Algae and stains can indicate leaks behind aluminum or vinyl siding © Daniel FriedmanSiding Leak Troubleshooting
Diagnose, repair or prevent leaky siding

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Siding leak diagnosis and repair:

Siding isn't waterproof; but how do we find the source of leaks in aluminum siding, vinyl siding or other building siding systems? How can we prevent or repair leaky building siding?

This article describes the causes of all types of leaks in building siding and wall coverings. Using detailed examples and photos of leaky building siding, siding stains, ice, or other leak clues for vinyl siding we illustrate the installation mistakes or subsequent siding damage that can cause troublesome, even dangerous leaks into a building wall or wall structure.

The article includes 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.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Leaky Vinyl or Aluminum Siding: cause, effects, cure

Damaged vinyl siding (C) Daniel FriedmanIs Vinyl Siding Waterproof?

No. While the actual face of vinyl siding is waterproof, a vinyl sided building wall is by no means waterproof.

[Click to enlarge any image]

Watch out: vinyl siding used on building exteriors is not nor is it meant to be a waterproof barrier.

Openings at the lap joints of vinyl siding sections as well as drain openings provided along the bottom edge of most vinyl siding products let the wall system breathe and allow wind-blown rain that may enter the siding to drain out of it as well.

But if house wrap and door and window flashing were not properly installed this same water that penetrates the siding can actually enter the building wall structure. Such leaks invite insect damage, rot, and mold contamination that can be quite costly.

In our photo at above left it looks as if there is a problem with moisture behind the siding, leaking out at two of the siding panel bottom edge vents, long enough to invite a pair of algae colonies on the wall.

We suspect that that top J-channel trip at the gable end protrudes past the gable board and is not sealed.

Leaks into Siding at Gaps or Damage Points

Gaps in vinyl siding leak into the wall (C) Daniel Friedman Gaps in vinyl siding leak into the wall (C) Daniel Friedman

Most of the vinyl siding problems we see appear to be due to poor installation details, though on occasion we see cracks and breaks that may be blamed on older, more brittle vinyl products. Installation gaps in siding of any type, if not back-flashed, invite leaks into the wall (above left).


Structural movement or damage, shown in a more extreme case at above right, can also allow significant leaks into the building. The horrible looking siding with a gaping siding-mouth below the window is discussed


Damaged vinyl siding (C) Daniel FriedmanWind-blown rain entering behind any building siding material can become a water problem.

In this article series be sure to

review J-CHANNEL MISTAKES in Vinyl Siding Cause Costly Building Damage

as this is one of the most common causes of leaks around windows and doors of vinyl-sided buildings.

Our photo of leak stains on a foundation wall below vinyl siding (at left) suggests that either we had wind-blown rain or other exterior leaks into the building wall or there was an in-building leak or flood such as from a burst pipe.

Water passing over wood and other building materials but behind the vinyl siding can easily pick up pigments that stain the foundation wall outside.

If the builder failed to install a weather barrier, this sort of staining is more likely.


Tracking Down the Leaks in a Vinyl-Sided Building

Vinyl siding leak diagnosis & cure (C) InspectApedia GSReader Question: buckling aluminum-wrapped window frames, damaged windows, siding leaks. The Siding Rep won't agree that there is a problem

To the Editor:

I recently wrote in that there is a problem with water/moisture getting into my exterior wall cavities. The top of one rectangular window frame had buckled outward causing the aluminum window wrap to ripple and come apart at the caulked areas.

This is when I first noticed there was a problem. The rectangular window photo shows the buckled window frame top with the now replaced aluminum wrap, bent over nail included.

The building has vinyl siding installed over existing wood lap siding.

A J-channel was wrapped around all the doors and windows to receive the siding.

A workman came out and applied a sloppy bead of clear silicone where the J-channel meets the window frames including the top of the frames.

The company's rep. stated that the American Siding Institute had changed the installation method and that this was now being recommended.

Vinyl siding leak diagnosis & cure (C) InspectApedia GS

I could find no reference to his claim on the Am. Siding Institute's website. I also looked at several other siding jobs in the area and none had caulk between the J-channel and window frames.

The worker also removed the old caulk from the window wrap, gouging it so you see the silver aluminum and re caulked it. It's the worst caulk job/workmanship I believe is possible.

The rep. won't acknowledge that there is any problem and is very elusive. He even suggested it was because of something I had done. And that I should be careful because he had recorded my conversations with him.

There's more but I won't bore you with further details. I have maintained my cool and friendliness throughout it all but am rather upset to put it mildly. - K.G. 1 Feb 2015

Reply: be polite, find the leaks

Troubling case report and revealing too.

If you were being polite and civil and an onsite company rep felt he had to record his conversation with you one might infer that the company or the rep have experiences with their product and field service that lead them to expect trouble.

It may help to keep in mind that for many building supplies you are not the manufacturer's client. The builder is their client.

Vinyl siding leak diagnosis & cure (C) InspectApedia GS

The addition of caulk along the vertical abutment of window trim and J-channel as well as across the window top (your photo at above left) is not unusual but isn't usually required and if flashing errors were made it won't stop leaks; one wonders if the top J-channel runs were cut short and not bent over the outside of the vertical J-channels.

Vinyl siding leak diagnosis & cure (C) InspectApedia GS

The open aluminum trim miter cut (your photo at above right) might be a leak into the window structure as well unless the vertical or right-hand trim was properly extended under the window top trim and sealed properly during construction. The joint looks open.

Extreme leaks behind and into vinyl siding (C) Daniel Friedman The claim by your contractor that caulking is now recommended for vinyl siding J-channel installations is a bit confounded (sounds like arm-waving to me) and is not supported by expert documentation and standards as I include below.

Proper as well as improper J-channel installation details are described

at VINYL SIDING J-CHANNEL MISTAKES in Vinyl Siding Cause Costly Building Damage.

At left we illustrate extremen leaks into vinyl siding on a New York home at which the siding installer had omitted building flashing. In driving rain so much water ran into the siding (and into the building walls) that it was literally pouring out of the siding bottom (blue arrow).

The Vinyl Siding Institute (cited below) provides a vinyl siding installation manual that provides some guidelines.

There the instructions recommend applying a bead of sealant to the nailing flange of window sills to cover nails and nail slots when flashing previously-installed nail-fin windows.

This case does not apply and cannot pertain to the application of vinyl siding atop existing building siding such as on your home.

Question: leaks into a siding gap my contractor left in this vinyl siding installation job

Wet area on foundation below siding may come from siding leak higher on wall (C) JJ

I’m hoping you may be able to provide me advice, though I realize your ability may be limited based on the photos and information I will provide.

Wet area on foundation below siding may come from siding leak higher on wall (C) JJ

I purchased my home in NY and closed on 3/30/17. The house had been remodeled by a contractor in 2016 including new windows, new siding, new roof, & new gutters.

Recently I noticed a spot on the exterior foundation wall under the kitchen window that is wet when the surrounding wall is not (when it rains). It will also be wet after for some time, while the surrounding wall will not. The window does not have a jchannel installed because, I’m told, it has one built in.

This is the side of the house that receives driving rain when it rains.

Should this have a jchannel installed? And could this be the reason for the water on the foundation wall? (Photos 1 & 2 above)

On this same side of the house, where there was an addition added years ago, there is about a 2” gap in the siding where it meets the trim where the addition roof meets the house. There appears to be no flashing here, or house wrap.

Wet area on foundation below siding may come from siding leak higher on wall (C) JJ

(Photo 3 is the full corner and photo 4 is an enhanced closeup allowing you to see the wood wall underneath)

Wet area on foundation below siding may come from siding leak higher on wall (C) JJ

This should be properly flashed, am I right? Additionally, since I do not see any house wrap here - I’m concerned. I know there is what appears to be foil backed 1/2 inch styrofoam under the siding at the bottom portion of the house,

however I also know that an entire wall of my garage has none of this foam, nor does it have any house wrap under the siding - it’s just the wood under the vinyl. Shouldn’t there be house wrap around the entire structure since it was just sided in 2016?

Do I need to pay to have the house resided?

I had an inspection and none of this was noticed during the inspection.

I appreciate any advice you can provide. - Anonymous by private email 2018/05/05

Reply: evaluate vinyl siding leaks and installation details to decide what steps are needed

Indeed you did a nice job of finding what could be the principal leak point sending water behind your siding - we don't want water to run off of the lower roof and into an area behind the siding where it might enter walls and lead to a mold, insect, or rot problem.

Your photos of the siding gap show bare plywood, suggesting that there was also no housewrap.

So there may be multiple siding-leak-related problems here:

I don't yet assume that it would be appropriate nor cost-justified to have the home re-sided, but we certainly need additional onsite inspection.

An astute onsite inspector armed with moisture meter and perhaps a willingness to cut a test opening at the most-suspect points of drywall inside - maybe behind floor trim - ought to help sort this out.

Let us know what you're told and we may be able to offer follow-up suggestions or questions to ask.

Also see the detailed flashing and housewrap suggestions in these three articles:

Proper Flashing & Sealing Around Windows & Doors

Here is how your windows (and doors) should have been prepared, excerpting from the Vinyl Siding Institute's Installation Manual for Vinyl Siding:

Flashing Previously Installed Window with Exterior Casing (Brick Mold) If a window with exterior casing (e.g., brick mold) has been previously installed without flashing, the following instructions should be followed:


That VSI standard does not explicitly address the situation of adding layers of siding that require building out the window trim, but the principles are basically the same: you need to flash and seal the window before installing the siding and J-channel.

I'd want to get a clear idea of where the problems lie and to document them with care before trying further repairs.

Reader follow-up: plans to remove siding to find the leaks

I had an old school established roofing/siding company owner look at it yesterday and he is going to take off the siding around the buckling window to determine the problem which he says is water infiltration.

Also says not only is the caulk job poor but the caulk used is not the right type.

All 21 window's aluminum wrap is badly scratched where the worker tried to remove the old caulk. Where he couldn't remove it he simply caulked over the old.

All windows will have to be rewrapped, etc.. What was a $200 job is now going to be between $1750 - 2500.

The owner of the company who did the work said through his salesman yesterday that he believes any damage to the aluminum wrap was preexisting. He refuses to come out to look at it. I feel as if I'm dealing with sociopaths and my best shot is going to be small claims court.

Reply: take photos

Be sure to take ​ ​plenty of clear photos of the siding as is now, during removal, and after removal; I'm particularly interested in water tracks and marks, missing flashing, improperly-installed housewrap, and in the end-cuts of the J-channel over windows. Send me what you can and I can comment further. All of that data may even be useful to you

Use this VINYL SIDING HOOK to Remove and Reinstall Vinyl Siding without ruining it.

Reader Follow-up:

Yes, the exploratory work begins tomorrow. I'll keep you in the loop.

Here are the photo's of the window being disassembled after the installer repaired it.

Stripping window trim to find leaks at walls (C) InspectAPedia GS

The installer tried to unsuccessfully bend the top casing board back with screws and then poorly installed a new piece of aluminum wrap.

Stripping window trim to find leaks at walls (C) InspectAPedia GS

He then silicone caulked all 21 windows where his J-channel meets the window casing effectively forcing any water/moisture into the wall cavity.

What I discovered is disturbing. The photos speak for themselves. I can literally reach into the wall cavity above this window and pull out the insulation. There is nothing to keep the water/moisture from entering the wall. - K. 9 Feb 2015


OK but I still can't see what I want;

Look at the J-channel over the top of the window.

Look at the end of that J-channel section and see how it was cut.

It should be cut so that the bottom or "flat" portion of the J-channel is left long and then bent down over the outside of the vertical J-channel that runs along side of the window.

If the J-channel top piece was simply cut off straight or say on a 45 degree angle to miter the corner (which is what I suspect) then water running out of the horizontal J-channel ends can leak down the inside of all of the top and side and bottom J-channel segments and enter the wall.

If that is what was going on, adding caulk where it was added won't help a bit.

Reader Follow-up: Vinyl Siding J-Channel Installation Details

Here are some photos where the window top J-channel intersects with the window side J-channel. I also looked at several other windows and all had a tab off the top J-channel turned down into the side J-channel.

Vinyl siding J-channel end-treatment to avoid leaks (C) InspectApedia KG


It looks as if that detail was correct, except possibly for a subtle leak opening if the tab was cut on an angle so that there is an opening on the horizontal surface at the end of the J-channel that lets water not run down the tab and onto the outside of the vertical J-channel and at its end out onto the siding exposed front. (That's the intended path).

On enlarging one of your photos I observed the leak points in the aluminum-wrapped window trim shown in the photo just below

. It seems likely that water would enter behind the aluminum wrapping on these sills and depending on how the aluminum was bent behind the window sill's J-channel or behind the vinyl siding below it could certainly direct water into the building walls.

More water might pass into these gaps than one would imagine, as blown rain running down the window face will flow onto the sill and a bit of it across the sill ends and right into those two gaps.

Leaks in aluminum-wrapped window trim (C) InspectAPedia KG

In any event, look closely at the exposed wood trim and walls for water tracking marks that may tell us where most of the water was running. For example was water entering the wall at the head of the window or along its sides or at the sills or below the sills.

And important as well is exactly why, if I understood you correctly earlier, the windows themselves were damaged. Where did water come from and how did it enter the window frame to cause damage?

Guidelines For Vinyl Siding Installation

Wood Siding Leaks & Damage

Downspout leaks caused this wall damage (C) Daniel Friedman

The wood clapboard-sided home shown above, located in Poughkeepsie New York was badly damaged by leaks into the wall that ensued from yankee gutter leaks and downspout leaks below a roof valley.

Ultimately water began to appear on the floor of the kitchen inside. Years of water leaking into the wall had also sent water into the brick foundation wall below where conditions invited carpenter ants into the structure.

The wall was damaged by both insects and rot. We had to remove and rebuild this corner of the building.

Downspout leaks caused this wall damage (C) Daniel Friedman

Factors Inviting Leaks Into & Behind Building Siding

Typar peel and stick flashing tape (C) D Friedman Eric Galow Galow Homes

See SIDING LEAK POINTS & CAUSES where we've moved this discussion.

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.

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.


Continue reading at SIDING LEAK POINTS & CAUSES or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see SIDING LEAK DIAGNOSIS & REPAIR FAQs - questions and answers about leaky siding that were posted originally on this page.


Or see these

Articles on Sources of Building Exterior Leaks

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SIDING LEAK DIAGNOSIS & REPAIR at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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