Q&A about what to do about flat & low slope moisture problems.

This article series discusses design details to avoid moisture and condensation problems under flat and low-slope roofs.

Flat Roof Moisture & Condensation Q&A

These questions & answers about causes of, effects of and cures for moisture problems in flat and low slope roofs were posted originally at FLAT ROOF MOISTURE & CONDENSATION FAQs - be sure to review the advice given there.

On 2018-12-29 by (mod) - venting snafus under a flat dormer roof

Sorry but from your narrative I don't understand this construction. It sounds as if there are multiple layers of insulation and building materials - a design that can potentially create a moisture trap.

The add image button will let you include any image of the construction or an image of a drawing but not a document.

Generally "holes" for roof venting are enormously-less-effective than continuous venting inlet and outlet openings, and on a flat or nearly-flat roof such openings will probably do nothing much at all. There are just no air convection currents that move an invertedf "U" shape.

On 2018-12-29 by Mr J Barritt

Hi
Please can you help,

I've have recently just had a flat dormer roof with a good slope constructed and fitted from solid 175mm x 50mm ceiling joists then 2 layers of 50mm king-span ridged foil backed insulation laid on top over lapping each other all with foil taped joints, Then on top a 18mm OSB board laid, then Fiberglass roof laid on top,

Underneath along the ceiling joists about central in depth approximately 40mm circular holes cut out along approximately every 800mm,

Then flush with the ceiling joists a layer of fiber insulation fitted then a breathable membrane stapled to the underside of the ceiling joists and then plaster boards fitted,
on the facias 70mm circular air vents were fitted just below the guttering height and to the center ends of each open joist void and every 600mm apart up along up each outside edge,

does this sound OK as I'm a Delivery man and no roofer?

The job looks really good with no shoddy gaps & solid, and they had finished it really neat,
Kind Regards
Mr J Barritt
i look forwards hearing from you on this

ps
The drawing is in a Microsoft Publisher format
Ah forgot i didn't draw vent holes along the joists in void areas

On 2018-12-03 by (mod) - we have mould and damp coming through on the facia boards

Lets find the moisture source.

Water travels on low slope roofs making leak source tracking a fun challenge

Some suggestions:

Inspect the roof surface for leaks, open seams, flashing errors or leaks or wrong type (holding water on the deck) ponding

Feel for soft areas on roof deck

Use moisture meter and IR ir thermography to map leak patterns

Pull off a fascia board

Attach some photos of your situation

On 2018-12-03 by nick jordan

Our new extension has a wide (750cm) soffit under a flat roof (the interior ceiling sits another 40cms below the soffit level.

There is venting all along one side where the soffit is much narrower. the wide soffit is on two sides.

On the southern facing side we have mould and damp coming through on the facia boards, which is waterproof ply. Why? And what do we do about it. We are anxious as to whether damp is coming through the fibreglass deck over the room proper but just hasn't penetrated the layer of insulation that the ceiling has and the soffit doesn't.

On 2018-02-19 by (mod) - flat roof venting in Edmonton CA

Alexandra

Thank you for an interesting question. I'm hesitant because from your note I'm not sure I fully/correctly understand how the building works, so my answer is a bit general.

I would focus first on points where warm moist air from the interior is escaping into the roof cavity and perhaps soffits.
Seal those meticulously.

In my experience it's practially impossible to vent a flat roof - there's just too little natural air movement by convection or stack effects. So warm moisture-laden air escaping into the roof cavity invites a collection of condensate and risks rot or mold damage. I suspect that particularly in your climate (similar to mine here in northern Minnesota), the frost spots you see outside are telltales of moisture inside the structure.

Adding new soffits over the old hasn't addressed the question of vents in those locations. I'd be sure not to reduce the present ventilation.

On 2018-02-19 by Alexandra

We have a 60 year old flat top in Edmonton. The roof was replaced in 2008 with a 2 ply SBS torch on membrane. The past several winters we have noticed moisture coming through the vents on the soffits. The soffits are original with some new vents put in 3 years ago.

Also 3 years ago we put a shower in the where just a tub had been. We have blocked off the fan as it didn't really vent to the outside.

This year during the crazy cold spells in noticed frost in several spots in the corner where the soffits and the stucco of the house meet. Would we be smart to seal off that joint between soffit and house to direct the moisture escape to the vents?

Should the vents had better air flow? Also we are thinking of putting up new soffits over the old that has many layers of paint on it! Thanks and your website is very informative.

On 2018-01-08 by (mod) -

Darrell

I can't pretend I actually understand the situation from just your note but I'd keep in mind that condensation on windows is a symptom of a moisture or venting problem, not its cause. It's common when blinds are closed to find condensation on windows because of reduced air circulation over the window surface. But when there is excessive indoor condensation we

- look for an excessive indoor moisture source from an activity or water reservoir (plants? cooking?) inside the building

- look for ventilation defects (bath fans not working, for example)

My OPINION is it is almost impossible to effectively vent the entire underside of a nearly-flat roof. I concentrate on finding and fixing moisture sources and, as you may be doing, keeping warm moist air from entering the roof cavity.

On 2018-01-08 by Darrell

We have a flat roof (with a 3degree slope to the north (truss design). The outside roof on the second floor is used as a deck. I have noticed that during the winter time after experiencing 2 weeks of -20-30 degrees and then a warming spell... that the condensation that exists between the truss area builds up

. I can tell this because in the bedroom there has been some water stains on the ceiling. I also tried to silicone the light sockets and the smoke detector. The bedroom has become a problem as there are huge windows that when the blinds are closed build up extreme condensation on the windows in the morning and overnight. I am assuming that the condensation from the bedroom windows and the ensuite bathroom are generating a lot of condensation that ends up in the cavity space above the roof. There is insulation above the gyproc along with a vapor barrier.

As well, the soffit along the outside of the house should be able to vent the dead space between the roof and the bedroom ceiling. However, I don't know what to do during the cold spell, that collects the condensation around the windows and that ends up in the ceiling and then disperses during a warm spell.

I don't know if I need to run the HVAC system all night, leave the blinds open so as to not build up condensation on the windows or cut some types of vents around the house along the soffit... or cut vents on the parapet walls. Any help in resolving this problem would be appreciated... as well we live in Regina, Sk. and I don't know anyone who would be capable of analyzing this problem.

On 2017-12-17 by (mod) -

Helen,

Glad you found the article useful.

For the case you describe you could perhaps put up with an add-on layer of clear glazing independently framed over and sealed to the outside window surface above the wired glass both to protect it and to reduce unwanted heat gain or loss.

On 2017-12-12 by Helen

Thanks for this article its great. I have an old house and just put Georgian wired glass on an outside toilet roof. It is on a slight slope but water just pours off it from condensation.

I will put an extractor fan in but this would be on all the time. When the snow fell last week the floor was soaking but only from condensation. I didnt use double glazed as it was very heavy have you any advice please

On 2017-11-26 by (mod) -

You could probably fill that Gap from the inside using a fire retardant foam sealant. However the more serious safety worry from your description is an unsafe gas-fired heating system that could kill everyone in the home. Be sure that you have working carbon monoxide and smoke detectors properly located and tested. And be sure that your system has been checked by a qualified heating service technician.

On 2017-11-12 by Scott

I have two mobile homes (not a double-wide) connected together without a common roof. Right now, I have a peel type of membranous type of sealant which rolls on on the seam of where the two homes come together. I have a 6" roll immediately over the seam and a 12" roll over top of it.

Now this keeps out the rain but the condensation from the inside arch-way is a problem. Constantly dripping. I tried insulation the cavity with no success. I was wondering if insulation it again after it dries out and then placing plastic sheeting over the site would be beneficial? So aggravating! Oh yes. I use LP unvented gas but this was an occurrence even when I used electric. Furthermore, the dehumidifier did not help. although it did collect some water. Thanks

On 2017-06-06 by (mod) -

Yolanda,

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.

I suggest taking a look at "Minimizing Water Intrusion Through Roof Vents in High-Wind Regions" [PDF] FEMA document - here's a copy:
https://inspectapedia.com/ventilation/Roof-Vents-for-High-Wind-FEMA.pdf

PS: we love your questions, but please, next time you write, do not SHOUT

On 2017-06-06 by YOLANDA FERRANDI

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
Problem fixed.

On 2017-06-06 by (mod) re: how to stop roof vents that leak in high wind + rain, hurricanes, windstorms

Yolanda,

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
Problem fixed.

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:
http://www.gbshrewsbury.com/share/index.php/s/GMdUnVloiu2HxQD

In summary:
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)

But--

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

Geoffrey

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.

Thanks

Hi,

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.

Thank you!

On 2016-12-31 by (mod) re: how much foam should I add on top of existing roof?

Tony,

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 https://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

tom:

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?

Ronnie:

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 ?

On 2013-03-06 by Anonymous

How long for rubber roof to cure

On 2012-11-08 by Shane

If you find a good contractor in Calgary for this I would like to know as I have similar problems in the Calgary area.

On 2012-09-06 by (mod) - leaky roof in Calgary

Judi, in addition to checking with experienced roofers and contractors in Calgary, you might check the EXPERTS DIRECTORY link at page top and therein go over to the Canadian directory where under Alberta you'll find several experienced inspectors listed for Calgary.

On 2012-09-05 by Judi

Referring back to LM in Calgary, we are still fussing the problem. We've got a quote for the 'hot roof design' and you're right, it's big bucks. We're thinking of trying ridge vents to be sure that the air flow is going across the supposed 75 mm of air space left abpve the rafters.

In spite of goose necks and whirlies, we're wondering if there might be something in the problem areas blocking the flow across the deck. But before I do ahead, I want to get a competent onsite inspection by an expert that doesn't have any skin in the game. How to find this type of help in Calgary? What would I look for in the yellow pages?

On 2012-09-01 17:02:12.029883 by Dan - leaking flat roofs with brick parapet walls

Any help/ideas would be greatly appreciated.

I have leaking flat roofs with brick parapet walls similar to the one pictured. It appears the parapet walls were laid directly on top of the wood roof and in those areas the water has wicked its way through over time and rotted out the wood. So to do it right I have to pretty much knock all the parapet walls off to pull up the roof and replace the rotten boards, right? Sounds very exspensive, are there any semi-correct, cheaper alternatives?

On 2011-12-08 by (mod) - how to Diagnose annoying night time noise from flat roof.

Steve:

re: Diagnose annoying night time noise from flat roof.

With no information and no building inspection, we can't say with confidence exactly what is causing the annoying roof noise you report. An onsite expert should be able to help however. Here are some things that I'd expect that person to consider:

- what are all of the materials in the roof structure: support, roof deck, layers and coatings - thermal expansion rate differences during night time cooling might explain movement between materials and thus the noise. For example movement of materials over a metal roof deck may indeed telegraph noises to the interior

- leak history of the roof; even a roof that is not leaking now may contain moisture from prior leaks or from condensation due to moisture penetration from the building interior (again material and installation dependent) that could in cold weather be forming ice or contributing to thermal movement and noises

- Roof insulation scheme - where has insulation been placed and using what materials. It may be possible to correct the noise problem by a review and improvement of the roof insulation. For example, adding insulation at the building interior may reduce the differences in temperatures experienced by the two sides of the roof structure - the interior side vs the exterior side. If thermal changes are
a key in movement that is the source of the roof oise problem.

- what else is on or near the roof and the rest of the building structure that might be moving or otherwise serving as a noise source: loose building components, antennae etc.

- what HVAC or mechanical systems are mounted on the roof or penetrate it

- at what times, weather conditions, temperatures, wind directions, operation of
mechanical systems etc. map to when the noises are observed; where, spatially, are the noises loudest? Do those locations track
possible noise sources to specific building components.

To invite comment I'll post this Q&A at ROOF NOISE TRANSMISSION in the ARTICLE INDEX

On 2011-12-08 by Steve Rickard

Roof noise - during the winter our torched-on membrane (180g. polyester base sheet & 280g. polyester SBS Cap Sheet) covered flat roof (installed 2002 on the apartment building) makes noises typically at night which disturbs the tenants and has caused some to move out. There are no leaks and the roof looks properly installed. What causes the noise and what can be done about it? Rgds, Steve.

On 2011-09-02 by mel

Great advice, thanks for the info

On 2011-08-24 by (mod) - how to find leaks in a low slope or flat roof

Mel, I just have no useful idea what's going on for sure but I know how I'd proceed to figure out what's happening:

1. borrow or buy a moisture meter and track the area of moisture in the ceiling. Water from any source on a flat roof can run quite a distance horizontally before it shows up inside.

Make a sketch mapping the bounds of the wet spot by measuring moisture out to where it's not detected; You might find a path pointing to a surprise source such as a roof penetration that leaks or even condensate from a cold water plumbing pipe or an HVAC duct that is incompletely insulated.

I've also found cool weather condensation in ceiling ducts due to unanticipated up-flowing air from the home into the ductwork.

Also look VERY closely at roof seams. A "torch on" roof probably means modified bitumen (or is it multi-ply asphalt and tar and gravel?). On a Mod-bit roof, the proper level of heating at every seam is important - you should see about 1/2" of runout at every seam where the heated underside of the upper layer of modified bitumen oozed out slightly at the lap seam.

If you don't see this some seams may have been a bit too cold and one of them or more could be open.

On 2011-08-24 by Mel

Hi, we have a six year old torch on roof, our roof is 100% flat. Our roof is tung and grove cedar with rigid insulation and two layers of torch on material. last winter we had several drip spots in different area of our ceiling. Our roofer seems to think it is caused by condensation.

The roof never leaks during rain storms but only in winter but the temp could be above freezing or below and it will drip. The roof does not pool and seams look good. Any idea what is happening ? Thank you.

On 2011-05-21 by Haylee

Very halpfull, thanks (:

...

Continue reading at FLAT ROOF MOISTURE & CONDENSATION - topic home, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Flat & Low Slope Roof Venting & Moisture Articles

• FLAT ROOF DRAINAGE SYSTEMS
• FLAT ROOF LEAK REPAIR
• FLAT ROOF MOISTURE & CONDENSATION
• LOW SLOPE ROOFING - home
• MOISTURE CONTROL in BUILDINGS
• ROOF LEAK SOURCE DIAGNOSIS
• VENTILATION, ROOF SPECIFICATIONS - home

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FLAT ROOF MOISTURE & CONDENSATION FAQs at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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