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Building A Roof Deck

Building A Roof Deck


Roof Deck Construction

This article, Building a Roof Deck was inspired after 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 ¼’ 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 for yourself 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 to 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 as the water damage stopped at the house rim joist. Once the framing and ½” plywood sheathing was replaced we installed a new maintenance-free Pella window and covered the entire house corner with ice and water shield.

Note: Ice and water shield is a self-adhering membrane that is mostly used as roofing under-layment 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 Wolf PVC trim and sided the wall with pre-primed cedar clapboards. Care was taken to prime the end grain of the clapboards; install rubber flashing at all vertical seams; and to use only stainless steel fasteners. We were looking for a long term fix here.

Dealing with the Gutters:

The half round galvanized gutters on this house were probably sixty-year-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 as a trough to channel the water from the downspout across the flat roof and out at the roofs edge where a lower copper gutter brought it to the ground. We believed that keeping the upper roof water off the flat roof would prevent future problems. Once the house and gutter situation was addressed we turned our attention to the old rubber roof.

Repairing the Water Damaged Roof Framing:

When walking on the rubber roof deck 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 three feet of ¾” 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 eight feet of the sixteen foot 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 rotted the ceiling joists were 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 we wanted to support the ceiling joists and our body weight as we walked above and repaired the rot.

Accessing the Outer Rim Joist To Replace It:

In order to access this repair and 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 this 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 2 x12 framing, nailing it into the vertical support posts. This accomplished two goals. It tied the posts together by supporting them and allowed 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.

Removing 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 material to the old material.

Reinforced Framing

In this situation we removed all of the rot and the rotted outside double rim joist 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” nails to fasten them to the old joists. We then went back and installed two 3” Truss Lock structural lag screws every 12” into the old joists where we had access. We slid on the joist hangers on 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 sistered we installed the first of the outer double rim joists and direct nailed the rim joist to each new joist end with three 16 penny nails. (too many joists in this paragraph – not sure it makes sense, or says what you want it to) The second outer rim joist was installed and nailed on allowing us to then install the joist hangers with 16d nails. The rim joist header was then attached to the screen porch support posts with long timber lock 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.

Replacing the Roof Sheathing:

We reinstalled ¾” CDX plywood over the joists with construction adhesive and coated decking screws.

Repairing the Trim:

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.

Installing The New Rubber Roof:

Considering that most roof decks leak due to seam or flashing failure, most often due to seam adhesive break-down, we chose to install a full sheet of EDPM [ethylene propylene diene monomer (M-class) rubber] with no seams.

Regardless of the roofing 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 you use care when sealing the seams.

Tip: Use a single sheet of membrane if possible in order to avoid having any seams. On this project we installed an EDPM rubber membrane over the top of the roof deck and installed it 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 French door.

Using 2×4 stock allowed us that very small but necessary step down from the master bedroom door onto the deck. In my neck of the woods I would have preferred a six or seven inch step down to keep drifting snow and sloshing water away from the door’s threshold.


The 2×4 “sleeper” or joist was left at 3-1/2” at the far end and taper cut to 2” 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 left over EDPM rubber roofing material to separate the deck joist from the rubber roof and protect the roof rubber from abrasion. We also installed Vycor, a thin rubber self-adhesive membrane, to the tops of the joists to protect the pressure treated deck joists it from rain, snow melt and organic debris, all of which could eventually rot the joist.

Using screws we attached a constructed joist every 16” 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.

Plan for rain and avoid roof leaks!

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.

Installing the Post Sleeve Supports:

The galvanized, steel surface mount, post sleeve supports are designed to be mounted over the decking and bolt through a built up blocking in the joist bay below. Once installed I was impressed at how they strong and secure they were.

Having a place to stand – Installing the decking:

All of the materials chosen for this deck are low maintenance except the decking itself. The homeowner wanted 1×4” mahogany decking. So we purchased all long length deck boards to eliminate any joints. Spacing of the deck boards was important. Some contractors might space deck boards too close and then once they swell they don’t drain snow or rain effectively and eventually there will be a nasty build up of organic materials between them.

I aim for a full 1/4” spacing between the boards – especially since this is a roof top deck. We used the thicker portion of our speed squares to provide a consistent space. To fasten the boards we used 2-1/2 inch stainless steel screws for most of the deck and then had to reduce to 2 inch stainless steel screws because the tapered joists reduce in size as they get closer to the house.

Installing 4×4 PT Posts:

Before we could install our 4×4 pressure treated posts into the surface mounts we had to trim all four sides on the table saw to get them to fit properly. It’s important not to have a fit here that’s too tight because the PT wood can swell and crack the post sleeve.

Azek Post Sleeves

Once the 4×4 posts were installed we checked and shimmed for plumb and then installed 5 x 5 AZEK post sleeves. The homeowner choose a post sleeve and railing system from AZEK Building Products for it’s low maintenance.

Choosing to use the 5” 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 x 4 post. The design of the new 5 x 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 x 5 Rail Post Sleeve, we used a decorative pyramid and island post caps and post skirts. We used small stainless steel screws and acrylic caulking to hold them in place.

Coat The Deck Before The Railings Go On:

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 allows moisture to pass in and out of the wood allowing it to breathe. The stain contains Brazilian Rosewood Oil, a strong, flexible, water and mildew resistant oil that is sustainable harvested from the seeds of the Brazilian Rosewood tree. No trees are ever cut to produce this oil. Rosewood Oil allows the wood to retain its flexibility and creates no surface film.

I also like this stain because it’s easy to maintain and resists cracking, bubbling and peeling. There will be no sanding required on next years application – just clean the deck and re-apply, a huge maintenance plus. I decided to apply the stain on the deck before the railings because it’s more efficient and 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 cutting in around the posts or under the bottom rail. A big time saver!

Railing Requirements:

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 in height 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 inch 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 Is 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 with in 8” of the deck to ensure 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, 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 it was to the earlier deck. The fewer nail hoes the better!

Finishing Up!

Once finished we caulked all joints and seams and did a thorough yard clean up, including running a magnet around the yard to capture nails from the demo work. For smaller projects like these we prefer to use a dumpster bag.

Using a flexible light weight bag that can instantly be transformed into a dumpster bag and perform just like a steel dumpster is changing the how I perform my remodeling services. It’s a perfect solution for small renovations or clean up jobs that are too small for a dumpster or where the location or the budget can’t accommodate a steel dumpster and its long term rental costs.

Before there were dumpster bags we would show up to a small repair project with trash barrels and then, depending on what we found, we might have to order a dumpster. Now instead of having to guess and scramble with unexpected repair and renovation debris, I just keep some dumpster bags in my trailer.

These super strong and durable dumpster bags are puncture and tear resistant. The best part of these bags 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 clean-up.

Read our article on Deck Safety – repair or replace

Building A Roof Deck
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