Wilmington’s mapped ground snow load of 20 psf is the number that shows up on truss calculations across northern Delaware, but it never lands on a rooftop unchanged. Code converts that ground figure into roughly 14 psf for an ordinary heated, sloped roof, and that converted number is what your permit application and truss drawings actually use. Ask your county building department which code edition it enforces before you assume that figure applies to your address.
What is the ground snow load in Delaware?
The number and where it comes from
At Wilmington, the design ground snow load is 20 psf, drawn from ASCE 7-16, the edition that IRC/IBC 2021 makes applicable in most states. This is a ground-level figure, measured before it ever reaches a roof deck. It’s the value an engineer or truss manufacturer starts from when sizing rafters, and it’s the number a plan reviewer checks against on a permit submission for new construction or a major re-roof.
Ground load is not roof load
The code doesn’t hand that 20 psf straight to your rafters. It runs the ground figure through exposure, thermal and slope factors first, and for an ordinary heated sloped house roof, the result comes out at roughly 14 psf. A steeper roof shheds more and carries less. This gap between the ground map and the roof number is the single most common point of confusion on this subject, and it matters here because the two figures differ by nearly a third.
A newer edition, ASCE 7-22, maps the same Wilmington point at 38 psf. That is not more snow and it is not a sign your existing roof is suddenly undersized. It’s a strength-level value built on a 1.0 load factor rather than the 1.6 factor behind the 7-16 number, a different accounting method for the same physical load. A few jurisdictions are beginning to adopt 7-22, but the figure your permit and truss drawings use today is the 7-16 value. Delaware’s coastline and any higher ground inland can differ from the Wilmington reference point, so a building department outside New Castle County may work from a different local figure entirely.
How much snow can a roof hold in Delaware?
Why there’s no single number
There’s no inches-of-snow figure that answers this honestly, because it depends on what the snow has become, not just how deep it is. Fresh, light snow runs roughly 5 to 7 pounds per square foot for every foot of depth. Settled or wet snow runs two to three times that. A layer of ice on top of either adds more weight again in a much thinner slice. That’s why a foot of powder in January and a foot of heavy, wet snow in March put very different loads on the same rafters, even though a tape measure reads the same number both times.
Drift is where roofs actually fail
Working from the Wilmington ground figure of 20 psf, an ordinary sloped roof design load lands near 14 psf, but that’s an average across a flat, unobstructed roof plane. Real roofs aren’t flat and unobstructed. Snow blows off a lower slope and piles against a wall, drops off a taller adjacent roof onto a lower one, or collects in a valley between two roof planes. In those spots the local load can run well above the design average, and that’s the load that actually cracks a rafter or buckles a ceiling. A parapet, a dormer, or a rooftop unit all create the same kind of drift trap.
Watch for signs a roof is carrying more than it should:
- Interior doors that suddenly stick or won’t latch
- New cracks or popped nails in ceiling drywall
- A visibly sagging ridge line or roof plane
- Creaking or popping sounds from the attic under load
If you see any of those, rake snow off from the ground with a roof rake rather than climbing onto a loaded roof yourself. A structure already carrying more than it was built for is not a safe place to add your own body weight, and a licensed engineer or your local building department is the right call for anything beyond routine raking.
What wind speed must a roof withstand in Delaware?
What the number actually measures
At Wilmington, the basic design wind speed under ASCE 7-16 is 114 mph, and that’s not a sustained wind or anything a weather forecast would call the wind speed. It’s a 3-second gust measured at 33 feet in open (Exposure C) terrain, for Risk Category II buildings, which covers ordinary houses. ASCE 7-22 maps the same Wilmington point at essentially the same 114 mph, so this is one of the rare cases where the newer edition doesn’t shift the number much. Comparing this figure to a hurricane’s headline wind speed will make the code look weak when it isn’t measuring the same thing at all.
Coast versus inland, and what the number governs
Wilmington sits inland, and Delaware’s coastline carries a materially higher design wind speed along with its own wind-borne debris region requirements, which the Wilmington figure does not capture. Where a wind-borne debris flag applies, the code requires impact-rated glazing or approved shutters on new construction, a real cost with a real safety purpose behind it. Whether that flag applies at a given coastal address is something only the local building department can confirm from the current wind map, since it isn’t a single statewide answer.
On an ordinary roof, this design wind speed drives the shingle wind rating a manufacturer certifies, the nailing pattern a roofer follows, how the sheathing is fastened to the framing, and the uplift connections tying the roof structure down to the walls. Edges, rakes and ridges take the worst of it, because that’s where wind uplift concentrates as air peels around a corner or over a roof edge. A shingle rated for high wind speeds performs only as well as the nailing pattern it was actually installed with, which is why installation quality matters as much as the product spec.
Does Delaware require an ice barrier under the shingles?
What the membrane does and doesn’t do
An ice barrier is a self-adhering membrane run from the eave edge up past the interior wall line, and its job is narrow: stop meltwater that backs up behind an ice dam from working its way under the shingles and into the roof deck. It does nothing to stop snow accumulation itself and nothing to prevent the dam from forming in the first place.
Delaware’s coldest-month mean minimum temperature at Wilmington runs 25.6°F under NOAA’s 1991-2020 climate normals, cold enough for repeated freeze-thaw cycling through a typical winter. Combined with a design ground snow load of 20 psf at the same reference point, the conditions for ice damming, meltwater refreezing at a cold eave below a warmer roof plane, are real in this part of the state, even if they’re less severe than states farther north or at higher elevation.
Where the requirement actually comes from
Delaware has no statewide residential building code. State law leaves adoption and enforcement to each county, and only the energy code is set at the state level. New Castle County and Sussex County are reported, by a non-official source, to be working from the 2021 International Residential Code; Kent County and the state’s municipalities were not verified against any edition. Whether an ice barrier is required, and any local amendment to how far up the roof it must run, is a question your county building department answers, not a statewide code section.
An ice barrier protects the deck from a symptom. The actual cause of an ice dam is heat escaping from a heated attic into the roof space, melting snow from underneath even while the outdoor air stays below freezing at the eave. That meltwater runs down the roof plane and refreezes right where the roof overhangs the wall and stops being warmed from below, building the dam. Air sealing the attic floor and adding Insulation are what stop the dam from forming at all. A membrane alone, with no attic work behind it, buys protection against the water without touching the reason the water is there in the first place.
What roofing material suits Delaware best?
Matching material to the load
The honest starting point for this question is the combination already on this page: a 20 psf ground snow load and 14 psf roof design load at Wilmington, a 114 mph basic wind speed, and a single statewide energy code climate zone of 4A under the 2021 IECC, with no county exceptions. That combination rules some choices in and others out, but it doesn’t crown one material as universally best.
| Material | Wind performance | Snow behavior | Added dead load |
|---|---|---|---|
| Architectural asphalt shingles | Rated by manufacturer, only as good as the nailing pattern used | Holds snow on the roof rather than shedding it | Low, but layering over old shingles adds weight the design didn’t plan for |
| Standing-seam metal | Strong uplift resistance when properly fastened at seams and edges | Sheds snow in a sudden slide, sometimes with real force | Low |
| Slate or concrete tile | Heavy and wind-resistant, but the fastening system is what actually holds it | Holds snow similarly to shingles | Substantial, the material’s own weight adds to the snow load the structure must carry |
Standing-seam metal sheds a snow load well, which sounds like an advantage until that slide lands on a doorway, a walkway or a parked car below. Where a metal roof is being considered over an entry or a driveway, snow guards or a planned discharge path are a design decision that has to happen before installation, not an afterthought once the first slide happens. Slate and concrete tile carry their own dead weight into the structural calculation on top of whatever snow load the code already assigns, which is why a re-roof to a heavier material often needs a structural check that a shingle-to-shingle replacement doesn’t.
Where wind speed governs the choice, the fastening and underlayment matter as much as the material itself. A high-wind-rated shingle nailed to a standard pattern performs to that lower standard, not to its rating. The same is true of a metal panel with seams that aren’t engineered for the site’s uplift pressures. None of this is a substitute for a conversation with a licensed contractor or your county building department about what your specific roof, at your specific address, is required to meet. For a broader look at how these same questions play out in other climates, the Indiana’s roof snow load and wind rules cover a heavier interior snow load working against a similar wind design basis, while the Nunavut’s roof snow load and wind rules show what changes when the ground snow and cold load numbers run far higher than anything on the mid-Atlantic map. For the fundamentals behind any of these choices, our main roofing guide covers the groundwork this page assumes.