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.