higher at the outside then middle and I can only use 2x6s for framing.
i think a 2x6 can only span 6ft but the deck is 12ft.
how do I put a support or beam in the middle when it's sloped?

 

That is your problem. You don't have enough elevation to make it work. A 2x6 is actually only 5 1/2" high. Take out 1 1/2" for your slope and you've only got 4" of wood. Not even close for a 12' span. If you want to go that route you're going to need steel.

 

No I mean it's 1.5" + 5.5" on top
middle joist would therefore be 8" wood or so because that's the deepest part where the drain is.
i was figuring that with a 2x6 I could put one piece of wood support halfway so each span is about 6'

But it would end up being little blocks of wood, posts, or some kind of metal block.

There is a structure of joists and subfloor under the deck substructure and flat roof so that provides some sort of support as well.

 

Will a blueskin membrane do the job instead of EPDM membrane?