Table of spacing for bolts or screws connecting deck to house (C) J Wiley & Sons, Steven Bliss Porch & Deck Ledger Flashing to buildings
Best Practices Construction Details to Avoid Leaks & Rot

  • DECK FLASHING - CONTENTS: Deck Flashing Details to avoid building leaks & rot. Deck & Porch flashing Materials Choices. Deck & Porch flashing location, connections, sealing methods. Deck & Porch flashing Details for Safety
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Flashing details for decks & porches:

This article explains critical deck flashing details for decks in order to avoid rot and leaks at the deck ledger board and at the building to which a deck is attached, and we provide deck or balcony design and construction details for waterproofing at door jambs and sills.

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Photos & Construction Details Show How & Where to Install Flashing between a Deck and the Building

Sketch of flashing details at a deck attachment to building (C) J Wiley Steve Bliss

Details of Proper Flashing & Waterproofing at a Deck or Balcony Ledger Board

[Click to enlarge any image]

As described in Best Practices Guide to Residential Construction, Chapter Four, Best Construction Guide for Building Decks and Porches:

Flashing details at the deck ledger connection to the building:

Proper flashing is critical since leakage at the band joists could lead to decay and failure of the deck connections.

The siding should be removed over the band-joist area, and a wide band of peel-and-stick membrane or metal flashing should run over the band joist and up under the building felt or housewrap.

A second cap flashing should direct water over the ledger and away from the house (Figure 4-8 at above left).

Some deck builders prefer to leave an air space at the ledger to assist with drying if the area gets wet.

Alternative: air-space deck or balcony ledger board

Flashing details for an air-spaced deck ledger are shown at left in Figure 4-9 just below.

Figure 4-9: Details of an air-spaced deck ledger (C) J Wiley, S Bliss

Watch out: Do not use aluminum flashings with pressuretreated lumber unless a durable barrier material, such as a bituminous membrane, separates the aluminum from contact with the wood.

Preferably use membrane-type flashings, G185 galvanized steel, or copper. However, copper flashings should not contact galvanized hardware or fasteners.

Details of Proper Flashing & Waterproofing at a Deck or Balcony Door or Slider

Rotted sill at balcony door (C) Daniel Friedman

Flashing details at doors opening onto decks & balconies: In addition to the copper or galvanized steel flashing shown in the sketch at left, additional flashing is almost certainly going to be needed below the door or sliding door that gives access to the deck from the building.

Our photo (left) shows a balcony door sill that has rotted away at a 20 year old sliding door opening onto a rooftop deck (the website author's).

While the peeling paint and rot at the bottom of the brick-molding door trim (photo left) shows that there has been little maintenance at this door opening, the rot below the door is due to omission of flashing of any sort when the door was installed.

The aluminum sliding door track was simply caulked to the pine door threshold.

Deck rot and flashing omitted (C) D Friedman, D Grudzinski

Our collapsing deck photo (left) illustrates how dangerous and serious rot and insect damage can be at a deck.

The omission of flashing may have combined with poor connections of the ledger board to lead to this falling deck.

Photo courtesy of David Grudzinski, Advantage Home Inspections.

Details about this deck collapse as well as about repairing wall-deck or wall-porch floor snafus are at

Best Practices Guide to Residential Construction explains:

Some contractors build their own custom door sill flashing on-site using a bending brake, but pre-fabricated sliding door flashings and water tight door pans for use at balconies and decks are available from building suppliers as well.


When installing flashing below the sliding or hinged door opening onto a deck, be sure that:

If deck flashing and door jamb sealing steps are omitted at this connection, failure of the deck ledger can be caused rot and decay in the ledger or band joist.

Lags or bolts provide little support when fastened to rotted wood. So proper flashing of the ledger and band joist area is critical.

Deck Collapse Case Study includes additional photos of improper connections between a residential deck and the building that led to a catastrophic deck collapse.

Specialty Deck Flashing Products

For deck flashing at ledgers, deck perimetrs, and sliding doors entering onto decks there are pre-fabricated specialty flashing products that can help assure a durable design (and are more reliable than betting on caulks and sealants alone).

Everflashing is a Minnesota company that produces stainless steel and aluminum deck flashing. A schematic of an Everflashing deck ledger flashing product is shown at below-left [click to enlarge], and a second photo (below right), also provided courtesy of Mark Morsching, Everflashing, shows a stainless steel ledger board flashing product from that company as well as other deck construction details.

Everflashing Deck flashing (C) Mark Morsching, Everflashing Deck flashing (C) Mark Morsching, Everflashing

Perimeter Flashing and girder top seal (C) EverflashingThe green "counterflashing" showin in the schematic at above left, may be installed using peel-and-stick flashing tape.

To avoid deck joist top rot or built-up deck beam rot, Morsching applies peel-and-stick flashing membrane material both in installing the deck ledger flashing, and importantly to the top edges joists (near the ledger board) and deck beam tops.

An example deck beam top sealing over the top of the built-up deck beam is shown in the image of Everflashing's perimeter flashing product (left).

Additional deck flashing images are also found at Deck Board Gaps, Spacing Guide where we describe protecting joist tops from rot.

Details on peel and stick flashing membranes are found at FLASHING MEMBRANES PEEL & STICK.

-- Deck flashing photos adapted and used with permission, Everflashing.

Leave an Air Space or Gap Between the Deck Ledger and the Building to Avoid Rot and Leaks

Deck ledger nails exposed (C) Daniel Friedman

Some deck builders prefer to leave an air space at the ledger to assist with drying if the area gets wet. Flashing details are shown in the figure at left.

Spacing the ledger away from the house helps prevent decay in the sheathing and band joist if the area gets wet, but this requires more bolts. A bituminous membrane across the band- joist area offers an extra layer of protection, useful in very wet or snowy regions.

Caution: Do not use aluminum flashings with pressure- treated lumber unless a durable barrier material, such as a bituminous membrane, separates the aluminum from contact with the wood.

Preferably use membrane-type flashings, G185 galvanized steel, or copper. However, copper flashings should not contact galvanized hardware or fasteners.

Also, when using a spaced ledger as shown in the sketch, the connection of ledger to building should be made using through-bolts to the building rim joist (as shown). Lag screws are not recommended.

Free-Standing Decks: an Alternative to Using a Deck Ledger to Attach a Deck to the Building

Free standing deck (C) J Wiley, Steve BlissGiven the leak and rot concerns inherent in supporting a deck with a ledger, one approach is to forgo the ledger altogether and support the deck on posts on all sides, keeping it structurally independent of the house.

In this approach, the deck may be braced to the house to help it resist racking, but all vertical loads are carried to the ground by posts, shown in the figure at left.

A conservative rule-of-thumb states that joists can cantilever one-fourth of their total length, assuming that the cantilevered end is not carrying any loads other than the normal uniform floor loading.

The cost of this deck design alternative will be higher than a deck attached to the building using the deck ledger board approach (above), because of the added labor and materials for:

-- Adapted with permission from Best Practices Guide to Residential Construction by Steven Bliss, courtesy of Wiley & Sons. Also see our BOOK REVIEW of that book.



Continue reading at DECK FLASHING LEAKS, ROT for examples of deck or porch rot and collapse due to improper construction and missing building flashing, and examples of how to repair leaks at the intersection of a deck, balcony, or porch wall and the deck or porch floor - or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.





Or see DECK DESIGN & BUILD - home

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DECK FLASHING at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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