I'm still in design phase(slow but sure, gathering info) for a modern passive solar lake home.  I'm planning on a flat roof design.  There aren't many around in residential and seem to be reserved for custom modern homes.   Conventional wisdom is that a flat roof is more apt to have problems.  Why do most commercial buildings have flat roof construction?  I'd think commercial would also go for most economical/less problem prone construction as well.

It is very easy to put a sloped roof on a relatively small building, like a 30*50 or so. But could you imagine a sloped roof on a builidng like a Wal-Mart? Even with a modest 4/12 pitch, just think of how high the peak of the roof would be. A 300' wide building would have the roof peak 100' in the air, on top of the already 20' ceilings.

As far as problems go, as long as you make sure the roof is done right, you shouldn't have to worry about anything, assuming your lake house doesn't have to deal with a whole lot of heavy snow loads.

Conventional wisdom is right, at least in my area. Flat roofs on residential projects almost always have water problems within a few years. This is true of commerical as well, but they seem to expect it moreso than the normal homeowner. Our local high school has had to have roof repairs every five years for as long as I remember, and they just keep on doing the same things over and over again. As to why commerical buildings have flat roofs, I think the reason is twofold. Putting a pitched roof on a footprint running into the tens of thousands of sq. ft. is difficult, to say the least. And secondly, where would you put all those HVAC units on a pitched roof? At least in new schools they seem to be getting away from this style, using smaller structural units, which can have more traditional pitched roofs, interconnected.

These guys are right, but if given a choice I would always go with a pitched roof at least 4/12. When I was inspected roofs daily, I swore flat roofs were destine to fail, but then if everything was okay, I wouldn't have been there.

"Flat" roofs generally are the most cost effective solutions for commercial buildings.  Even a "flat" roof should have a 2% slope, although 1% is commonly used.

Construction technology has come a long way from multi-ply coal tar and asphalt roofs.  Today, Frank Lloyd Wright could build his houses without leaks.

The devil of a "flat" roof is in the details and ponding should be avoided.  Minimize penetrations and pay careful attention to the flashing details. Workmanship is critical.  Keep the details as simple as possible.

Always have multiple drains (or scuppers) and it is prudent to have an overflow or secondary drain at a higher level to prevent a structural overload if the main drain(s) are plugged.

If you have a "small" area, the amount of perimeter flashing to the roofed area is high and working in small areas can sometimes be difficult.  These can be more problematic than a large area.

Bruce
(who hopes to install a "flat" extensive green roof on the new house currently being designed.)

Bruce, Do you think a EPDM or thermoplastic membrane would be a better choice than built-up asphalt? TPO not PVC, I still remember looking at all those PVC membranes that shrunk so much that it pull away all around the perimeter, I even saw a couple PVC that pulled brick parapet walls down.

Thanks for the info! I guess for top level I could go with a sloped roof back into hillside which would give me more light to southern exp/lake view anyway. The only "flat" roof I'd have is for two levels that would have 10-15 ft setback to create deck space.

slenzen,
That is a new set of problems, foot traffic/deck on a roof. Roofing membranes are not meant to have repeated foot traffic.

How I have seen it done is "sleepers" resting on the roof membrane and the deck attached to that. But there is very small movements that wear on the membrane and to replace the roof, the deck has to come off.

And I have seen attachment points fastened to roof structure and the membrane going around that. But that is more penetrations and flashing = more problems. To replace that roof the deck has to come up. Sometimes the deck is made in removable panels.

Work closely with a good roofing company, and post here often. Bruce sounds like a roofer, keep this going until you get it worked out. In our area, I highly recommend Grady at Allstar Construction 763-479-8700, very experienced roofing contractor.

I am going to start a thread on the ICF page, that is flat roofing related and see where it goes.

What about sprayed urethane foam roofing? It does not have the shrinkage problems associated with EPDM membranes and it adds insulation value. Last time I checked, the cost was about $4 to $5 per sq. ft. I am not sure about hail resistance though. I have heard it was both good and bad.

As far as placing EPDM under a deck structure, I have seen where an extra layer was placed under each sleeper supporting the deck. This extra layer provided some wear resistance as things expanded and contracted with temperature changes.

I don't deserve that much credit.  I work for a large real estate development firm.  My function in life is to hire the design team and consultants, oversee the design, hire the contractor and make sure it gets built correctly, on time and on budget.  You could say that I am a "jack of all trades" and master of none, but I do deal with a lot of different and interesting issues.

I monitor GreenBuildingTalk to learn about green residential construction issues in preparation for my "final" project.

Bruce

Right John, I forgot to mention the scraps placed under the sleepers. I have not heard of the spray urethane roofing.

This really isn't a big problem, but you do need to design it accordingly with some extra protectiion so the main membrane is not damaged.

Precast concrete pavers are good option, too, as long as the weight is not a problem.

In any case, make sure that the drain is always accessable for cleaning.  If leaves and other tree droppings can get trapped in joints, etc., make sure that you can remove the deck or pavers to clean.

If you do make a thick paved "patio" you might consider a 2 level drain.

Bruce

The design phase is the most important. I would like into finding a roofing company to ask some of your questions.
This is a list of roofing companies that might be helpful for you.

You know, there are cities where hundreds of thousands of houses have flat roofs Philadelphia and Baltimore. It is not that much of a design challenge. The big problem with a flat roof is insulation and ventilation. A problem that I will not explain.

For a flat roof just do this: EPDM membrane (white or black depending where you live) glued down to poly iso insulation (at least 1 1/2") screwed with plates, over tar paper, onto 3/4" T&G plywood roof sheathing. Then on the inside you can use spray foam which is better or fiberglass batts. If you use fiberglass then use more poly iso on top. R-40 should be your absolute minimum design goal. R-60 is nice.

For areas with regular traffic put the roofing board (I forget what it is called - like impregnated homosote) between the poly-iso and the roof membrane. For this membrane I would use modified bitumen. For the deck, put it on sleepers that rest on strips of the modified bitumen. Since there is not a lot of ventilation you would have to use a manufatured deck board or a South American hardwood like Ipe or Garrapa screwed down to the sleepers. Or you can use pavers which I don't know much about.

Another reason why commercial buildings ave flat roofs is that they don't care if the building falls down in twenty years. Their financing runs in ten-year cycles where as residential is usually financed for 30 years.

A pitched roof is easier for residential construction but a flat roof should not be out of the question. There is one on my house.

Our extreme weather in MN is harder on flat roofs. With Lake Minnetonka views, a Lite-Deck roof would be ideal.

you are right but i think sloped roof is suitable only for small buildings. But flat roof is most effective roof which i had ever seen. So i prefer flat roof.

ever considered green roof. You wont have the water issues. Our LEED project isnt doing a green roof, but a Johns Manville TPO membrane mechanically attached roof. Grey/white color to reflect the light.

I have a 1700sq.ft. FLAT roof on my newly ICF house ( not finished yet )

my goal is to use it as a terrace on wich we'll have our greenhouse, a spa, table chairs ETC ...
it is also 40ft above ground, so the view and tranquility is astonishing

my floors are all steel decks on steel joists, including the roof

i just installed EPS with drainage forms on the roof for this winter

i haven't decided yet on how i am going to waterproof the terrace,

but i am doing reserach on concrete finishes and it seems very promising
VS single ply membrane

there is a company in the UK called RoofKrete wich finished flat roof with only 8mm of concrete
and i've read a few stories that it holds up pretty good
i would probably go with something like 1.5-2" instead of the flimsy 1/4" they are using
and 10-20% metal content + fibers + latex hydrophobic admixture ...

what do you think ?


all in all, the cost of a regular roof would be exorbitant on my 55 by 45 house,
and the building code don't permit for such high building on my street ( 37actual ft + roof )
so i still have $$$ left to invest in a good waterproofing setup