Porch & Deck Ledger Flashing to buildings - Construction Details to Avoid Leaks & Rot
InspectAPedia^®  -    

• 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
• Questions & answers about deck flashing to avoid building leaks, rot, damage

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 left).

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

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

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.

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.

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.

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:

• The door flashing pan is bent up and caulked under the door sill so that water will not be directed into the building by the flashing
• The door flashing pan extends outwards over the deck ledger just as the simpler flashing is shown in the sketch above
• The door flashing is constructed of copper or galvanized steel, or you use an adhesive flashing membrane in order to avoid corrosion through the aluminum flashing where it touches pressure-treated wood used for the building rim joist, sill plate, or deck ledger board.
• A water-tight flashing membrane (Moistop®) is set in place around the door sill and lapped over the metal deck ledger flashing discussed above
• The deck doorway rough opening is sealed with caulk as the door frame (jamb, header, sill) is set into the R.O.
• Building paper above and around the door opening should lap over the opening between the door jamb set and the rough opening.

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.

The 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.

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.

Given 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:

• an additional set of deck piers and posts to support the deck close to the building
• additional bracing below the deck to avoid racking (such as the 2x4 angle brace shown in the sketch).