Rebuilding a Rotted Deck on a Flat Roof
By Rob Robillard
A client recently asked if I would replace her badly worn-out roof deck. This wooden deck was built over her screen porch and on top of a flat roof.
It’s not really flat: It’s pitched 1/4 inch per foot, but without a spirit level you can’t really tell.
Years of neglect and lack of gutter coverage above the deck had destroyed not only the deck but a window and the sidewall as well.
The Value of a Roof Deck
Besides being a great place to take the love of your life for a romantic interlude, flat roof decks are very popular among homeowners because you can use your otherwise unused roof space to create a private retreat and increase your outdoor living space. Many condos and apartment buildings have roof-top decks, patios and balconies instead of backyards or ground-level patios and decks.
First Things First: Replace the Rot
First order of business was to remove the old deck and repair the rotted house wall and replace the window in the same process. The window was in a corner where two roofs met and formed a valley. Water runoff from the roof valley was overshooting a damaged gutter and soaking the window. Water eventually entered the window sill area and rotted the 2×4 sill-framing and plywood sheathing behind the cedar clapboards.
Luckily we only had to replace a few 2×4 studs and window framing since the water damage stopped at the house rim joist. Once the framing and 1/2-in.
plywood sheathing was replaced, we installed a new maintenance-free window and covered the entire house corner with Grace Ice and Water Shield.
Note: Ice and Water Shield is a self-adhering membrane that is mostly used as roofing underlayment because of its ability to provide leak protection for sloped roofs and to resist water penetration due to water back-up (ice dams).
We trimmed the window with PVC trim and sided the wall with pre-primed cedar clapboards. Care was taken to prime the end grain of the clapboards; to install rubber flashing at all vertical seams; and to use only stainless steel fasteners. We were looking for a long-term fix.
Repairing the Water-damaged Roof Framing
When walking on the roof there were obvious soft spots, and you could see that some of the seams were opened a bit. We were unsure what we would find when we peeled back the rubber, but fortunately the water damage was concentrated mostly at the low end of the roof pitch, near the outside wall.
We found that pretty much the entire last 3 feet of 3/4-in. plywood roof sheathing was rotted and needed to be removed.
At this point I was hoping that we only had rotted plywood but experience told me that was wishful thinking.
Rather than cutting we simply pulled up the plywood back to the nearest seam, using nail pullers to remove the nails.
Once exposed we saw that about 8 feet of the 16-ft. outside structural beam was rotted. This beam or rim joist is supported by the screen porch posts below.
We continued to pull up plywood back towards the house corner, chasing rotted plywood and ceiling joists. We stopped when we got to solid framing, opting to continue a bit further to the nearest plywood seam.
Once all of the rot was exposed it was apparent that a prior repair had been attempted and not successfully so.
Supporting the Roof
Once we saw how badly the ceiling joists were rotted we stopped and built a temporary 2×4 wall underneath the roof and against the screen porch tongue-and-groove ceiling boards. Since the structural rim joist was rotted, this was necessary to support the ceiling joists and our body weight.
Accessing the Outer Rim Joist
In order to replace the outer rim joist and insert new framing into the joist bay, we needed to be able to actually stand on the outside of the roof.
Because all of the surface area beneath this roof is screening, access for ladders was difficult, and staging would take too much time and effort.
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.
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.
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.
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.
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.
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.
Installing the New Rubber Roof
Considering that most roof decks leak due to seam or flashing failure, most often due to seam-adhesive breakdown, we chose to install a full sheet of EDPM with no seams. EDPM is ethylene propylene diene monomer (M-class) rubber. Using a single sheet of membrane without seams is the best protection against leaks.
Regardless of the material you choose for your roof deck, proper roof slope should be added either during construction or, in post-construction, using a tapered (sloped) insulation system. I recommend using the thickest membrane and carefully sealing the seams.
On this project we installed the EPMD rubber membrane over the roof deck and up the side wall of the house, 18 to 22 inches.
Framing the Roof Deck
Because this roof was slightly pitched, we determined that we could use 2×4 pressure treated lumber for our deck joists and then frame the deck level while still leaving an acceptable step down from the master bedroom’s French door.
Using 2×4 stock allowed us that very small but necessary step down from the bedroom door onto the deck. In my neck of the woods I would have preferred a 6- or 7-in. step down to keep drifting snow and sloshing water away from the door’s threshold.
The 2×4 “sleepers” or joists were left at 3-1/2 inches at the far end and taper-cut to 2 inches at the house end. We snapped lines with a chalk line and cut them with a circular saw.
We installed these tapered “sleeper joists” on top of scrap strips of leftover EDPM material to separate the deck joist from the rubber roof and protect it from abrasion.
We also installed Vycor, a thin rubber self-adhesive membrane, to the tops of the joists to protect the pressure-treated lumber from rain, snow melt and organic debris, all of which could eventually rot the wood.
Using screws we attached a constructed joist every 16 inches on center with a free-floating ledger board and a double rim joist.
This roof deck frame is not fastened to the house wall and sits on top of the flat roof. Weight alone holds this beast down; only the railings attach to the house.
Avoid Roof Leaks with Post Sleeve Supports
From a waterproofing standpoint, we chose not to penetrate the rubber roof membrane with our posts. Some contractors achieve this by bolting their posts to the outside rim joist but that would have ruined the crown molding and copper gutter detail.
We took a different approach and purchased Azek’s post sleeve connectors. The galvanized steel surface-mount, post sleeve supports are designed to be mounted over the decking and bolt through built-up blocking in the joist bay below.
Once installed, I was impressed at how strong and secure they were.
Installing the Decking
All of the materials chosen for this deck are low-maintenance in nature except the decking itself. The homeowner wanted 1×4 mahogany decking. So we purchased deck boards long enough to eliminate any joints.
Spacing of the deck boards was important. Some contractors might space deck boards too closely, and then once they swell they don’t drain water effectively and eventually there will be a nasty buildup of organic materials between them.
I aim for a full 1/4-in. spacing between the boards—especially since this is a rooftop deck. We used the edge of our Speed Squares to provide a consistent space.
To fasten the boards, we used 2-1/2-in. stainless steel screws for most of the deck and then had to reduce to 2-in. stainless steel screws because the tapered joists reduce in size as they get closer to the house.
Installing 4×4 PT Posts and Post Sleeves
Before we could install our 4×4 pressure-treated posts into the surface mounts we had to rip the sides on the table saw to get them to fit properly. It’s important not to have a fit that’s too tight because the PT wood can swell and crack the post sleeve.
Once the 4×4 posts were installed we checked and shimmed for plumb and then installed 5×5 post sleeves. The homeowner chose a decorative PVC post sleeve and railing system from Azek Building Products for its low-maintenance requirements.
Choosing to use the 5-in. rail post sleeve allowed us to achieve a highly durable, sleek, consistent looking post. It also allowed us to architecturally mimic the look of a traditional wood-trimmed 4×4 post.
The design of the new 5×5 post sleeve contains specially located inner “chambers” that allow the sleeve to stay in place securely while allowing a traditional wood post to expand.
We plumbed the post sleeves against the 4×4 posts with shims and were then ready to install the railing system.
Note: To finish off the new Azek 5×5 post sleeve, we used a decorative post caps and post skirts. Small stainless steel screws and acrylic caulking held them in place.
Coat the Deck Before Railing Installation
We cleaned the deck and applied a coat of oil-based Penofin Exotic Hardwood Stain.
I prefer Penofin because it penetrates the wood’s surface and permits moisture to pass in and out of the wood allowing it to breathe. The stain contains Brazilian Rosewood Oil, a flexible, water- and mildew-resistant oil that is sustainably harvested from the seeds of the Brazilian Rosewood tree. No trees are ever cut to produce this oil. It’s easy to maintain and resists cracking, bubbling and peeling. There’ll be no sanding required on next year’s application—just clean the deck and re-apply.
I decided to apply the stain on the deck before the railings because it saves time and labor. Without the railings in place, I was able to apply stain to the entire edge of the deck from the deck itself and not from a ladder on the outer perimeter. This cut at least an hour from my time and by not having the railings in place I didn’t have to worry about trimming around the posts or under the bottom rail.
The beauty of the Azek system is that the baluster spacing conforms to the International Residential Code (IRC) calling for on-center baluster spacing to be less than 4 inches.
The IRC also requires that a guardrail be 42 inches high and the entire assembly be strong enough to resist a 200-pound horizontal load (or 20 pounds per linear foot, depending on the local code).
The Azek railing system meets this requirement and can be ordered so the balusters do not need to be cut to reach the 42-in. requirement. By eliminating the need to cut the balusters we saved time and labor.
Note: Check local code requirements prior to installation. Most building codes require that a 4” sphere shall not pass though the rail at any point.
The Devil in the Details
We spent time making sure the siding and trim were installed properly by flashing all of our joints and not nailing anywhere within 8 inches of the deck, which ensured we did not penetrate the rubber roofing that ran up the side wall.
One detail that we changed from the original design was to run a horizontal trim board—called a skirt board—along the deck.
This board allowed us to install our fasteners up high as opposed to installing and fastening clapboard siding down to the deck, as was done to the earlier deck. The fewer nail holes, the better!
Once finished we caulked all joints and seams and did a thorough yard cleanup, including running a magnet around the yard to capture nails from the demo work.
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These super strong and durable dumpster bags are puncture- and tear-resistant. The best part is that they fold up to the size of a small tarp and store in my trailer for those situations when I need a quick solution for job-site cleanup.
Editor’s Note: Visit Rob Robillard’s website at www.aconcordcarpenter.com.
Dealing with the Gutters
Water damage had caused this roof to rot, which meant we needed to address the drainage situation. The original half-round galvanized gutters on this house were probably 60 years old. They had holes and were not pitched adequately. The homeowner opted to replace them with half-round copper gutters.
We cut a Schedule 20 landscape drain pipe in half and used it beneath the deck surface as a trough to channel the water from the downspout across the flat roof and out to the roof’s edge where a lower copper gutter brought it to the ground. We reasoned that keeping the upper roof’s water off the flat roof would prevent future problems.