Cold weather & winter weather roof problems, troubleshooting, repairs:
This article describes problems caused in building roofing systems by cold weather and winter weather conditions, and additional roofing troubles that come during roof maintenance (or lack of it) and during re-roofing, roof-overs, or roof replacement jobs.
We also describe a case of severe roof leaks blamed on the roof contractor (who omitted roof flashing) but for which the homeowner also bore some responsibility.
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Winter & Cold Weather Roof Troubles: Q&A 
Reply: List of winter problems, risks, and damage to building roofs besides ice dams
If you restrict comments to hurting the roof itself, it's a too-narrow scope question, because some cold weather and winter roof problems can cause serious trouble in other building areas and with other building components too.
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Question: If a roof needs to be re roofed or if shingles need to be replaced, is November too late?
Reply: year round roofing procedures
In our opinion, no, not if proper cold-weather roofing procedures are followed. Experts install roofs throughout the year. However if your roofer is not an expert in cold-weather roofing the roof job may indeed not be properly executed and may leak or have a shorter life.
It is our OPINION however that the best roofing work is done in cool dry weather.
Too wet roof surfaces may result in materials being installed in violation of the roofing manufacturer's instructions, industry standards, and good practice, resulting in reduced roof life. Don't install roofing in the rain nor install roof coverings over wet surfaces.
Too cold roof surfaces and cold weather roofing conditions have a different set of concerns. Some examples of common cold weather snafus during re-roofing or new roof installation include:
Question: Should a contractor remove leaves and sticks OR should a homeowner include this as part of their regular maintenance?
Reply: who is responsible for roof maintenance on a residential building?
This roof maintenance question needs some clarification before we can write a cogent answer.
A roofing contractor who is hired to install or repair a roof will have to remove debris as part of the job. If the roof does not otherwise need maintenance and repair, it is the homeowner responsibility to care for their roof just as for their yard, and their gutters. But as our roof debris (and moss) photos below show, some roof cleaning tasks are probably beyond the scope of an ordinary homeowner and would be dangerous for the homeowner to attempt, while other seemingly easy tasks still risk dangerous falls or damage to the roof itself.
Watch out: working on roofs and from ladders is inherently very dangerous. For people who lack the physical ability, knowledge, skill, and for high and steep-slope roofs, it's safer to hire an expert.
Watch out to make sure that your roof "expert" really is one. If you hire an amateur handyman who does not know safe procedures nor know just what's needed, the dangers include improper, inadequate work, reduced roof life, or worse, an injury.
See ALGAE, FUNGUS, LICHENS, MOSS on SHINGLES for one of several articles about moss and debris found on roofs.
Watch out for improper cleaning too: aggressive measures like power washers and even stiff brooms can damage shingles, removing their protective granules and reducing roof life.
See DEBRIS STAINING on ROOFS for examples of causes and steps to cure debris problems on building roofs.
Question: "What's newer"? Shingles installed 5 years ago or 2 years ago?
Reply: how do we predict the remaining age of a roof?
This is a trick question, right? Perhaps you are implying that five years ago roof products were better quality so would be "newer" in condition than poorer products on a chronologically newer roof? Perhaps you mean, which roof is likely to have a longer anticipated life? Or you are suggesting that more recent roof products are inferior to ones made five years ago? Nonsense.
Roof covering product quality varies by specific product, manufacturer, and even by occasional variations in the manufacturing process for a given roofing product.
Similarly, wide variations in the expected roof live may be traced very often to the roof installation and workmanship.
While we may be interested in knowing the chronological age of a roof as a way to guess its remaining life, a better rule of thumb used by home inspectors and roof inspectors is the wear age of a specific or individual roof.
In addition to the the quality of the shingles or other roofing materials, lots of factors influence the life of a roof covering
Even where workmanship and roof covering product quality are identical, not all roofs wear at the same rate. There are quite a few factors that affect the rate of wear on a roof (after excluding roof product quality or defects) including:
For a more meaningful prediction of roof life: ask what is the roof's "wear age" ?
So if we see what looks like (or was claimed to be) a 20-year asphalt shingle roof, and in the judgment of an experienced roof inspector, a close inspection of the roof from outside and inside suggests that it's about half-way through it's predictable or usually-expected life, then we'd call the roof's wear age 10 years, even if the chronological age of the roof were different.
Watch out against giving a low "wear age" (longer remaining life) to an older roof without a close inspection.
Some wear signs such as shingle cracking and porosity and other shingle defects such as thermal splitting (CRACKS in FIBERGLASS SHINGLES) can be difficult or impossible to spot from ground level or even from a ladder at roof edge if the roof has other slopes or sections not accessible. (Roofs do not always wear equally on all slopes).
See ASPHALT SHINGLE LIFE / WEAR FACTORS for a list of factors that affect the life expectancy of an asphalt shingle roof.
See AGE OF ROOFING for help in determining the chronological or wear age of a building roof and for more factors that affect just how long a roof can be expected to last.
Question: How would a homeowner know if their shingles are newer and if algae growth is a risk?
Reply: AR - algae resistant shingles, & factors that affect algae stains on building roofs
Check to see if algae-resistant shingles or other algae-resistant roof covering materials were used. The manufacturers of mineral-granule-covered asphalt shingles indicate on on the package and/or in their product literature the algae resistance of various shingle products.
Nevertheless, algae-resistant is not "algae-proof" and green, brown, or black algae may show up on some roofs even where AR shingles were installed.
Other factors affect algae growth on roof surfaces include:
There may also be variations even within a given shingle brand and product, as manufacturing consistency might vary, or as shingle storage-conditions (before the shingles were installed) may have varied from one building to another.
How to diagnose and remove or prevent algae stains on roofs
There may be other causes of black stains on roofs, including roof stains or discoloration from
See STAIN DIAGNOSIS on ROOFS for a complete catalog of roof stain sources and cures.
See BLACK STAIN REMOVAL & PREVENTION for suggestions about cleaning black stains off of roof surfaces.
Question: What happens if old roof flashing isn't replaced with new flashing during re-roofing?
Reply: increased risk of roof leaks during a roof-over that relies on existing or old roof flashing
As our friend Mark Cramer, a building inspection educator likes to say, ... "It depends". 
Old flashing that is heavy, high quality material (such as heavy weight copper) might itself be fine depending on the location at which it is being left in place (for example at chimneys or roof sidewalls) for re-use by careful interleaving with the new roof shingles when they are installed. Other roof flashings (plumbing vents, valleys) really won't be properly integrated with the new roof surface if they are simply left "in place" below old roof materials during a re-roof (roof-over) job. (Some of these might be re-usable if the re-roof job includes removal of all of the old roof covering.)
What are the problems with relying on old roof flashing when re-roofing a building?
Roof flashing that is corroded, thin, worn, cracked, or otherwise damaged should not be relied upon when re-roofing. It should be replaced completely if a tear-off roof job is underway, or new flashing might be installed over old valley flashing during a roof-over.
Re-roof snafus at roof-sidewall and roof-chimney intersections
A basic problem with relying on old flashing to prevent roof leaks in the new roof covering is illustrated by flashing requirements at the roof-sidewall (the intersection of a lower roof surface with a vertical building wall) or at the abutment of the roof surface to the sides of a chimney.
Perhaps the old roof flashing and roof was not leaking at all in that location so the roofer and owner were happy just placing a new layer of shingles atop the old, with no new flashing whatsoever at the roof-wall abutment.
Leaks at roof-wall abutments or chimneys when relying on old roof flashing and when leaving old shingles in place
But properly installed step flashing at a roof-wall intersection is designed to bring water at each individual shingle back out onto the upper surface of the next shingle course below. Water that runs down the vertical wall or chimney side or that falls on the roof surface by the wall is constantly directed out onto the outer uppermost surface of the roof where it can drain away safely.
Chimney flashing details at left illustrate the positioning of step flashing and counterflashing along a vertical side wall or chimney surface. Courtesy of Carson Dunlop Associates.
When a new layer of roof shingles is installed along the same abutting wall without its own step flashing and counter flashing, all of the water that runs down the wall or falls on to the roof surface in that area easily enters under the new layer of roof shingles where, depending on the conditions below water may run anywhere under the new roof covering and on top of the old roofing layer.
Thus if there was a leak anywhere down-slope on the original roof, water entering at this roof-wall intersection has a better chance of finding it and leaking into the building, even though a new roof was installed above.
During a re-roof job, just gobbing on roof mastic or sealants is not a reliable substitute for properly-installed roof flashing
Later someone comes along and blobs flashing cement or mastic along the roof wall intersection to try to seal this un-flashed area. But because the various building materials expand and contract at different rates (wall, flashing, roofing, chimney side, roof deck, etc.), even a flexible mastic material is not a long term reliable substitute for properly installed flashing.
Roof flashing is intended to serve as a mechanical means of diverting water back onto the upper surface of roof shingles rather than permitting it to pass under the shingles where it finds a place to leak into the building.
When the installer stops relying on the mechanical drainage properties of flashing and relies instead on globbing on sealant, the risk of future leaks is increased. Even though there are some very good, durable, stick, flexible sealants on the market, the sealant has a difficult job to do: it has to bridge a variety of materials during times of widely changing temperatures.
Wood, copper, aluminum, asphalt shingles, roofing felt, roof decking plywood or boards, masonry chimneys, wall siding materials all have different coefficients of thermal expansion. That means pulling and tearing at the intersections where the installer relied on sealant. And often that leads to cracks or separation at the sealant, caulk, mastic, or other "flashing cement".
Re-using old flashing when old shingles have been removed may be OK
If the old roof shingles had been removed, and if the new shingle courses line up perfectly with the old step flashing left in place, then the old step flashing and counter flashing might be re-used with success, though there too there are some troubles to overcome. Removing the old roof shingles can leave the step flashing bent up away from the roof surface. If the roofer doesn't take steps to flatten out the old step flashing and leaves it curled up the new roof looks bad and may leak, especially during conditions of wind-blown rain. And the lifted shingles may fail prematurely.
Worse, "fixing" this curled lifted old, re-used step flashing by nailing down the flashing corners risks leaving a shingle penetration in the wrong location, where that too may leak.
Don't rely on thin, corroded, worn or damaged roof flashing materials when re-roofing
Old flashing that is crimped, cracked, leaky, punctured by nails now withdrawn, improperly installed in the first place, coated with roofing mastic (that corrodes through copper flashing even though it's a short term fix), is going to leak, especially in heavy rains, behind built-up on-roof debris, or when the roof is covered with wet snow (in such climates).
Distinguish between a tear-off re-roof job and a "roof over" re-roof job
A complete roof tear off won't leave any flashings on the roof except perhaps the counter-flashing at a chimney. In our photo (left) the roofer is removing all of multiple layers of roofing before installing a new asphalt shingle roof on this home.
So leaving old flashing isn't much of an issue except that when the new roof and flashing are installed around the chimney (or at the abutment of an upper sidewall or dormer to the lower roof surface) the counter flashing may need to be carefully bent down and sealed against wind-blown rain leaks.
A roof-over job too often leaves all old flashings in place for the obvious reason that to remove it means removing part of the old shingles that means in turn an uneven surface under the new shingles (unless it's built-up level using shingle pieces). At a roof valley where the old flashing is in excellent condition, both old and new layers of shingles may need to be sealed along the valley. That approach leads to come confusion about what flashing is, what it's for, and how it works.
See FLASHING ROOF-WALL SNAFU for roof-wall flashing SNAFUs to avoid
Roof Snafu case history: "My new roof is leaking like mad every time it rains! ... "
An pediatrician acquaintance in Poughkeepsie, NY, H.L., called me in desperation.
The low-bid roofer got the job.
Now I have to add that H.L. is a tightwad who pressed every bidder for his roof replacement to come in at the lowest possible price. He gave the job to the lowest bidder.
No roof flashing was installed anywhere.
I inspected the roof and found that following a complete tear-off, NO roof flashing had been installed anywhere. No flashing at the plumbing vents, no flashing at the chimneys, no roof flashing at the upper sidewall abutment to roof locations.
The roofer said that roof flashing would be an extra charge!
Who really caused this leaky roof-job?
The roofer desperately wanted the job.
The homeowner desperately wanted the lowest price.
In my OPINION the homeowner was partly responsible for causing his leaky roof trouble because of his insistence on squeezing the last dime out of his roofing contractor.
But the roofing contractor was responsible for not making sure the client understood that flashing is required at roof penetrations and abutments and that without it the roof would sooner or later leak. In this case it was sooner- at the first rain.
The roofer should make clear to the client the significance of the different roofing choices being offered - in terms not just of cost but of roof durability.
The lowest price for your re-roof job is not always really the lowest cost.
More advice about choosing a roofing contractor is at CHOOSING A ROOFING CONTRACTOR or select a topic from the More Reading links shown below.
Continue reading at ICE DAM PREVENTION on ROOFS or select a topic from the More Reading links shown below.
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