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  1. #1
    Join Date
    Jan 2012
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    United States
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    32

    Default looking at a house with a flat roof... what do I need to know?

    Hello inspectors... I'm looking at buying a 1961, 2 story, ~2400 SqFt single family house. It was built (for unknown reasons as this is Oregon) with a flat roof (mid-century modern "mansard" roof similar to this one: http://3.bp.blogspot.com/-IzXX-qF3KW...at+mansard.png).

    For obvious reasons (rain), this is not a common style in Oregon and I'm pretty unfamiliar with the concept. However, the previous owners have been there since 1971 and have apparently had no issues until very recently, and those issues are being repaired.

    So I'm here looking for advice. Is this a terrible idea and the previous owners got lucky, or is this no big deal? Are they expensive to replace/maintain as compared to standard comp shingles? What about typical lifespan? Will I have a hard time finding a roofer willing to work on it?

    Thanks for any advice.

    NHIE Practice Exam

  2. #2
    Join Date
    Feb 2009
    Location
    Southern Vancouver Island
    Posts
    4,549

    Default Re: looking at a house with a flat roof... what do I need to know?

    If that roof is original, built up roofing such as tar and gravel and the sellers have had it patched, you probably don't want to buy that house. Built up roofing is layers of tar paper 2 or 3 layers, and mopped-on hot tar. Gravel is usually laid on top of that to protect the tar. Sometimes the tar layer is painted with a reflective paint. The roof would now be at the end of its service life. There are limits to how many times a new layer can be added, and a topping of gravel makes it a difficult expensive operation.

    A better roof is a torched-on roof membrane, modified bitumen. Mod bit can be patched or re-covered fairly easily and it makes a good tight roof.

    The other issue is ponding. The original drain or drains will be in at a supporting wall. The roof sheathing sags between the walls, so there you have a pond or an ice rink.

    Lastly, you need to be able to access that roof twice a year to clean the drains, expecially if there are trees around. The flared 2nd storey section means your ladder has to follow the slope of that Mansard section. Too much flare and the feet of your ladder want to slide out.

    I am no fan of that style but have clients that bought them and survived.

    John Kogel, RHI, BC HI Lic #47455
    www.allsafehome.ca

  3. #3
    Join Date
    Mar 2007
    Location
    Orlando, FL
    Posts
    1,340

    Default Re: looking at a house with a flat roof... what do I need to know?

    Quote Originally Posted by thaddeus cox View Post
    However, the previous owners have been there since 1971 and have apparently had no issues until very recently, and those issues are being repaired.

    ... Will I have a hard time finding a roofer willing to work on it?

    What exactly are they repairing? Are they re-roofing? Scraping off old tar & gravel is a mess, may need new sheathing. Check the accessible portions of the attic carefully, I bet there are plenty of past leaks. especially around penetrations, pitch pockets, etc.

    BUR roofs are difficult to patch. Mod Bit is easier to repair, but has a shorter life span compared to a comparable shingle roof. Maintenance is important, and random roof leaks are common with this style; ponding or slow drainage will encourage water to "find" a way inside (loose seams, small defects, etc.)

    Your market likely has many commercial buildings that use flat roofs, so finding a roofer shouldn't be that hard. Call a few roofers and ask them, you'll know in 20 minutes if this is an impossible to repair roof system for your market. Most residential roofers know very little about repairing flat roofs, and incorrectly applied patches are common in my area.

    Dom.


  4. #4
    Join Date
    Jan 2012
    Location
    United States
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    32

    Default Re: looking at a house with a flat roof... what do I need to know?

    Quote Originally Posted by Dom D'Agostino View Post
    What exactly are they repairing? Are they re-roofing?
    Thanks for the helpful replies guys. At this point I don't know what kind of roof it is, and this gives me some background to start asking questions.

    I know they've had it re-roofed at least once. The root of the problem is a set of hot-water solar panels that have been up there for decades, and apparently during the last re-roof they chose to "work around" them, which lead to the leak. They are removing the panels and repairing some dry rot/mold. Not sure whether it will be a patch or a complete re-roof but I will find out.

    One thing to note, there is apparently *no* attic, so no access to inspect for past leaks.

    Dom, you mentioned maintenance being important - other than keeping the drains clear, what else is required?

    So identifying mod bit vs BUR - BUR has gravel, mod bit does not?

    Thanks again!


  5. #5
    Join Date
    Feb 2009
    Location
    Southern Vancouver Island
    Posts
    4,549

    Default Re: looking at a house with a flat roof... what do I need to know?

    Quote Originally Posted by thaddeus cox View Post
    I know they've had it re-roofed at least once. The root of the problem is a set of hot-water solar panels that have been up there for decades, and apparently during the last re-roof they chose to "work around" them, which lead to the leak. They are removing the panels and repairing some dry rot/mold. Not sure whether it will be a patch or a complete re-roof but I will find out.
    Almost certainly a patch of the affected area only. If the roof is getting replaced the owners would be braying this out loud and clear.


    One thing to note, there is apparently *no* attic, so no access to inspect for past leaks.
    Sometimes there will be a dropped ceiling so panels can be lifted to view the ceiling joists which double as roof support. You are looking for moisture stains and also how much insulation. In 1970, thin fiberglass batts are likely, about 3" and no more than 6". IMO.


    So identifying mod bit vs BUR - BUR has gravel, mod bit does not?
    That is generally correct, but a BUR could be minus gravel, and a torched-on membrane could have gravel.
    Not likely to see gravel on a Torched-on roof on that house, tho. It is done on larger buildings.

    IMO I would hope to see torched-on mod bit. Ponds can be drained with small pumps. Done that way in the UK, unheard of here.

    You are hiring a home inspector anyway, so ask if he can do a one hour roof inspection in advance of the full inspection later. Just an idea.
    Ask the owner if you can put a ladder up. Or the pole camera, a paint pole out the window. Finally there is the drone which picks you up and deposits you on the roof with a camera.

    John Kogel, RHI, BC HI Lic #47455
    www.allsafehome.ca

  6. #6
    Join Date
    Aug 2011
    Location
    Bennett (Denver metro), Colorado
    Posts
    1,394

    Default Re: looking at a house with a flat roof... what do I need to know?

    Quote Originally Posted by thaddeus cox View Post
    They are removing the panels and repairing some dry rot/mold. Not sure whether it will be a patch or a complete re-roof but I will find out.
    Wood rot in inaccessible rafter channels is a red flag waiving in a strong wind. It's your dad saying "Son. Stop and think carefully about what you're doing."

    Flat roofs can be fine, but poorly maintained ones can have all kinds of hidden problems. IR camera and moisture meter may not tell the whole story. I've seen dried out rotted planking under flat roofs that appear normal on IR and with the moisture meter. Occasionally, someone has decked over the rotted wood, so the roof feels solid under foot.

    If you choose not to decide, you still have made a choice.

  7. #7
    Join Date
    Mar 2007
    Location
    Lake Barrington, IL
    Posts
    1,363

    Default Re: looking at a house with a flat roof... what do I need to know?

    Flat roofs need significant attention to detail and with solar panels you've now got more penetrations through the roof surface to keep an eye on. Since there's no attic to check things out from time to time, it can be difficult to know if a problem is developing and then heading it off before it becomes a big problem.

    Eric Barker, ACI
    Lake Barrington, IL

  8. #8
    Join Date
    Aug 2011
    Location
    Ottawa
    Posts
    7

    Default Re: looking at a house with a flat roof... what do I need to know?

    Looking at the picture you offered inspired me to offer a few thoughts. "Flat" roofs are rarely flat and when they are flat they are usually sloped. The roofs that are not flat tend to be concave with a drain in the middle. I inspected a bungalow like this a couple of weeks ago and floating pine needles had accumulated around the grate that was over the drain and frozen solid. The next rain or snow melt will run into this ice damn and the roof will pond. The potential problem is that the weight of the accumulated water will put pressure on any openings and make small leaks much larger.

    The flat but sloped roofs drain to the lower edge. If there is no downspout collecting the runoff it can run down the exterior wall looking for openings and potentially collecting around the foundation. In the picture you showed it appears that the garage drains towards the main section of the building. Not ideal. Where the garage roof joins the main structure must have proper flashing and be sealed to avoid water damage.

    Flat roofs work great until they don't. They are harder to troubleshoot leaks because the actual penetration can be far from where the water ends up and access to the sheathing can be limited. So back to the original question. Check where the water flows for possible points of entry. Designs which shed water to the outside edges are preferable. Keep the drains clear. Inspect all flashings. And as always, make sure the grade runs away from the foundation.

    homeXam.ca - Home inspections in Ottawa, Canada

  9. #9
    Join Date
    Dec 2008
    Location
    Maryland
    Posts
    2,778

    Default Re: looking at a house with a flat roof... what do I need to know?

    Quote Originally Posted by thaddeus cox View Post
    H........ previous owners have been there since 1971 and have apparently had no issues until very recently, and those issues are being repaired. .....
    .
    You hit on the problem with flat roofs. You don't know that there is a problem until you have one. Typically owners with flat roofs do not inspect the roofs and they are lax on their maintenance. They typically ignore them till there is a problem. Then typically there is deck that has to be replaced and interior damage also to be repaired which is what lead to looking at the roof.

    Flat roofs (residential) are cheaper to build but they will cost you more in the long run than a pitched roof. In Baltimore, Maryland we have them all through the city. My personal opinion is to stay away from them if possible, ownership I mean. Just not worth the hassles involved.


  10. #10
    Join Date
    Mar 2007
    Location
    New Mexico
    Posts
    1,258

    Default Re: looking at a house with a flat roof... what do I need to know?

    [QUOTE=Garry Sorrells;253477] Typically owners with flat roofs do not inspect the roofs and they are lax on their maintenance. They typically ignore them till there is a problem. Then typically there is deck that has to be replaced and interior damage also to be repaired which is what lead to looking at the roof.

    That pretty much sums up 95% of owners and roofs, whether they be flat or pitched.

    Jim Robinson
    New Mexico, USA

  11. #11
    Join Date
    Dec 2008
    Location
    Maryland
    Posts
    2,778

    Default Re: looking at a house with a flat roof... what do I need to know?

    [QUOTE=Jim Robinson;253481]
    Quote Originally Posted by Garry Sorrells View Post
    Typically owners with flat roofs do not inspect the roofs and they are lax on their maintenance. They typically ignore them till there is a problem. Then typically there is deck that has to be replaced and interior damage also to be repaired which is what lead to looking at the roof.

    That pretty much sums up 95% of owners and roofs, whether they be flat or pitched.
    But with a pitched roof you can see it from the ground,
    you often can see them from the underside,
    you can observe(from ground) if there is early deterioration or something unusual going on.
    You c an see them with green eggs and ham.
    Also, pitched roofs are covered with materials that typically have 3+ times the life expectancy of a flat roof material. So typically by doing nothing you have 3 times the length of time before you are in trouble.

    Biggest is "out of site, out of mind".


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