InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.
Flat & low slope moisture problems:
This article discusses design details to avoid moisture and condensation problems under flat and low-slope roofs. Our page top photo shows the view into the cavity of a wood-framed low-slope roof covering a building that we (DJF) inspected for mold contamination sources.
Some of the fiberglass insulation kraft paper was visibly moldy; leaks over the life of the building had repeatedly wet the roof/ceiling cavity of the "cock loft" - a space between the under-side of the roof deck and separate ceiling framing below. Lab tests showed that the insulation itself had become quite moldy - a potential problem for the building occupants.
The accompanying text is reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.
This article, "Beware the Flat Roof, it calls up a whole new breed of moisture problems" explains how to avoid moisture and condensation problems under flat-roofed and low-slope roofed buildings.
The text below paraphrases, quotes-from, updates, and comments an original article original article "Beware the Flat Roof" (see links just above) by Steven Bliss.
It's a fact that flat and low-slope roofs demand careful detailing and good workmanship.
While a flat or low-slope roof can offer a long service life, 20 years or more, a small mistake can lead to a big leak. Flat roofs and low slope roofs also face potentially serious condensation problems that can in turn lead to costly rot or mold damage in buildings.
Our photograph at left shows severe alligatoring on a nearly-flat "low slope" roof that also was relying on tar and roof cement to try to stop parapet wall leaks.
The most common flat and low slope roof leaks occur at flashings and roof penetrations such as at plumbing vents, chimneys, and roof-mounted air conditioners or heat pumps.
Very common also are leaks at parapet wall flashing and parapet wall caps. Roofing industry spokesmen say that up to 90 percent of flat and low slope roof leaks occur because of poor detailing, poor workmanship, or abuse by other tradesmen working on the roof.
Roof flashing details that are not designed to absorb thermal or other building movement ( THERMAL EXPANSION of MATERIALS for a table of the coefficient of expansion of common building materials including brick, concrete, mortar, and stone) can lead to cracked broken metal flashings that leak badly into the building.
Other flat and low slope roof leak problems are caused by lack of expansion joints or counterflashing where needed.
Relying on sealants and caulks at joints and flashing terminations on any roof, flat or sloped, is asking for trouble.
[Click to enlarge any image]
Another common source of flat and low-slope roof leaks is ponding (standing water more than 24 hours after rainfall, also see our ponding roof photo) because areas of the roof lack sufficient slope to drain. "Flat roofs" should never be built dead level.
Sketch at left showing common flat roof leak points is provided compliments of Carson Dunlop.
While a well-installed flat or low slope roof can keep outside rain or snow-melt out of the building, water entering the roof cavity from inside the building in the form of water vapor can be more troublesome.
For example, moisture collecting as condensation in fiberglass roof insulation may leave the insulation with serious mold contamination even though the insulation still looks "clean".
See FIBERGLASS INSULATION MOLD. Under a flat or low-slope roof, the usual rules about roof insulation and ventilation don't apply.
This article explains methods for avoiding moisture condensation problems in compact insulated roofs that have no roof cavity space, and in steel or wood framed roofs that have a roof cavity space and that usually include insulation within the cavity space. A third flat roof insulation design approach, Inverted roof membrane systems place the roof insulation on top of, rather than below the roof membrane; these roofs have similar moisture condensation performance as the compact insulated roofs discussed just below.
The first roof insulation method (sketch at left) has the roof deck, rigid solid foam insulation, and the roof membrane all sandwiched tightly together in a compact "hot roof" system.
These roofs have few inter-material condensation problems for two reasons: first there is little air movement within the roof system to pump moisture-laden air into the roof; and second, the roof deck and rigid insulation form a reasonable interior vapor barrier.
Often in the tight sandwiched insulated roof design, the insulation is installed on top of the structural roof deck (typically wood, corrugated steel, or poured concrete), and the roof membrane is secured through the insulation to the roof deck itself.
Fear of condensation problems has led some roofers to add special breather vents to these compact roofs. Although breather vents are recommended by the National Roofing Contractors Association (NRCA) - one vent every 1000 square feet is specified - NRCA technical manager Wayne Tobiasson, who has studied flat roofs extensively for the U.S. Army Corps of Engineers Cold Regions Research and Engineering Laboratory (CRREL) [ca 1985 and prior], goes further and says that vents are "foolishness," particularly in roofs without vapor retarders.
In these roofs, Tobiasson said, if the vents do anything, they will create problems by inducing airflow up through the ceiling from below.
What about the rooftop vents? There are two types. One-way vents only let air out under pressure, but won't let air enter the roof cavity or space. These were developed originally to cure roof membrane blistering, which was common in built-up roof membrane roofs before the advent of glass felts.
The roof blistering, however, has since been linked to voids left between the roofing layers during the roof installation process. These roof blisters are not related to moisture trapped within the roof insulation - the space that these roof vents are theoretically designed to ventilate. The solution to roof blisters seems to lie in improved roofing materials.
Sketch at left showing how roof blisters occur in built-up roofing membranes is provided compliments of Carson Dunlop.
NOTE-DJF: Roof membrane blisters are seen, for sure, on some membrane roofs into which water has leaked to enter between membranes and insulation.
On the question of vapor retarders, Tobiasson said that roofs with non-permeable insulation tightly sandwiched between the deck and roofing are usually free of condensation problems except in the far north or in buildings with high moisture levels.
OPINION-DJF: However even a compact-roof with good indoor vapor barrier design can suffer from under-roof moisture condensation, that is, condensation under the roof inside the occupied space, if the building interior moisture levels are excessive and proper ventilation or dehumidification are not provided. We have seen that interior condensation problem above suspended ceilings below roofs that did not have a particularly high R-value, for example. Indoor moisture contacts the cool under-side of the concrete or metal roof decking where it condenses.
see MOISTURE CONTROL in BUILDINGS for approaches to avoid excessive indoor moisture.
In roofs with vapor retarders, Tobiasson conceded that the two-way vents may have a role to play in avoiding the creation of a vapor trap between the roofing membrane and the vapor retarder. Even in these, however, he thought that the vents are unnecessary and may do more harm than good since they penetrate the roof surface - making potential roof leaks.
An airspace is left above the ceiling insulation and below the under-side of the roof decking, and the roof is vented either around its perimeter with soffit vents (a "flat" roof) or the roof may be include intake-venting at its lower-edge through a soffit and outlet venting through a half-ridge vent or similar outlet vent along the roof's uppermost edge (a low-sloped roof).
The sketch at left shows a method for providing effective ventilation beneath a flat or low-slope building roof, using 2x4 strapping to assure that there is an airspace between the insulation and the roof deck underside.
Not shown are air inlet and outlet openings to assure that this vent provision is effective. Similar to our illustration and note at the top of this page, this flat roof ventilation design also avoids moisture condensation problems between the building material layers. However even that building design can suffer from under-roof moisture condensation if the building interior moisture levels are excessive and proper ventilation or dehumidification are not provided.
The problem with "flat" roofs is that there is no chimney effect, or in a very-low-slope roof, there may be an inadequate chimney effect, to drive outside air through the vented space. On flat roofs with soffit vents, the only mechanism that might drive air thorough the vented space would be occasional wind conditions that happen to blow air against one side of the building and up through the soffit vents, across the roof, to outlet on the opposite side - a rather speculative roof venting system you'll probably agree.
Of all roofs, the framed, insulated, and poorly-vented roof is the most prone to roof-cavity and in-insulation moisture problems. Anything that can promote air movement inside the roof cavity can help reduce this moisture trap.
One approach to venting flat framed cavity roofs that was developed in Canada is to create a full roof plenum, sometimes 2 to 3 feet high above the ceiling insulation. We have seen this roof design in many New York City buildings where the space is often called a "cock loft" and where it may actually be passable as a crawl area. This plenum area is then vented, aided by a vent fan or by one or even a series of cupolas or metal roof vent towers.
A more moderate roof venting approach for the flat and low-slope roof cavity design that we have seen used successfully is shown in the sketch above: 2x4's are run across the tops of the roof rafters (the rafters are also the ceiling joists in this building design). The rafters are placed 16" on center across (at right angles to) the rafters (ceiling joists) and below the roof sheathing. This provides a 1 1/2" high air space above the rafters, permitting air to flow along the under-side of the roof decking.
For this design to work well on a low-sloped, not dead-flat roof, an outside air inlet is provided by a soffit or roof overhang built at the low end of the roof, and a roof cavity air vent outlet is provided along the high or up-slope end of the roof using a built-up half-ridge vent or, where the roof construction provides a parapet wall or even a cosmetic "gabled roof" on the very front end of the building (something added by the designer for cosmetic reasons), that space can provide an ideal vent air outlet path provided you make sure that the roof space over the building has an open air path into and through that taller component to the outside.
The real key to avoiding moisture and condensation problems in low slope and flat roofs, though, is to keep moisture out of the ceiling in the first place.
Here we include solar energy, solar heating, solar hot water, and related building energy efficiency improvement articles reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.
Our photo (below left) shows a small clue traced to a roof leak and insect damage in a New York home. This example of subtle leak signs that lead to moisture troubles in low slope roofs is an example but is not the particular home discussed just below.
I have a duplex which faces North & South the roof on the west has no real visible problems of moisture. This roof has 2 wind turbines approximately 1/3 of the way up from the soffits and in about 15 feet in from the gable ends dividing the ventilation of the roof into roughly 3 equal areas. there is also one gooseneck vent near the high side or (ridge). the side of the roof gets any prevailing wind & sun during the winter months.
Now the other side of the roof is vented the same way but is a totally different matter the freeze thaw cycle deposited ice and water along the eave side of the roof up to three feet wide.
There was so much moisture in between the vapor barrier and the roof sheathing that water was acutely running out of the electrical boxes in the walls as well as extensive staining around ceiling junction boxes etc. The entire ceiling was remove there was so much water in the insulation that the vapor barrier could not carry the weight.
A new torch roofing membrane was installed along with new insulation R20 and a new 6mil vapour barrier & new drywall. The discharge line for the bathroom fan had come loose and was venting directly into the airspace this was re affixed and three more 10 inch by 3 inch goose necks installed at the high side of this side of the roof to increase air flow. It was believed the problem had been resolved. Alas this was not the case as this spring with the freeze thaw cycle the issues reoccurred although not as extensive. What can we do to permanently resolve the moisture issue???
A competent onsite inspection by an expert usually finds additional clues that help accurately diagnose a problem, and in this case that might have to include looking into the roof cavity for degree of water or even mold.
The roof leak indicator shown just above resulted in the little leak into the wall cavity shown in our EPDM roof leak photo at below left. This wall cavity leak was not visible from inside the building until we removed the drywall in this area.
Our second photo (below right) was visible when we peeled back the EPDM roof, roof insulating board, and edge flashing to reveal the wall top: carpenter ants were having a big party in the roof structure. Inside the building below this roof we found carpenter ant activity attacking about 15 feet of this wall, all attracted by this little leak. The ants didn't have to go downstairs for water.
. That said, here are some things to consider:
Continue reading at VENTILATION, ROOF SPECIFICATIONS - home, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
Or see LOW SLOPE ROOFING - home
Or see ROOF LEAK SOURCE DIAGNOSIS
Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia
Try the search box below or CONTACT US by email if you cannot find the answer you need at InspectApedia.
Questions & answers about moisture problems in flat or low slope roofs, posted originally at FLAT ROOF MOISTURE & CONDENSATION
On 2017-06-06 by (mod) re: how to stop roof vents that leak in high wind + rain, hurricanes, windstorms
Indeed heavy wind and rain can cause water entry at a roof vent if the vent flashing designed to prevent that is missing, bent-down, or otherwise damaged or inadequate in height for your climate. You don't give your location, but if you are in a coastal area you may indeed need a different ridge vent.
At RIDGE VENT LEAK PREVENTION we give a detailed answer to your question.
On 2017-06-06 by Yolanda Ferrandi
2017/06/06 Yolanda Ferrandi said:
We have a low sloped roof over our carolina room which has a raised vent at the top of the roof and ther roof slopes down from the vent at both sides of the vent. Rain does not come in when we have heavy rain, however, with strong winds with last october's hurricane, the tape at the seam of our sheetrock ceiling in the carolina room opened, but still no water came into the room.
We've had heavy rain several times over the past month, but no water enters from the ceiling.
I believe the raised vent at the top middle of roof over carolina room has a problem only when we have a hurricane or very heavy winds, m before we have the ceiling tape re-sealed and the ceiling repainted, I need to have the raised vent
On 2017-04-29 by Geoffrey
My math was off.
The 1" holes provide roughly 3.75sq in venting per joist channel (on each end of the flat), approx 90sq in total (on each side of the flat)
Thanks so much!
Yes, 6 1" holes were the max I could cut in the rim joist. But to be clear, that's 6 1" holes in each joist channel. That's about 10.5sq in in each channel, and 254sq in -- on either side of the roof.
Unfortunately, no one makes a product that will allow me to cut the roof vent holes at the very top of the pitch. By and large, the best I can do is cut the vent holes (and install product accordingly) about 18" below the top of the pitch.
I feel good about the soffit vents and off-ridge vents for the pitch. But combining that with the flat has proven to be the challenge.
I suppose when it comes down to it, will this be better than it was, which was fiberglass packed in the rafters and joists with no air gap or venting at all?
On 2017-04-28 by (mod) re: amount of venting needed under roof versus vent opening size
The drawing helps alot.
Assuming your exit vents are at the top of the sloped area or just inside the start of the flat area, even with 14 feet, I suspect that vents anywhere in that uppermost flat roof will show some exiting air because of the chimney effect of the sloped roof planes on either end of it - as those are warmed by sun and air wants to move up, IF there is enough air passage from sloped into flat area from the sloped sides (which is questionable) then the flat ought to vent. If an examination of the space found it was not venting may be some central turbine vents would help.
But in truth, some objective data by inspection or temperature measurement or airflow measurement would be helpful. Clearly the area to inspect and monitor is the central flat section.
What is a 6x1" drilled hole? 6 1" diameter holes won't move an appreciable amount of air.
Ultimately if you can't vent the roof AND if you see moisture problems there you may have to go to hot roof design for that section.
On 2017-04-28 by Geoffrey
Thanks a ton for the reply. I should've thought to include a diagram. This makes tons of sense to me, but I'm thinking about it every day.
I did look at hot roof design and this seems to be a mix and I'm trying to figure out the final steps
Here's a (crude) diagram:
Soffit vents installed
1.5" air channel between fiberglass and decking in pitch and flat
holes are drilled through flat ceiling rim joist to let flat sections "breathe" into rafter channels with off-ridge vents
**off ridge vents planned (the max height will be bout 18" below top of pitch)
Are there concerns that the off-ridge vents will act like baffles blowing air across the flat and interrupting the chimney effect with the soffit vents?
Are there concerns that an air channel above the insulation in either the pitch or flat will have unwanted condensation? The majority of AC ducts are below the fiberglass.
Thanks a ton
On 2017-04-28 by (mod) re: venting a low slope roof
If your design, which I can't quite follow, moves air in along the whole house eaves and out along the whole ridge, between all rafter pairs, that's good venting. That flat area "in between" is unclear to me; but sure flat roofs are very difficult to vent effectively - which could be an argument for going to an insulated, hot roof design - search InspectApedia for HOT ROOF DESIGN to see opinions and methods.
On 2017-04-28 by Geoffrey
I would like to add that I am averse to spraying foam in due to the cost, and the fact that closed-cell won't reveal leaks until it's potentially dangerous. With such an old structure, I am concerned about not seeing leaks.
I have a 1924 wood-frame home in Memphis, TN. The roof is approx. 12/12 pitch on either side with a 14' flat span in between. Where the rafters meet the ceiling joists is a rim joist.
In the late 50's, the attic was converted into an apartment. In doing so, they blocked the gable vent with a crawl space. Furthermore, they packed insulation in the rafters and ceiling joists covered with drywall. From then on, there was NO venting whatsoever in the roof.
I had to remove a large portion of the ceiling for repairs when I discovered this issue. I've since removed the entire ceiling and set off to fix this the best I could.
Keep in mind: this is a retrofit I am hoping to improve the best I can.
For all intents and purposes, we can ignore the knee walls for simplicity, and treat this question/issue like a 12/12 cathedral ceiling that meets a flat for 14' span, then 12/12 down the other side. (there are crawl spaces behind the knee walls). I have also furred down all the rafters and joists to allow for r21 and r30c, in addition to a 1.5" air channel above the insulation and below the decking. In the future a re-roof of the flat section may be an option, or adding a crawl space for venting, but right now it is not. The flat section is asphalt roll.
I installed soffit vents around the house, then ran rafter vents up to the flat section. With advice from an engineer, I drilled 6x 1" holes through the ceiling rim joist, allowing air to escape from the flat section into the pitch rafter channels.
My intent at this point is to install off-ridge vents as close to the top of the pitch as possible on either side of the flat section. The vent hole int he roof will likely be 12-20" down from the "ridge" depending on the product. This is less than ideal, since the holes at the end of each joist channel won't be able to "see" each other, but it's the best I can do.
The chimney effect *should* bring air up through the soffit vents and out the off-ridge vents. However, the off-ridge vents will function as intake vents when wind blows against them, and could ultimately force air down the rafters. I realize this is a complicated problem, and difficult to determine a guaranteed solution.
All that being said, is there any reason I shouldn't proceed with what I have to work with at this time?
What are the dangers of the venting process I have described. Keep in mind, as a retrofit for a 1924 home with a 1950s attic conversion, air-sealing the ceiling and roof as-is would be nearly impossible given my resources.
On 2016-12-31 by (mod) re: how much foam should I add on top of existing roof?
It is a common practice in commercial roofing jobs to install a layer of solid insulation bonded to the roof deck over which a new layer of flat roofing is installed.
The choice of solid insulation (thickness, type of foam, brand) depends on where you live, the climate, and thus the heating and cooling loads. Your roofer will also consider the roof covering product to be installed and what insulation boards the manufacturer recommends for use with their product.
For example some materials may be incompatible due to the bonding adhesives or solvents used to adhere the roof membrane. So sorry, but from a brief e-text nobody with any sense would pretend she could give a sure-fire "right" answer.
You might also want to take a look at http://inspectapedia.com/ventilation/Un_Vented_Roofs.php on hot roof designs.
On 2016-12-31 by Tony O'Donnell
I have a house with 75% flat roof and 25% cathedral ceiling. . The house was cosmetically finished by the previous owner, nothing appears to have been done regarding insulation. I am interested in the concept of installing insulation on top of the flat roof, the finishing of the roof appears to be in good condition. Would I be correct in having ridgid foam insulation on top of the existing roofing finish? What thickness of foam insulation should be used? What finish should be used?
On 2016-01-12 by Anonymous
I don't see any signs that the roof is leaking. It seems to me that humid air in between the tin is condensing and leaking down the walls. There is about 3-4 inches of sprayed foam insulation under the top sheet of tin. I also don't think there is a vapor barrier in the ceiling.
On 2016-01-09 by (mod) re: what to do about water leaking down walls
First let's figure out where the water is originating: is this a roof leak, melting snow from an ice dam leaking in at a plumbing vent, or some other external problem, or is this condensation from inside the building? If the latter look at the source of moisture indoors and look at the amount of insulation under the roof as well as the vapor barrier condition.
On 2016-01-08 by Tom
I have a tin roof on a heated shed that has a very low slope. The underside has sprayed insulation on it and then there is a tin ceiling below the insulation. There is no ventilation and in the winter I have a ton of water leaking down the walls. Can I ventilate the ceiling from the inside by cutting a hole and mounting a fan on one end of the ceiling and putting a vent on the other end?
On 2016-01-04 by (mod) re: can ice dams happen on flat and low slope roofs
Absolutely, George, ice can form on any roof shape, size, or slope depending on where there is heat loss, a drain backup, ponding, or a roof drainage obstruction.
On 2016-01-04 by George
I have read that ice damming does not happen on flat or low slope commercial roof. Then I have read that if the drain or drains freeze over on theses types of roofs ice damming can happen.
Can anyone tell me if ice damming can occur on flat or low slope roof?
On 2016-01-04 by (mod) re: how long should water stand on a flat roof after rain stops?
If there is a pond on a roof surface 24 hours after the last rainfall most people will agree that constitutes "ponding" that should be addressed.
On 2015-10-01 y ronnie
What is the industry standard for evaporation of water on flat roofs ?
Questions & answers or comments about the cause, cure & prevention of moisture problems in flat roofs and similarly hard-to-ventilate structures. .
Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.
Search the InspectApedia website