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Deck over Garage

zindpr's picture

hello all:

We're in the very early stage of planning an attached garage to be added to our house.  The garage is to be built at the back of the house (we have access through a back alley).

I was thinking of putting a deck on top of the garage - until tonight when the fellow, (who may) be drawing up the plans said that if we did that, it is going to leak.

Some background.  The house is a brick veneer built in 1870 in Ottawa Canada.  We get far more snow than I like to think about.

He claims that the combination of a flat roof and an original brick facade will cause water leaking.  He wants to put in a sloped roof and build the deck above the roof.

I'm not really crazy about the idea - wouldn't a properly applied membrane and a floating deck work?



(post #93508, reply #1 of 8)

It could, but the contractor is probably trying to talk you out of it because it is not easy to pull off. What happens if it does leak? I would like to hear your thoughts about what you as a homeowner would consider fair if the contractor tries to talk you out of doing something for fear of failure, but you insist on doing it anyway, then it leaks. Who do you hold accountable? Yourself? The contractor?<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />




Custom Cabinetry and Furniture<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />    

(post #93508, reply #2 of 8)

Thanks for the reply.

He's not a contractor but an Architectual Technologist and he's just doing the plans for me.   I'll be doing most of the contruction myself, and therefore I will be accountable.  I have no problem with that.

I have a friend that had an addition put on about 20 years ago and put on a deck above it and it has never leaked.  He also has an old brick veneer home and the deck is attached to a rolled tar roof.

I was just surprised that this would be such a problem.  His concern was the seal between the brick and the roof.  Is there not a way to solve this problem?


(post #93508, reply #3 of 8)

Since you doing it yourself try this--

     Slope to one end with no scupper or parapet wall on any 3 sides (snow will push out the walls or trap water).

     Use at least 2 layers of Buthathane Membrane (the stuff used for ice dams, the the peel and stick roll of vinyl with sticky tar backing).

    Install trex decking made in removable sections (for easy access to repair waterproof membrane.)

   Make removable railing to make snow removal easier.

(post #93508, reply #4 of 8)


Quality flashing details, membrane installation, and maintenance will give you the results you want out of a low slope roof.  There are many issues you will need to consider.

The flashing details will be important, flashing into the existing brick wall will require the temporary removal of brick to install thru wall flashing and weeps above the new flashing for you flat roof.  This lets any vapor or water to exit the wall, assuming your wall is a veneer and not a solid three wythe brick wall.  The remaining three new walls of the addition I am assuming are a parapet design which should be a typical detail for a roofing contractor.

The max. warranty for the roofing warranty from a large rubber roofing company is twenty years.  This is where a maintenance plan comes into play.  Make sure the the roofing contractor follows the manufacturer's detail for the warranty you desire, and the plan to pay your roofer for annual maintenance checks (1, 2, or 5 years depending) to check the roofing membrane seals and flashing details, caulking, etc.

The floating deck also will need to be researched for the best fit of material.  There rubber roofing pavers that are similer to a stone paver and use pedestals to raise the paver above the roof system to allow for drainage.  This system is good because you remove the pavers to fix any leaks and for maintenance.  But weight will may be an issue.   A floating wood or fiberboard deck will be more permanent and will require post supports thru the roofing.  The more penetrations thru your roof the better chance of a leak, but as long as you check roof periodically I would not worry about the deck system.  I would completely float the deck about the roof with out connecting to any walls.  Plus the deck system would add less weight to the roof framing.

Structural deck needs to support the weight of the roofing, decking and people loads and you may want a structural engineer to review or design the system.  The design will include possible larger foundations, footings for the weight.

Provide a minimum of two roof drains no matter how small your addition is, or provide overflow scuppers thru the parapet in case the drains clog.  Also, the roof framing could be sloped to the center or to an outside wall to cut down on tapered insulation saddles.  The roof insulation can be the minimum also.

I think this is a do-able project, and I hope I did not scare you out of this idea.  I just wanted to give you the areas of concern so you can check the contractor and designer during the construction. If the flat roof doesn't work for you, I would consider a metal roof because you can achieve a low slope of 1 or 2:12 with metal and a good warranty.  Sorry this was so long.



(post #93508, reply #5 of 8)

Thanks for the detailed response. I'm probably not wise enough to get scared off.  The flashing detail makes sense, and it sounds like a job for the roofer.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

Would it be better to slope the roof to one side and not have a parapet on that side, that way avoiding the drains and penetrating the roofing system?  I could run a line of eavestrough along the side to move the water away.

For the decking system, I was thinking of building it using a series of pallets which floated on the deck. They would be running parallel to the slope allowing proper drainage.  Sungod may have a good idea with the trex.

Does this make any sense>


(post #93508, reply #6 of 8)

Yes, if you have the ability, if no one will see the non-parapet side, to slope the deck to one side and either do your idea or just a gutter system that would be ideal.  Depending on the roofing system you put down, if you choose a rubber membrane with seams arrange your deck so nothing is covering the seams and weakening them.  Otherwise, One idea is to put another piece of material over your roof where you are placing the deck supports to help protect.  One place to find some of the roofing details is /technical/ details, and/ or look into a manufacturer like firestone for rubber roofing.  Also, leaving the methods and materials to your roofer is a good idea.  Please respond with any other questions if you have them, and good luck in your project.


(post #93508, reply #7 of 8)

Thanks, I'll probably take you up on that.  I pulled an article on membranes from the site. 

Once we start breaking ground next spring, I'll post some pics.


(post #93508, reply #8 of 8)

Very good, I will look forward to seeing your project, and good luck with your plans and bids, etc.


Steider Residential Designer

Architect II, Assoc. AIA

Bloomington, IL