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Rooftop decks & walk-on roofs:
This article discusses rooftop deck construction details to avoid building leaks or rooftop deck safety hazards.
We include walk-on roof surfaces, roofing membrane to use below a rooftop deck, and construction of rooftop deck railings and posts.Also see WALK-ON ROOF SURFACES.
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[Click any image to see a larger, detailed version]
Decks over living spaces can be detailed various ways, but all should have the following elements:
Single-Ply Membranes for Rooftop Decks
Our rooftop deck photo (above left) shows how the installer provided for future cleaning of debris accumulating below the nearly-flat roof deck. But it is the choice of roof membrane and how the deck panels are supported so as to avoid damaging the membrane that make a rooftop deck work ... or not.
Continuing as detailed in Best Practices Guide to Residential Construction:
A variety of roofing membranes have been used successfully under rooftop decks, but EPDM remains the most widely used on residential roofs.
EPDM owes its popularity to its longevity and durability. It is dimensionally stable and strong over a wide temperature range (it will not expand and contract or tug at flashings), is highly UV-resistant, and can stretch without tearing. In most residential jobs, EPDM is fully adhered to the roofing deck with a contact-type adhesive rolled onto both surfaces.
Seams can be avoided on small jobs, since rolls come as wide as 50 feet. If necessary, however, seams are relatively easy to seal, using a special double-sided seam tape and lap caulk at the exposed joint. To seal around corners and penetrations, use flexible uncured EPDM, which will conform to irregular shapes and seals well to the main roofing membrane (see EPDM, page 96).
EPDM roofing membranes typically come in .045- and .060-inch thicknesses and carry at least a 10-year warranty in residential work. If properly installed, however, the .060 material should last for decades. EPDM’s chief weakness is its vulnerability to petroleum products, such as oils, grease, and solvents.
If used under a deck, warn the homeowners not to splash oil or grease from cooking or solvent-based wood finishes on the decking.
EPDM is often installed over a base layer of fiberboard or rigid insulation, but most EPDM membranes can bond directly to a sanded wood underlayment such as a 1/4-inch AC plywood. If bonding directly to plywood or oriented-strand board (OSB), check with the roofing manufacturer regarding treatment of joints in the sheathing and the need for a primer.
To allow for some movement at sheathing joints, some manufacturers recommend holding the adhesive back one inch from the joints.
Also see WALK-ON ROOF SURFACES.
Terminations and Flashings for Rooftop Decks
Where the roofing membrane meets the house, run it 10 to 12 inches up the wall under the sheathing wrap. At outside edges, leave a 3- to 6-inch flap, depending on the edge treatment. A large metal drip-edge usually caps the roof along the fascia boards.
Deck Flooring Construction for Rooftop Decks
The decking sits on 2x sleepers, usually laid flat. If you want the decking surface to be level, the sleepers will need to be tapered to compensate for the slope in the roof deck. For a large deck where flat sleepers are too thin to taper, you can use 2x4s or 2x6s set on edge.
To protect the roofing membrane, set the sleepers on strips of EPDM or rooftop walkway matting (available from commercial roofing suppliers). Attach the decking with screws so it can be removed later for inspection or repairs.
The rooftop deck shown in our photo (left) is installed in a restaurant in Mexico City where it must endure very strong sun and intermittent very heavy rain (and an occasional earthquake).
Shading and sun protection are provided by canvas awnings. - DF.
Post and Railing Construction for Rooftop Decks
As with other decks, guardrails must be a minimum of 36 inches high, infill balusters less than 4 inches apart, and the whole assembly strong enough to resist a 200-pound horizontal load (or 20 pounds per linear foot, depending on the local code). Use the longest pieces available for railings.
If the deck is small enough, use a single continuous top railing on each side. Then tie the side railings to the house with steel angle brackets to create a rigid railing assembly.
From a waterproofing standpoint, it is desirable to keep the posts outside of the roofing membrane. This can be achieved by bolting the posts to the rim joists or sub fascia in the roof framing (see Figure 4-17 below).
[Click any image to see a larger, detailed version.]
If this is not practical or if it is unacceptable from a design standpoint, then the EPDM will need to be sealed around each post. This is best left to a professional roofer.
Typically, the roofer will fashion a metal or membrane flashing collar around the base of each post and seal this to the roofing membrane, as with the boots used for plumbing vents.
-- Adapted with permission from Best Practices Guide to Residential Construction.
Deck & Porch Resources, Products, Manufacturers: Where to Buy
See Deck & Porch Products, Manufacturers for our complete/updated listings of manufacturers of porch & deck products, materials, coatings, fasteners, lumber, tools.
Reader Question: minimizing sound transmission through a rooftop deck
(May 22, 2014) Jared said:
Sure Jared, we have a series of articles on noise transmission & control.
Take a look at
Rooftop Deck Height, Walkout Decks, Door Clearances
(June 16, 2014) rich said:
(9 January 2015) Anonymous said:
I have a question about a walk-out roof deck with composite decking and 2x sleepers floating on the membrane roof. For best practice, how much lower should the finished decking elevation be below a door threshold. Is there a IRC code to reference. Thanks for the help!
Rich, I can't say from just the details in your note.
5/4" lumber sleepers + a 2x6 (which is 1 1/2" in thickness) will give you approximately 2 3/4" of overall thickness or height gain over the existing roof surface.
Your design doesn't mention the use of any padding material that is often applied below sleepers to prevent them from damaging the roof surface.
IRC code on height of door sill to rooftop deck surface
Anon I'd pass the question to your local building department who may tell you what applies in your area. Interestingly some codes want an exit porch or walk to be the same level as the indoor floor - worrying about trip hazard. In my opinion that's a terrible design that invites leaks from rain or melting snow if you're in a snow area. Ideal would be a standard step height down - say 7" but less, even a couple of inches, can if properly flashed, at least address the leak risk at this location.
In a follow-up comment I'll paste in some relevant portions of the IRC on Exit doors:
IRC citations on exit door details:
Deck & Porch Wood Treatment Companies
Suppliers of Composite Structural Lumber for Decks & Porches
Plastic Decking Systems
Metal Decking Systems
Prefabricated Deck & Porch Railing Systems
-- Adapted with permission from Best Practices Guide to Residential Construction
This article series discuss best porch & deck construction practices, including choice of framing materials, decking or flooring choices & installation, how to select and use deck and porch structural and flooring fasteners, actual deck & porch framing construction details & connections, deck joist & beam span tables, how to build leak-proof rooftop decks, construction of covered & screened porches, deck & porch railing construction & materials, choices of finishes and stains for decks & porches, and past & current deck lumber preservative treatments with related health & environmental concerns. This article series includes excerpts or adaptations from Best Practices Guide to Residential Construction, by Steven Bliss, courtesy of Wiley & Sons. Also see our BOOK REVIEW of that book.
Continue reading at GUARDRAIL CONSTRUCTION, DECKS & RAMPS or select a topic from the More Reading links shown below.
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