Algae and stains can indicate leaks behind aluminum or vinyl siding © Daniel FriedmanSiding Leak Causes
Find the cause of leaky building siding

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Siding leak points & causes on buildings:

This article series describes the causes of all types of leaks in building siding and wall coverings.

The article provides a catalog of all sorts of causes of leaks into building walls and provides links to in-depth diagnostic and repair articles for each of them. Our page top photo shows water leaking out of gable end siding on a condominium complex in Maine.

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13 Factors Inviting Leaks Into & Behind Building Siding

Typar peel and stick flashing tape (C) D Friedman Eric Galow Galow HomesAt JAMES HARDIEPLANK® FC SIDING we asked: how likely is it that gaps or deteriorated Hardieplank® siding (or any other siding) will result in leaks into the building structure and concomitant damage?

Gaps, poor nailing, wind damage and other and loose siding are defects might result in actual loss of siding from the building but that's uncommon and certainly less common than leaks from rain or wind-blown rain that is pushed or leaks into building walls.

While gaps and openings permit leaks behind the siding, some siding openings are normal, such as aluminum or vinyl siding bottom-edge vents.

Most siding products are not defined by their manufacturer as a water proof barrier, though there are some excptions such as WERZALIT SIDING & RAINSCREENS, a system that includes interlocking, gasketed, waterproof siding boards intended for use in areas of severe weather exposure.

The presence of a housewrap on modern buildings or felt on older buildings combined with proper flashing at windows, doors and other openings are relied-upon to keep water from entering the structure itself.

The complete exterior cladding system, including wall sheathing, housewrap, flashings, trim, and siding installation must work properly together.

Provided that in addition to the leak at an opening in siding there are accompanying errors, omissions or damage in or to the building's house wrap and/or in building flashing around windows, doors, or other locations or omissions of sealant where it is required then water indeed can enter the actual building structural wall cavity where it can cause expensive damage such as rot, mold contamination, wet insulation, insect damage.

Without these additional defects, water that passes behind building exterior siding, cladding, or trim should be remain on top of the house-wrap and be directed down the wall to a point of exit at the wall bottom. Sometimes that works. When it doesn't, then water can enter the building wall cavity at butt joints between building plywood or OSB or other sheathing.

See these examples of key materials that must be installed properly beneath most building siding products if air and water leaks and related damage are to be avoided.

The peel-and-stick flashing membrane shown in my photo is discussed at PEEL & STICK FLASHING MEMBRANES.

Class action litigation that asserts premature failure of building siding products often includes both cosmetic issues (peeling surface, loss of color, some gaps) as well as the cost of structural damage from leaks.

Algae and stains can indicate leaks behind aluminum or vinyl siding © Daniel FriedmanIn this excerpt from In re Hardieplank Fiber Cement Siding Litigation, Court File No. 12-md- 2359 (D. Minn.), page 3 paragraph 6.: discussed at JAMES HARDIEPLANK® FC SIDING we find:

The Siding is susceptible to premature failure, causing damage to the underlying structures and property of Plaintiff by allowing water and moisture to penetrate into the structure.

How often will this water penetration "into the structure" actually ccur and what sorts of damage will it cause?

What is the actual vulnerability of buildings to the claim made above: separating theory from actual field experience.

The following leaks can occur and often do on residential building exteriors covered with a range of lap siding products including shingles, shakes, hardboard siding with shrinkage gaps, fiber cement siding with shrinkage gaps or warping openings.

Conditions that Permit Water to Pass Behind Building Siding

  1. Wind-blown rain entering the siding bottom edge vent drain openings present on most aluminum or vinyl siding can pass behind the siding through those openings.

    This is a normal condition of these siding products (not present on hardboard, fibercement board, shingle type siding, nor plywood sheet siding) and is a reason that these siding products are not billed as "waterproof".

    Our photo above of stains below courses of siding indicate the presence of water running behind the siding itself, entering higher on the wall and exiting between various siding clapboards.
  1. Water entry at siding butt-joint gaps: water from rain or melting snow running down a building exterior wall can pass behind the siding where it encounters a gap in siding butt-joints IF the gap is not backed by flashing and IF water entry at the gap is not blocked by the choice of siding exposure depth and by having staggered butt joints so that the gap is closed by the siding course below.

    Excessive exposure, beyond the manufacturere's specifications is likely to create leakage at this point if the butt joints are open.

    Siding shrinkage such as occurs in some fiber cement products opens such butt joints and increases the risk of leakage if other defects are also present at that location.

Gaps at butt joints of HardiePlank siding on an 8-year-old home  (C) Daniel Friedman

Take a close look at the fiber cement siding gap above. [Click to enlarge any image]

You can see a bit of the upper edge of the next-lower siding course in the bottom of this 1/4" wide fiber cement siding butt joint gap. There is no question that wind-blown rain or rain running down the building wall will have no trouble entering behind the siding at gaps like this one.

When that happens, the building has to rely on a perfectly-correct installation of housewrap and flashings below this point if it want to avoid soaking the exterior sheathing itself and if it hopes to avoid water leaking into the actual wall cavity at sheathing joints.

On this home, when all of the gapped siding was removed, we found extensive rot around most of the windows and doors, generally due to improper window flashing and trim. At some locations the sheathing had been damaged and water had penetrated behind into the wall cavity.


  1. Water entry at the abutment of siding lower edges to horizontal trim above windows and doors, IF the trim was not properly flashed and sealed.

    A notorious example of leaks at this location is at improperly-cut (too short, not properly positioned) J-channel used with aluminum or vinyl siding and placed around windows and doors or other wall penetrations.

    See VINYL SIDING J-CHANNEL MISTAKES Cause Costly Building Damage
  2. Water entry at the abutment of siding to edges of vertical trim at windows, doors, and building corners, IF the trim was not properly flashed (at windows and doors) and/or sealed at those locations and at building corner trim.
  3. Water entry at the abutment of siding to the upper edge of horizontal trim located under windows and doors or along the water table or other building exterior decorative trim on walls, IF the trim was not properly flashed or sealed at those locations.
  4. Water leaks at soffit or eave overhang penetrations for chimneys that permits water to leak at the roof surface, then pass behind the siding IF the chimney is not properly flashed.

Ice on siding and wet areas traced to improperly installed sidewall vent high efficiency gas furnace (C) InspectApedia Carole Rowley

My photo above of ice below some courses of vinyl siding traces a moisture problem traced to improper installation of a through-wall vent for a direct-vent heating appliance.

  1. Water leaks at other wall penetrations such as mounting points for exterior lights, doorbbells, receptacles, or the passage through the wall for vents or hose bibbs, IF the penetration was not properly sealed.

Wind damaged siding, Maple Shade New Jersey (C) Daniel Friedman

  1. Water leaks where siding has fallen off or been blown-off of the building, shown above on a New Jersey home where vinyl siding is missing, can happen during severe weather or may occur because the siding wasn't properly installed in the first place.

Photograph of deteriorated James Hardiplank hardboard siding.

The area of missing fiberboard siding shown above might be blamed on a product failure, as we can see what looks like a nail hole in the lower corner of the bottom-most siding at the right side of the picture. But this siding loss will be more-likely to occur when the siding was not nailed properly to the building wall sheathing or framing or where the sheathing itself is defective (or omitted in some buildings).

Our photograph of a Wardcraft home on which James Hardieplank siding has apparently fallen-away, contributed by our reader cited above, shows such an opening.

Outside of extreme storm damage, the most common cause of this fallen-off siding is improper nailing or use of the improper type of nail.

But some types of siding, particularly fiberboard, can be loosened by deterioration too. Extreme deterioration of wood-based fiberboard (likely and shown below) or fiber-cement siding (unlikely) that becomes so soft that it pulls away from its fasteners is also possible.

Photograph of deteriorated hardboard siding.(C) D Friedman Hardboard siding deterioration at nail (C) D Friedmanj

Above: badly-deteriorated wood-based hardboard siding deterioration that is sufficient that siding is at extra risk of coming off of the building entirely. This siding is described at SIDING HARDBOARD.

Below: poorly-installed vinyl siding that is loose and risks coming off of the building.

Horrible vinyl siding job in New York - many defects cataloged (C) Daniel Friedman

The terrible vinyl siding job shown above is discussed in more detail at VINYL SIDING INSPECTION & REPAIR.

Buckled torn vinyl siding due to building movement (C) Daniel Friedman

  1. Structural movement or damage that bends or flexes siding causing it to break or create openings and leaks.

    The bent broken leaky vinyl siding shown above is discussed at VINYL SIDING MOVEMENT DAMAGE. Also see VINYL SIDING BUCKLED WARPED.

Damaged vinyl siding (C) Daniel Friedman

Above: sagging of the lower roof has pulled away siding below this building's windows. It looks as if there was no housewrap behind the siding.

Below: broken asbestos cement shingles on a Poughkeepsie home damaged by the TV cable installers hammer.

Photograph of - damaged vinyl siding

  1. Mechanical damage to siding causing breaks, punctures, holes then permit leaks from wind-blown rain or rainwater that runs downt the wall surface.

    Fire damage, storm damage, impact from lawnmower-blasted rocks, vehicle impact, other impacts (photo below), or work on the siding by an idiot cable installer who breaks all of the fiber cement shingles around the building (shown in our photo above) are examples of mechanical damaged siding.


Broken vinyl siding risks leaks into the structure (C) Daniel Friedman

  1. Other improper siding installation conditions such as installing siding in contact with the ground or even extending below grade level, inviting water damage, insect damage, and rot.

    My photo below shows fiberboard siding against which soil backfill was placed, a virtual guarantee of deterioration and damage.


Hardboard siding disintegrating (C) D Friedman

  1. Other building maintenance or site preparation defects, such as failure to maintain roof gutters can cause roof runoff spillage close to the building walls, accelerating siding deterioration from rain splash-up.

    My photo below illustrates rotted plywood siding in an area that was wet by roof spillage splash-back.

    OPINION: on the other hand, it has always seemed crazy to me that we install an exterior building cladding that includes a specification that it cannot tolerate getting wet.


Roof splash-up damage (C) Daniel Friedman



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