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Rebuilding a Rotted Deck on a Flat Roof

Decks, Outdoor Living, Porches and Gazebos May 22, 2013 Sonia

We decided to span the entire screen porch with temporary 2X12 framing, nailing it into the vertical support posts.

This accomplished two goals: It tied the posts together by supporting them while allowing us to place ladders all around the screen porch.

Once we were able to stand at chest height, we could then access the areas in need of repair and focus on removing the rotted rim joist.

8 Installing deking 2 Rebuilding a Rotted Deck on a Flat Roof

After sheathing and trim boards were installed, the deck boards came next.

7 Deck frame with rubber 3 Rebuilding a Rotted Deck on a Flat Roof

After new sheathing was installed, the roof was covered with a single sheet of EDPM rubber roofing. Then, Tapered 2x joists were installed to accommodate the slope of the roof.

Repairing the Rot

In order to repair the rotted ceiling joists we needed to cut out all of the rot and apply new framing material against the old material. This is called “sistering.”

Note: Sistering a joist means attaching a second joist to the side of the damaged joist. It can also involve sandwiching the old joist between new material on both sides. Typically this involves framing lumber, but it could involve engineered lumber, structural steel or formed steel joists that are made of heavy-gauge sheet metal.

Sistering the ceiling joists was the most cost-effective solution because these joists were supporting a finished tongue-and-groove screen porch ceiling below that was in great shape.

Sistering joists can usually be difficult to do. While a certain size of lumber may physically fit in beside the old joist, getting the new board into position usually poses serious problems, due to walls, ceilings, and floors that were added after the original joists were dropped into place.

When sistering, try to go full length to support the ends of the new material being added. If that is not possible, attempt to get your joints as far from the middle of the span as possible. Glue, nail and bolt or lag the new material to the old material.

10 deck boards cool pic 2 Rebuilding a Rotted Deck on a Flat Roof

To prevent penetrating the roof membrane, the deck’s rail system was designed with post sleeve supports, which fasten onto the decking and into built-up blocking in the joist bay.

9 Install decking TIP using speed square to space boards Rebuilding a Rotted Deck on a Flat Roof

The mahogany deck boards were installed with stainless steel screws and spaced 1/4″ apart.

In this situation we removed all of the rot and were able to slide full-length ceiling joists into the ceiling bay. These new ceiling joists would be directly nailed as well as mechanically supported and connected to the new double rim joist with galvanized joist hangers.

11 Ripping 4x4 posts to fit in post sleves Rebuilding a Rotted Deck on a Flat Roof

Some of the width had to be ripped off the 4×4 posts so they fit into the post sleeve supports.

To sister the joists together we applied a liberal amount of construction adhesive and used a few 3-in. nails to fasten them to the old joists. We then went back and installed two 3-in. Truss Lock structural lag screws every 12 inches into the old joists where we had access.

We slid on the joist hangers before installing the outer rim beam, leaving the hangers loose inside the joist bay. There was no access below to add these hangers later.

13 Deck stain 6 Rebuilding a Rotted Deck on a Flat Roof

Before completing the rail system, exterior wood stain was applied while the crew had unobstructed access to the decking

12 Leveling 4x4 posts Rebuilding a Rotted Deck on a Flat Roof

The posts were plumbed and shimmed as necessary.

After the joists were completed we installed the first of the outer double rim joists and directly nailed the rim joist to each new joist end with three 16-penny nails.

15 Assembling railing 3 Rebuilding a Rotted Deck on a Flat Roof

The PVC post sleeves were trimmed to height on a miter saw.

14 New PVC fascia and crown Rebuilding a Rotted Deck on a Flat Roof

New PVC fascia and crown molding were installed for a decorative enhancement.

The second outer rim joist was installed, allowing us to then fasten the joist hangers with 16d nails. The rim joist header was then attached to the screen porch support posts with long structural screws.

Note: A flat roof must be strong enough to support 55 lbs. per square foot. Our roof was certainly that strong when originally built, and I’m confident that our repairs made it even stronger.


Sheathing and Trim

We reinstalled 3/4-in. CDX ply-wood sheathing over the joists with construction adhesive and coated decking screws.

Next, in order to repair the outer rim joist we had to remove trim boards and crown molding.

We decided to replace all of the trim and crown molding with Azek PVC trim boards and PVC crown molding, filling the nail holes with PVC Bond and Fill filler. PVC will not rot from weather exposure.