Deck flashing errors & deck collapse & deck/balcony flashing repair advice: here we provide a case study of the effects of missing or improper deck connections and deck flashing details leading to structural movement, risk of imminent collapse, rot and building leaks at the deck ledger board and at the building to which a deck was attached.

We include an explanation & illustrations showing how to install or repair flashing at the intersection of a deck or balcony when water can not be allowed to pass below that structure.

Our page top photo shows a rotting, collapsing residential deck whose deck ledger was separating from the building, threatening imminent collapse. Readers will also see that the deck railings were unsafe (open) and that no building flashing was installed.

This residential deck was very unsafe and should not be walked-on nor should people stay below it as it could collapse at any moment. This photograph and other photographs and text in this article were provided by ASHI home inspector David Grudzinski.

Dangerous Rot & Collapse Risk When a Deck is Not Properly Flashed

This where most of the water enters the house [blue arrows]. It is a narrow section of porch that is fully exposed to the elements. We are in [deleted], so the weather is not extreme.

In my posts, I proposed

1. removing two to three rows of siding
2. and cutting back to the OSB.
3. Then using a flashing membrane to cover the gap.
4. The Tyvek would overlap the membrane.
5. The foam, strapping, and siding would be reinstalled.

Whatever material I used would need to be durable and capable of adhering to OSB and cement.

I am less concerned about the rest of the porch. The overhang from the second story does a pretty good job of keeping things dry below.

This photo shows the area directly underneath the short section of front porch. The concrete slab rests on metal pan material. Most of the water enters where you see the light fixture [red arrow].

Reply:

For this installation to work it looks as if you need L-shaped flashing that extends up behind the lower course of siding (and under any housewrap) for its vertical portion, and that extends out onto the upper (exposed) surface of the concrete walking surface (that forms a roof over the area below).

[Click to enlarge any image]

1. Remove the lower two courses of siding - with care so as to permit re-use. You may need to remove more siding for working space.

2. Clean and seal the concrete to house wall juncture with high quality silicone caulk.

3. Slice the housewrap if needed, horizontally, then install L-shaped aluminum or copper flashing along the exposed length of this patio-to-house- abutment.

I'd like to see 6" or more of vertical height, higher as needed if you have deep snow in your area.

The bottom of the "L" shape should be slightly open - that is bent to less than 90 degrees so that when you install the flashing onto the sidewall the bottom flashing base that is in contact with the concrete surface will be snug and neat (and will of course cover the caulk in step 2).

I'd like to see 3" of horizontal L-flashing along this surface.

This installation will redirect any wall leakage (such as wind-blown rain penetrating the siding) out on top of the concrete rather than behind it and into the space below.

Watch out: this repair will only work if the house wrap and window flashing higher on the wall are all properly installed.

Provided the concrete it itself sloped down away from the house this design should work well.

Send me photos of the job as described if you can and I may be able to comment further.

Watch out: even with this repair, it is important that the ground surface also drains away from this structure or you may find water ultimately leaking into the underground storage area through its outer foundation wall.

See the sketch above. I note that the old, improperly-routed flashing (that sends water below the concrete slab and into the storage area) can be left in place as the combination of (presumably) intact housewrap and properly placed L-flashing will direct any water penetrating the wall back out onto the top of the concrete walking surface.

And it'd be hell trying to get the old improper flashing out anyway.

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

Question: Cause of severe rot at 5 year old deck rim joists

Please see the attached photos of band board/joist rot found on my deck that is only 5 years old!

The boards were pressure treated, however, I did not receive a warranty. The contractor is claiming ants were the problem and is taking no responsibility. Orkin has inspected and contradicted this assessment.

The contractor used composite stair boards with a scalloped backside as the fascia.

It appears to have trapped water, which is my assessment. What are your comments?

I have to get my deck replaced and don’t want this to happen again. Any comments would be helpful. - Anonymous by private email 2020/10/02

Moderator reply: that's gross wood rot, not insect damage

that is not carpenter ant damage it's brown rot - a typical wood rot.

"Rot" is in essence a fungal attack, not an ant attack.

Yours is characterized by both the roughly rectangular cracks in the softened wood and the dark gray color.

Something is very odd here. I see the white painted end of boards under these rim joists in your first photo - no obvious rot in what's visible there.

I infer that you have removed boards that previously covered these rim joists.

It also looks as if your deck is quite close to ground and that tongue-and-groove decking using composite lumber means that there will be no air flow or ventilation of the framing supporting this deck. Water that leaks through the deck surface to wet the framing below is going to stay trapped there, risking shorter life and rot of the deck joists and girder, not just the rim joist.

There can be no doubt that whatever was covering the rim joists trapped moisture therein.

The combination of trapped moisture and probably not actually treated wood but some other lumber has produced the extreme rot deterioration we see in the photos.

"Treated" wood is a bit vague; But the "sharp" edges of the lumber don't look like typical pressure-treated 2x lumber commonly sold by building suppliers.

So there were probably two errors: using improperly or inadequately treated wood or even the wrong type of lumber as rim joists and the covering of that with material that trapped water.

I've seen this sort of rot on other decks that used synthetic lumber

eg at COMPOSITE LUMBER DURABILITY

If you agree we can post this discussion and photos (our default is to keep you anonymous unless you ask otherwise) at DECK FLASHING LEAKS, ROT - the article above on this page.

where we may invite further reader comment that will be useful.

You might want to also see the Rot Damage and Preservative Treated Lumber articles cited below at our RECOMMENDED READING links.

Reader follow-up:

Thank you for your reply. The deck boards and stair boards were composite lumber with a plastic coating. I have attached photos of the trim board that the contractor used to wrap around the rim joists. It was stair boards. As you can see the back side was scalloped.

Two boards were installed horizontally around the joists therefore the channels ran horizontally. I suspect, where there is rot, the top row of screws were not as tight as they should be and water was let in. It collected in the channels and soaked into the wood.

Also, since my first email two contractors have come out to give me quotes to repair and we’ve looked closely at the boards underneath and we believe that none of the wood was treated lumber.

I can’t believe I did not notice that when it was installed. I think I was just so happy to see how good this deck looked compared to the old rotting wooden deck that was there before. The contractor’s drawings state these boards would all be treated lumber and the drawings were approved by my city for the permit.

The city also inspected and approved the completed project and did not note the wood was not treated either.

I have learned a big lesson to pay more attention and check everything. You can use my photos and this discussion, however, please keep me anonymous.

Moderator reply:

I agree that this does not look like treated lumber - for the rim joists;

About the fastening of the horizontal trim to the rim joists, no amount of tight nailing or screw-fastening would guarantee to keep the area behind those boards dry; flashing could but isn't normally used;

Better deck design and construction to avoid rot would have included:

• Run the deck floor boards out past the rim joist and trim by about 3/4" or more to give a drip edge. As installed rain or melting snow are directed behind the trim board. With an overhang past the band joist or rim joist you don't normally need a drip edge flashing around the deck except where the deck abuts the building and the ledger board there.
  
  See DECK FLASHING - Best Practices Guide - for details of ledger board flashing
  
• If using a fascia board on the rim joists, secure the trim with a deliberate gap or drainage plane-path so that water getting behind it can get out and the rim joists can dry.
• The deck framing rot problem is exacerbated by the use of synthetic boards as trim as those are pretty-much waterproof, holding the maximum moisture behind the board and against the un-treated joists.

Other factors that shorten deck life and increase the rate of rot include lack of ventilation: a deck built close to ground and deck boards butted too closely together. See DECK BOARD GAPS & SPACING in the recommended reading below.

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Continue reading at DECK FLASHING - Best Practices Guide, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

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• FIVE MOST COMMON BUILDING DEFECTS in PORTUGAL - one of which is balcony leaks
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• DECK CONSTRUCTION BEST PRACTICES - home
• DECK FLASHING - Best Practices Guide
• DECK FLASHING LEAKS, ROT
• DECK & PORCH CONNECTIONS: LEDGER BOARDS
• DECK & PORCH CONSTRUCTION - home
• PRESERVATIVE TREATED LUMBER
• ROT, FUNGUS, INSECT DAMAGE
• ROT, TIMBER FRAME for tools and approaches to assessing the extent of wood timber rot or insect damage.

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