Problems with a Residential Flat Roof

[Pete Sol]

I was called in to take a look at flat roof. There are a lot of things wrong with this roof. It goes beyond the limits of incompetence or inebriation, probably a grudge or something. The company that did this roof is well established in my area.

Most of the problems I can deal with and provide solutions for. There is one that I'm not so sure about though.

One of the flat roof areas (it is it's own roof, on op of a small penthouse 20'X20') has the wrong slope. The precipitation drain is the highest point on the roof, by as much as 3" in some places. As you can well imagine, there is considerable puddling.

My question is, does this roof need to be ripped off and redone completely? It was only done last year and the people did not get a good settlement claim. Are there any other options; such as some way of putting another layer of roof over the existing one to correct the slope and therefore drainage issues? The roof is made of hard insulation, covered with large sheets of what appear to be asphalt tile.

I can provide more details if that can help. There are also ventilation issues with this roof right now. And I'm not sure yet if the insulation was done to code.

Thank you

[kurt]

Post some pics if you can.

That said, it sounds like there is no way to fix this roof without tearing off materials and starting over.

You may have to relocate the drain; non-draining roofing (with a few material exceptions) can't have ponding water. Correcting a 3" negative pitch doesn't sound possible; moving the drain may also present it's own huge problems.

Part of this is wondering what the materials are....(?) Asphalt tiles on a flat roof sounds completely goofy.

So, take a few pics and put them up so we can know what you're talking about.

[Pete Sol]

First, thank you for the reply Kurt.

I can post some photos of the roof in question in about a week, when I go on there to fix the Gooseneck HVAC vents (right now they are sealed with aluminium muffler tape, that is not a typo) and put covers on the stack vents (precipitation is going right in there now).

If re-leveling the roof is not an option, then relocating the plumbing drain is not going to be an option either based on cost-vs-results. I'd rather recommend a new roof all together.

The surface material is not asphalt shingle. What I meant before is that it is a bituminous material; large sheets, spread over the roof, with what looks like a tar between joints. I'm posting below some photos that I found online that look the same.

Thanks

[kurt]

That's modified bitumen. It's not "tar" between the laps; the material is torched in place with high heat; the goo is melted material.

Replacing the roof without having it drain is spinning your wheels. You have a mildly complicated repair involving re-pitching the roof.

Without a pic, it's hard to know what to tell you.

[hausdok]

The roof is made of hard insulation, covered with large sheets of what appear to be asphalt tile.

Roofing supply companies make tapered insulation sheets. If the parapet is high enough, I should think you'd be able to go over the existing roof with a layer of foam sheathing that's draining in the right direction and cover it.

ONE TEAM - ONE FIGHT!!!

Mike

[caryseidner]

The roof is made of hard insulation, covered with large sheets of what appear to be asphalt tile.

Roofing supply companies make tapered insulation sheets. If the parapet is high enough, I should think you'd be able to go over the existing roof with a layer of foam sheathing that's draining in the right direction and cover it.

ONE TEAM - ONE FIGHT!!!

Mike

They do sell tapered insulation, but a tapered insulation system is very difficult to engineer properly. Many things to take into account, but the 2 biggest factors are the slope of the existing roof and the installers ability to follow the installation instructions and diagram. That being said, they do work when properly done.

I would never recommend installing a tapered system over an exiting roof. If there is no slope or negative slope, the better solution is to replace the roof with a roof covering that is designed for that. CTP (Coal Tar Pitch) and EPDM both can be installed over a structure that has no/negative slope. Mod Bit, as identified in you sample photos, will not remain watertight. It needs positive slope towards a drainage point.

The specifications are a little different for either of those systems when they are installed over a no/negative sloped substrate, and as home inspectors we can never see if those have been followed.

The bigger problem is finding a good roofer to install either of these systems in a residential application.

[kurt]

Yep. That's right.

Although, I'd still hate an EPDM with a big pond; I don't care if it's approved for ponding, or not.

I've been involved in a few tapered foam nightmares (well, not me, I just got to see it after it was bolloxed).

Fine for minor alterations, a problem for major problems. I'd put 3" differential in the major problem category.

[caryseidner]

Fine for minor alterations, a problem for major problems. I'd put 3" differential in the major problem category.

Absolutely, and on second thought, I suspect this roof was not framed to slope the water as described. I'll bet there are some layers of "things" (more roofing, tapered insulation boards, something else) beneath the top layer causing this. Tear it off. If there are indeed other layers, there might be a load issue.

[Pete Sol]

Hi guys (gals),

I really appreciate your insights into this. Just for clarification, the precipitation/plumbing drain is approximately located in the center of this roof. So in essence, somehow the center ended up being the highest point on the roof.

I do think it's modified bitumen, I am not a roofer though. Although, one look at the material and I was fairly certain it could not stand up as a barrier to water with this amount of ponding and would surely one day fail prematurely.

Other info: If I hadn't specified already, the building is not new, just the roof. Therefore, the rafters are the same as before and this problem wasn't. It would also be rather unbelievable that somehow the ply-wood was sloped wrong. Reading your posts has aided me to diagnose what I do think the problem is. The layer of angled sheets, probably was installed backwards. With the thickest portion near the center drain and the thinnest towards the parapet.

I wanted to know if I could contact a roofer in my area and reasonably ask him about a re-slope (yes, the parapet is high enough, it's about 14") for this degree of correction. In my experience as a builder, sometimes just because someone agrees to do a job a certain way, doesn't mean they should. Maybe a roofer will say "fine, yeah, re-slope no problem" but in fact it is a problem, because of some factor; strength, ability to seal, weathering, ect. I also don't want to be told we can't when we can, because I will need to make a proposal on this work.

I think already and along the way I have always favoured a new roof to re-sloping this one, because of other issues with ventilation. However, when I make my proposal, I inform my clients of all their options and the results they are likely to get with whichever course they chose. It's my job to weigh costs and risks for them, but its their job to decide what course we will take (within reason, I have no issues with walking away from a bad job)

I will mention another roof defect for those interested. The ventilation ratio for a roof such as this is 1/150 nfva. Because, there is no adequate vapour retarder below the attic and the HVAC runs up and through the roof. I can sense there is considerable stack effect going on (when I go over the existing vents I feel warm moist air). Right now there are a total of 3 vertical pipe vents for the attic area of approx 1200sqft.

All the pipe vents are on the same side of the building, I don't see that aiding to circulate air through the attic. I think one solution to this can be to cut boxes in the brick fascia of the attic, on the side opposite the vents and install powered ventilation units. I'll figure out the strength using the ratio. Two or three units should be enough. They will also help reduce stack effect by blowing neutral temp air into the attic. Does this sound reasonable?

Thanks. This is my first post on the forum and I am really enjoying to correspond with your community. I like the atmosphere, I like that you've asked me questions so as to clarify the situation, it's informing and fun.

[Erby]

My suggestion would be to get a real roofer out there to tear it off and redo it right. Patches are just that, patches. Get it done right and enjoy the life of the material.

Incorporate your proper ventilation work while the new roof is being put on.

Do YOU want to own the leaking and failure that's just down the road?

I'd bet not.

-

[randynavarro]

How does one solve the inadequate draining on this roof?

Click to Enlarge

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[Erby]

Install drains (and drain piping in the attic) at the low spots would likely be most economical.

[kurt]

OK........ducts in the "attic", inadequate venting, non-draining roof.

How about recessed lights in the top floor ceiling?

There's a bunch of stuff going on here; now I really don't know what to tell you.

Beware improvised ventilation schemes with mechanical solutions; there's something about this I really don't like.

Pics are the order of the day: I'm curious what we're really talking about.

[Pete Sol]

I can't wait to put up some photos next week.

I'm sold on the new roof. I just had to be completely sold, beyond any shadow of thought that something more economical could be improvised. I don't think now that anything improvised in this case will end up being economical. Not to mention peace of mind.

[plummen]

First, thank you for the reply Kurt.

I can post some photos of the roof in question in about a week, when I go on there to fix the Gooseneck HVAC vents (right now they are sealed with aluminium muffler tape, that is not a typo) and put covers on the stack vents (precipitation is going right in there now).

If re-leveling the roof is not an option, then relocating the plumbing drain is not going to be an option either based on cost-vs-results. I'd rather recommend a new roof all together.

The surface material is not asphalt shingle. What I meant before is that it is a bituminous material; large sheets, spread over the roof, with what looks like a tar between joints. I'm posting below some photos that I found online that look the same.

Thanks

Are talking about covering plumbing vents? [:-bigeyes