Ontario's Plain-English Guide to Custom Home Buildingπ 705-533-1633 | βοΈ 1-866-868-6606 | βοΈ info@icfhome.ca
Water Behind Siding: Stains Inside and Nothing to See Outside, Where It Gets In, What the Door-to-Door $90,000 Hidden Moisture Damage Pitch Gets Wrong, and What a Real Diagnosis and Repair Cost
Exterior Finishes
Water Behind Siding: The Wall Looks Fine, the Drywall Says Otherwise, and the Man at the Door Says $90,000
Siding hides what it covers. Water can run down the inside of a vinyl wall for ten years and the vinyl will look exactly as it did the day it went on, which is the whole problem with vinyl and the whole opportunity for the fellow who knocks on your door with a moisture meter. Real water damage behind siding is common, especially around windows and under roof-to-wall corners on houses built from the 1980s to the mid 2000s. So is the sales pitch that borrows it. This page is how to tell them apart, what a real diagnosis looks like, and what a real repair costs, with numbers.
Where it gets in
Almost never through the field of the siding. Water gets behind cladding at the interruptions, and the list is short enough to check in an afternoon: window and door openings with no sill pan or no head flashing; the bottom corner of a roof that dies into a wall with no kick-out; a deck ledger bolted through the siding; a band of stone or a trim board with no metal over it; penetrations that were caulked instead of flashed; and siding buried in a flower bed or run down tight to a roof so it wicks. Every one of those is on the flashing details page with what should have been there.
The other way in is from the inside. Warm, humid house air leaking out through a wall in January condenses on the cold sheathing and freezes there. In March it melts and runs down to the bottom plate. A bathroom with a fan that vents into the wall cavity, a leaky poly at an outlet on an exterior wall, a top plate with a gap at the attic: these put water into the wall with no rain at all, and the stain shows up in April, which fools everyone into looking for a spring leak.
What you see inside, and what each usually means
| Symptom | Most likely cause | First thing to check |
|---|---|---|
| Brown stain at the top corner of a window frame, inside | Head flashing missing or taped at its bottom edge; wrap lapped behind it | Outside, above the window: is there metal, and does the siding above it have J-channel that collects water and dumps it at the ends? |
| Stain or soft drywall below a window sill | No sill pan; the window itself leaking at the corner welds; or a window with the weep holes blocked | Hose test on the window alone, with the siding below it watched. Window leaks. |
| Stain on a wall below where a lower roof meets it | No kick-out flashing. Nine times in ten. | Stand outside and look at the bottom end of that roof edge. If there is no diverter, you have found it. |
| Stain at the bottom of an exterior wall, basement ceiling wet below it | Siding at or below grade, deck ledger, or water from the eavestrough downspout dumping at the wall | The downspout first. Then the grade. Then the ledger. |
| Musty smell, mould line behind furniture on an outside wall, no stain | Condensation from air leakage or a cold wall with the furniture holding moisture against it | Humidity in the house, the bathroom fan terminations, the poly at the outlets. |
| Paint bubbling on an exterior trim board | Water trapped behind the trim; no Z-flashing over it | Press the trim. Soft is the answer. |
The $90,000 at the door
The pitch goes like this. A crew is “doing a house nearby”, they noticed your siding, they will do a free inspection. A man with a pinless moisture meter waves it over the wall, it beeps, he says there is hidden moisture damage behind the siding and the whole house has to come off before it gets into the framing, and he can do it this month for $90,000, or $84,000 if you sign today. Sometimes there are photographs of someone else’s rotten wall on a tablet. Sometimes he has pulled a panel off and shown you a dark patch of sheathing, which is what sheathing looks like after thirty years in the dark whether or not it is wet.
Here is what is true in that pitch. Hidden moisture damage behind siding exists, I have repaired plenty of it, and the man may even have found a real wet spot. Here is what is not. A pinless meter on the outside of vinyl reads the vinyl, the air gap and whatever is behind it as a blur; it cannot tell wet sheathing from a metal flashing, a foil-faced foam or a damp morning. A whole-house tearoff is almost never the repair for a problem that lives at two windows and a roof corner. And the number: re-siding my standard 2,400 sq ft two-storey in mid-grade vinyl, with every square foot of sheathing replaced and every window reframed, which has never once been the case on a house I have opened, comes to about $41,000 before HST. The arithmetic is below. $90,000 is not a repair estimate; it is a test of whether you will sign.
Ontario gives you ten days to cancel a contract signed at your home, in writing, no reason needed, and the door-to-door siding page goes through that and the rest of the pitch. Use the ten days to get the diagnosis below.
What a real diagnosis looks like
- Start inside, with the stain. Where is it, which wall, what is directly above it outside? A stain maps to a detail. A pin meter on the drywall and the baseboard gives a number: wood or gypsum under 15% is dry, 15 to 19% is damp and worth watching, above 20% is wet and above 28% has been wet long enough to be rotting.
- Walk the outside with the list from the top of this page. Kick-outs, head flashings, sill condition, ledger, grade, downspouts. Most real problems are visible from the ground once you know what you are looking for.
- Probe, do not wave. A real moisture inspection puts two pins through the siding into the sheathing at the suspect spots, below window corners, at the roof-wall corner, at the bottom plate, and reads the sheathing itself. Small holes, sealed after. That is the only number that means anything, and it is what a $90,000 pitch never includes.
- Open one spot. Pull three courses of vinyl under the worst reading. Twenty minutes, and the vinyl goes back on. Now you are looking at the sheathing, not guessing at it.
- Write down what was found and where, with photographs and the meter readings in them. That is your scope. A siding quote based on this is a repair; a siding quote based on a beep is a sale.
A proper exterior moisture inspection with probes, by a home inspector or building envelope consultant who owns the tools, runs about $400 to $900 for a house, more for a stucco or EIFS house that needs twenty or thirty probe locations. The moisture inspection page covers who does it, what the readings mean and when it is worth paying for. On a stucco house the questions are different and so is the test; moisture behind stucco is that page.
What the repair actually costs
The common case: two windows and a roof corner on one wall. Strip 400 sq ft of vinyl at $2.00, $800. Sheathing found rotten at two window sills and the roof corner, 64 sq ft at $6.50, $416. Framing repair at the two openings, sill plates and cripples, two at $700, $1,400. New wrap on the opened wall, 400 at $0.85, $340. Flashing kits at the two windows, $170, and a kick-out, call it $50. Vinyl back on, 400 at $9.50, $3,800, and if the old colour is discontinued the whole wall gets new vinyl so it matches, which this already assumes. Bin $550. Total $7,526 before HST, $8,504 with. That is the repair I do most often, and the house that got the $90,000 pitch usually needed this.
The bad case: the whole house. Say the inspection finds trouble at most openings and the siding is at the end of its life anyway. Tearoff 2,400 sq ft at $2.00, $4,800. Sheathing replaced over a quarter of the wall, 600 sq ft at $6.50, $3,900. Framing repair at six openings at $900, $5,400. Wrap 2,400 at $0.85, $2,040. Flashing kits at 20 openings at $85, $1,700. Mid-grade vinyl 2,400 at $9.50, $22,800. Bin $550. Total $41,190 before HST, $46,545 with. That is a house with a quarter of its sheathing gone and six rotten window frames, which is a house with a story, and it is still less than half the number at the door.
Changing the cladding while it is open. If the vinyl is coming off anyway, this is the day for fibre cement or foam; the siding cost page has every option by grade and the same house worked in each. The repair lines above do not change.
Questions people ask
How do I know if there is water behind my siding?
Stains or soft drywall inside at window corners, below sills or under a roof-to-wall corner; a musty smell on an outside wall; bubbling paint on exterior trim. Confirm with a pin meter through the siding into the sheathing at those spots, not a pinless meter waved over the outside.
Is hidden moisture damage behind siding real?
Yes, and it is usually at two or three details: a window with no sill pan or head flashing, a roof corner with no kick-out, a deck ledger. What is rarely real is a whole house of it. A proper probe inspection, $400 to $900, tells you which you have.
Can a moisture meter read through vinyl siding?
A pinless meter reads a blur of the vinyl, the air gap, any foil or metal and the sheathing together, and it beeps on all of them. It is a screening tool at best. A pin meter pushed through the siding into the sheathing reads the sheathing. That is the number that matters.
What does it cost to fix water damage behind siding?
About $7,500 before HST for the common case, two windows and a roof corner on one wall, including re-siding that wall. About $41,000 for the rare whole-house case with a quarter of the sheathing and six window frames replaced. Sheathing is $6.50 a square foot, framing repair $400 to $900 per opening, vinyl $9.50 back on.
Someone at my door says I have $90,000 of damage. What do I do?
Do not sign. If you did, Ontario gives you ten days to cancel in writing. Get a probe inspection from someone who is not selling siding, read this page against what they found, and price the repair from the scope, not the beep.
Price the repair from the scope
Once you know which wall and which openings, the exterior cladding cost calculator prices the tearoff, sheathing, framing, wrap, flashing and new siding for just that wall or the whole house. If you already have a quote with “hidden moisture damage” on it and a big round number, the $99 review will tell you what is actually in it.
Sources and further reading
- Your rights when starting home renovations or repairs: Ontario government: the 10-day cooling-off period on a contract signed at your door, the written contract rule, estimates, and the door-to-door rules.
- Addressing moisture and mould in your home: Health Canada: what to do when the sheathing under the siding turns out wet.
- Canada Mortgage and Housing Corporation: the Canadian wall-assembly and moisture research behind rainscreens and the second plane of protection.
