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ROOFING INSPECTION & REPAIR
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ATTIC LEAKS, CONDENSATION & MOLD
BEST CONSTRUCTION PRACTICES GUIDE
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BUILDING SAFETY HAZARDS GUIDE
BUILT UP ROOFS
CATHEDRAL CEILING INSULATION
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CHIMNEY INSPECTION & REPAIRS
CHIMNEY FLASHING Mistakes & Leaks
CHOOSING A ROOFING CONTRACTOR
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COLD WEATHER ROOF TROUBLE
COOLING LOAD REDUCTION by ROOF VENTS
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ENERGY SAVINGS in buildings
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EXTRACTIVE BLEEDING on SHINGLES
FELT UNDERLAYMENT REQUIREMENTS
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FIRE RATINGS for ROOF SURFACES
FIRE RETARDANT PLYWOOD
FLASHING on BUILDINGS
FLASHING, ASPHALT SHINGLE VALLEYS
FLASHING, CHIMNEY Mistakes & Leaks
FLASHING, CLAY TILE ROOFS
FLASHING MEMBRANES PEEL & STICK
FLASHING for METAL ROOFS
FLASHING ROOF WALL DETAILS
FLASHING ROOF-WALL SNAFU
FLASHING SIDING DETAILS
FLASHING WALL DETAILS
FLASHING WINDOW DETAILS
FLASHING WOOD ROOF DETAILS
FLAT ROOF MOISTURE & CONDENSATION
GALVANIC SCALE & METAL CORROSION
Green House or Solarium Roof Leaks
GUTTERS & DOWNSPOUTS
HAIL DAMAGED SHINGLES
HEAT TAPES & CABLES on Roofs for Ice Dams
HOT ROOF DESIGNS: Un-Vented Roof Solutions
HOUSEWRAP INSTALLATION DETAILS
HUMIDITY LEVEL TARGET
ICE DAM PREVENTION
INSECT INFESTATION / DAMAGE
INSULATION IDENTIFICATION GUIDE
INSULATION INSPECTION & IMPROVEMENT
LEAD POISONING HAZARDS GUIDE
LEAKY ROOF DIAGNOSIS & REPAIR
LEED GREEN BUILDING CERTIFICATION
LOW SLOPE ROOFING
MASONITE WOODRUF FIBERBOARD ROOFING
MEMBRANE & SINGLE PLY ROOFS
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NOISE CONTROL for ROOFS
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ROLL ROOFING, ASPHALT
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ROOF DORMER TYPES - PHOTO GUIDE
ROOF INSPECTION SAFETY & LIMITS
ROOF JOB PROBLEMS, RESOLVING
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ROOF NOISE TRANSMISSION
ROOF REPLACEMENT SNAFUs
ROOF SLOPE DEFINITIONS
ROOF VENTILATION SPECIFICATIONS
CATHEDRAL CEILING INSULATION
CATHEDRAL CEILING VENTILATION
HOT ROOF DESIGNS: Un-Vented Roof Solutions
Inspect Attics for Moisture or Mold
Inspect Attics for Blocked Soffit Intake Vent
Inspect Basements for Moisture or Mold
Inspect Building Exterior - Roof Venting
Inspect the Ridge Vent System from the Attic
Inspect the Soffit Vent System from the Attic
INSECTS & FOAM INSULATION
HOUSE DOCTOR, how-to be
Insulation Air & Heat Leaks
Roof Venting: Intake - Outlet Area Ratios
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Roof Venting: Soffit Intake Vent-Continuous
Roof Venting: Un-Vented Hot Roof Solutions
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ROOF VENTING NEEDED?
SKYLIGHT VENTILATION DETAILS
ROOFING FELT UNDERLAYMENT REQUIREMENTS
ROOFING MATERIALS, Age, Types
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STAIN & BIODETERIORATION AGENT CATALOG
STAINS on & in BUILDINGS, CAUSES & CURES
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STAIN DIAGNOSIS on ROOFS
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STANDARDS for ROOFING
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STRESS SKIN INSULATED PANELS
TEST LABS - ROOF SHINGLE
Thermal Expansion Cracking of Brick
THERMAL EXPANSION of HOT WATER
THERMAL EXPANSION of MATERIALS
THERMAL IMAGING, THERMOGRAPHY
THERMAL IMAGING MOLD SCANS
THERMAL MASS in BUILDINGS
TREES & SHRUBS, TRIM OFF BUILDING
TRUSS UPLIFT, ROOF
TRUSSES, Floor & Roof
UNDERLAYMENT REQUIREMENTS on ROOFS
VENTILATION in BUILDINGS
WALK-ON ROOF SURFACES
WARRANTIES for ROOF SHINGLES
WALL CONSTRUCTION BARRIER vs CAVITY
WATER ENTRY in buildings
WIND DAMAGE to ROOFS
WINTERIZE A BUILDING
WOOD SHAKE & SHINGLE ROOFING
WORKMANSHIP & ROOF DAMAGE
ZINC METAL ROOFING
Flat & low slope roof moisture & ventilation problems & solutions: this article describes roof structure, insulation, and ventilation design details to avoid moisture and condensation problems under flat and low-slope roofs.
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.
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.
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.
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.
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 HUMIDITY CONTROL & TARGETS INDOORS and 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.
Original article in PDF form:
Frequently Asked Questions (FAQs) about moisture & condensation in flat & low-slope roofs
Question: What's the best way to fix flat and low slope roof moisture problems?
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 reaffixed 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???
We live in Calgary Alberta this winter has been colder, longer and with more snow any thought on this matter would be greatly appreciated - L.M., Calgary Alberta
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:
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