How flat can a flat roof be?

[JohnJ]

Having obsessed about foundations for a week, I'm now visualizing the roof of my man cave/Little House.  Our home is built on a piece of ground cutout of a steep mountain.  We have about 20-30 feet of space behind the house that's flat, then you start going up.  That's where my structure is going.

1)  How flat can a roof be to still allow drainage?

2)  Even more ambitious . . . assuming the roof was flat enough, I was wondering if I could put a surface down on the roof that would allow you to walk around up there (with a railing, of course) and use it as a deck.  Is that possible?

Thanks in advance

[Don_P]

Yes you can, for a price.
Is there any way you can push the building over the edge and save the flat spot for outdoor space? The building doesn't really require the flat spot.

[Redoverfarm]

There is no such thing as a flat roof.  Meaning that there is no pitch.  What appears to be flat in some construction has enough pitch to drain the water some ever so slight.  If you live in a snow region this is not a good idea.  You will have to build the roof somewhat stronger than usual and use a waterproof membrane followed by a floating deck atop that. Even with the extra precautions there is no assurance that it will not leak. 

I am like Don_P if you want a deck I would move the build over part of the steep grade and utilize the flat for the deck area.

[MushCreek]

Frankly, I've never understood the appeal of a flat(ish) roof. Many commercial buildings have very flat roofs, and most of them seem to leak, given enough time. What's worse, it's often very hard to determine where the actual leak is. If you want a deck on top of a roof, i would go the other way- make the roof as steep as you can and still incorporate the deck. You don't want to walk on the actual roof membrane itself; this will promote leakage. What is normally done is to build a low pitch roof and cover it with sheet rubber- EPDM, I think. Then you lay tapered joists on top of it, and attach the decking and railings to that. Flat deck, pitched roof.

[Squirl]

I've seen it done.  I've been on a few.  Most that I know of are between 1 in 12 and 1 in 20 pitch.  My understanding of most "flat" roofs are roofed in asphalt rolls and tar.  They don't last as long. They have to be done on a 7-10 year schedule.   They are built up over the years with one layer over top of the last.  They should be designed to handle a heavy load.

I have seen decks put up over a flat roof.  IIRC, The decks I saw had a deck type of block.  I can't remember if there was another type of connector. It was designed like any other deck, but sized so there was the least amount of posts and roof contact.  Most were clear spanned across thinner buildings so the posts fell over load bearing walls. A full deck is supposed to extend the roof life to 15 years or more, according to the people who owned them.  There is an expectation that it will have to be torn off and reroofed at that time.

[JohnJ]

The building doesn't really require the flat spot.

What kind of foundation would it take for a 24 x 14 structure built on steep grade?  Are we talking about sinking beams into the ground?  Using more blocks on the low edge to match the height at the flat edge?

I guess I could do it but I'd have to figure out how the logistics of getting from the house to the shed and vice versa . . . especially if I'm right on the edge of a drunken stupor . . .

[JohnJ]

I have seen decks put up over a flat roof.  IIRC, The decks I saw had a deck type of block.  I can't remember if there was another type of connector. It was designed like any other deck, but sized so there was the least amount of posts and roof contact.  Most were clear spanned across thinner buildings so the posts fell over load bearing walls. A full deck is supposed to extend the roof life to 15 years or more, according to the people who owned them.  There is an expectation that it will have to be torn off and reroofed at that time. 

What I saw when we were shopping for this house, was a country home in southern Virginia that had a deck outside the home's master bedroom.  The actual roof was covering an enclosed walkway between the first floor den and the garage.  It was built using that plastic Trex decking . . . I don't know if that was a factor.   Unfortunately, I didn't pay close attention to where the roof ended and the deck began, but nothing abnormal caught my eye.  It looked as smple as just putting decking and railing on a flat roof.

I did not notice any slope to the roof but I'm sure there had to be some . . . it was just imperceptible, at least to this rookie.

[Don_P]

This is as low angle a view of our place as I could get a good shot of on google earth;


The sawmill is in the lower foreground, the first shop timber truss is under construction behind it, I'm at the left corner of the truss. The house is at the top of the pines/ head of the pasture facing south (right). The black shadow up the pines is the (helicopter) cleared path for the power line. The house sits pretty centered on a narrow shoulder of the hill. It would have cost more at the time but I've wished as the years have gone by that I had moved the house further forward toward the grassy slope, walkout basement, and left the top of that shoulder for parking and yard. Just one opinion and every mix of circumstances is different.

As long as I'm rambling the entire layout of my place is for a young person. Follow the sawmill road uphill to the garden, coop and barn then switchback and up to the house. Vertical rise between buildings is about 180'... I used to walk it all the time, I do less and less. Lining the buildings along an elevation contour is much easier for a life of tending things.