Maryland Snow Load for Roofs: What Contractors Must Design To
Maryland roof snow loads vary by county. See how to find the ground snow load, calculate roof load and drift, and document it for permits on metal roofs.
There is no single snow load for Maryland. The ground snow load (Pg) that governs your roof is set by the county or city building department, and it can be higher than a national map suggests. In Prince George’s County it is 35 pounds per square foot. Other jurisdictions publish their own minimums, and the mountain counties in the west often need a site-specific determination. This guide is written for roofing contractors, builders and GCs who have to put a number on a plan sheet and get it through review.
TL;DR
- Start with the local jurisdiction’s adopted ground snow load, not a generic map. Prince George’s County states 35 PSF, and Baltimore County and Prince George’s both specify a 30 PSF minimum roof live load.
- Roof design load is not ground load. Exposure, thermal, importance and slope factors reduce or raise it, and drift can add more.
- Plan reviewers reject notes that lack the actual coefficients (Ce, Ct, Is) and drift surcharges. Do not leave them to software defaults.
- Switching to a metal roof is not automatically a permit-exempt re-roof. Check the county before you quote.
- Snow retention, panel gauge, seam profile and clip spacing all follow from the load, so settle the number before you order panels.
Table of Contents
- Ground snow load is a local number
- From ground snow load to roof snow load
- What this means for a standing seam roof
- Permits: is a metal re-roof exempt?
- Documenting snow load so plans get approved
- Our perspective: settle the load before the takeoff
- Ordering panels once the load is set
- FAQ
- Sources
Ground snow load is a local number
Maryland’s Prince George’s County answers the question directly: 35 PSF ground snow load, with design snow loads determined under the ASCE 7 chapter on snow. The county’s own FAQ is the document a plan reviewer will hold you to, which makes it more useful than any third-party map.
Baltimore County and Prince George’s County both specify a 30 PSF minimum roof live load alongside their snow load requirements. Contractors often get caught here. The assumption that the national ASCE 7 map applies as printed breaks down when a county has adopted its own amendments, and those amendments tend to set higher minimums. ASCE 7 is the nationally adopted loading standard, but the adopted local code decides which version and which values apply.
The practical rule: before you price a job, find the building department’s stated Pg for the address. If the county does not state one, ask the plan reviewer or the project engineer. In the western mountain counties, expect an engineer to determine the load for the specific site rather than hand you a number.
Does Maryland actually get that much snow?
The design load is deliberately conservative because it covers rare, heavy events, not an average winter. NOAA 1991 to 2020 normals put Baltimore’s normal season at about 19 inches, with February the snowiest month at roughly 7 inches on average, and big year-to-year swings. Averages hide the problem. A single wet storm, or rain falling on an existing snowpack, can load a roof far beyond what a normal season suggests. Maryland’s mix of snow, sleet and freezing rain is why the design number matters more than the climate average.
From ground snow load to roof snow load
The design load on the roof is derived from Pg. Under ASCE 7, the flat roof snow load (Pf) is calculated from the ground snow load with these factors:
| Factor | Symbol | What it reflects |
|---|---|---|
| Exposure | Ce | How much wind sweeps snow off the roof |
| Thermal | Ct | Heat loss through the roof that melts snow |
| Importance | Is | Risk category of the building |
| Slope | Cs | How readily snow slides off the surface |
The base relationship is Pf = 0.7 × Ce × Ct × Is × Pg, and the slope factor is then applied to get the sloped roof load.
A worked example
Take a heated, single-family house in a jurisdiction with Pg = 35 PSF, a partially exposed site and a standard risk category. Using Ce = 1.0, Ct = 1.0 and Is = 1.0:
Pf = 0.7 × 1.0 × 1.0 × 1.0 × 35 = 24.5 PSF
If the same building were in a higher risk category with Is = 1.1, the flat roof load becomes 26.95 PSF. The factor values must come from the code tables for your actual conditions. The point of the example is the arithmetic: the number on the drawings is rarely the county’s Pg itself, and it is not necessarily lower either, because the 30 PSF minimum roof live load may control on a low-snow site.
Drift matters more than the base number
Structural engineers stress that roof snow load must include drift surcharge (Pd) and exposure effects, not just ground snow. Drift builds where wind deposits snow against a taller wall, a dormer, a chimney, a parapet, or where an upper roof sheds onto a lower one. Those locations can carry far more than the uniform load.
Prince George’s County’s plan submission requirements ask for drift surcharge loads to be documented, and this is a detail frequently missed in preliminary takeoffs. If your quote assumes a uniform load across the whole roof and the drawings show a two-level roof with a valley, the structure and the panel attachment at the drift zone may not match what you priced.
What this means for a standing seam roof
A metal roof is lighter than many alternatives, but the load path still runs through the panel, the clips and fasteners, and into the deck and framing. A few things change with a higher snow load.
Panel profile and gauge
Standing seam comes in two common forms: mechanical lock and snap lock. Mechanical lock panels are seamed on the roof into a double-lock closure, and snap lock panels engage by pressure. Both can be specified in 24-gauge or 26-gauge steel. Heavier gauge, a taller seam and tighter clip spacing are the usual levers when load or wind uplift calls for more capacity. The exact allowable load for a given assembly comes from the manufacturer’s tested data and the engineer of record, not from a rule of thumb, so ask for the load tables before you finalize the panel.
Our 1-inch mechanical lock and 1.5-inch snap lock standing seam panels are produced from Englert metal systems in both gauges, so a spec change between 26 and 24 gauge does not mean changing systems.
Snow retention and sliding snow
Metal sheds snow quickly, which is good for the roof structure and bad for whatever sits below the eave. Entries, walkways, lower roofs, mechanical units and parking areas all need thought. Snow guards or rail systems are specified by layout, not by feel. Plan them at the takeoff stage, because the retention hardware has to be compatible with the seam profile and its loads have to be added to the roof design.
Low-slope and flat sections
Local guidance for Baltimore notes that flat and low-slope roofs do not shed snow as easily as pitched roofs, leading to heavier accumulation. The ASCE slope factor reflects the same idea. Standing seam is a good fit for low slopes only when the minimum slope and seam type in the manufacturer’s data match the project. Our article on IBC metal roofing checks covers that side.
Ice dams are a detailing problem, not a load problem
Ice dams come from heat escaping into the attic, melting snow that refreezes at the eave. They are solved by insulation, air sealing, ventilation and proper underlayment and eave details, which is why trim and flashing belong in the same conversation as snow load. Our trim and flashing packages are ordered with the panels so eave, rake, valley and wall details are coordinated to the same layout.
Permits: is a metal re-roof exempt?
Not necessarily. Permit rules differ by county, and the differences affect how much structural documentation you need.
| Jurisdiction | What the county says |
|---|---|
| Baltimore County | Permit required for roof repairs on residential or commercial buildings when 50 percent or more of the sheathing or deck is being replaced, and for structural component replacement |
| Harford County | Replacing roofing with material in kind for maintenance is treated as an ordinary repair; changes beyond that are handled differently |
| Talbot County | Lists total roof replacement among projects using its general building application |
The trap is the phrase “ordinary repair.” A material change, such as moving from shingles to standing seam, or replacing structural sheathing, can bring plan submission and structural review into play. Check with the building department before you promise a schedule. If a permit is required, the reviewer will want the snow load documented properly.
For Maryland code details beyond snow, see our guide to Maryland metal roof code checks for contractors.
Documenting snow load so plans get approved
Building officials often reject roofing submissions that lack specific structural notes. Rockville’s residential submittal requirements are one example of a city asking for detailed structural information, and a county may add its own amendments. A clean submission usually includes:
- The jurisdiction’s ground snow load (Pg), with the source.
- Exposure factor (Ce), thermal factor (Ct) and importance factor (Is), each stated, not left to software defaults.
- Flat roof snow load (Pf) and the sloped roof load.
- Drift surcharge (Pd) and where it applies, with the drift locations shown on the roof plan.
- The roof live load, noting the 30 PSF minimum that certain counties specify.
- Panel manufacturer, gauge, profile, clip type and spacing, tied to tested load data.
- Snow retention layout, if used.
In practice, the rejections come from missing items, not from wrong ones. Put the coefficients on the sheet and the reviewer has nothing to send back.
Our perspective: settle the load before the takeoff
We supply standing seam panels to contractors across Maryland and the rest of the Mid-Atlantic, and the snow load conversation almost always happens too late. A contractor orders panels based on a quick takeoff, then the engineer or the plan reviewer comes back with a drift zone or a changed clip spacing. By then the panels are cut.
That is the reason we work the way we do. Cut-and-drop means panels are cut to your measured lengths and delivered to the job site, so lengths and layout need to be right. Settling the load, the profile and the gauge first means the cut list reflects the real roof. We would rather answer a spec question on Monday than re-cut a panel on Friday.
Our practical advice for Maryland jobs:
- Ask for the county’s snow load in writing before you quote, and keep it in the job file.
- Mark every roof-to-wall, dormer and upper-to-lower roof transition on the takeoff as a potential drift zone.
- Decide on snow retention while measuring, not after the panels land.
- When the engineer changes gauge or clip spacing, send us the revision so the order and trim package change with it.
— Matt Catino
Ordering panels once the load is set
Once the design load and profile are confirmed, the order is straightforward. We offer cut-and-drop delivery of custom-length panels to the job site, which reduces waste, and on-site roll forming for long runs or hard-to-reach projects. Contractor accounts, takeoff support and fasteners and accessories are on our contractor supply page. For regional specifics, see our Maryland metal roofing panels page.
FAQ
How much snow load can a roof handle?
It depends on the framing, deck and roof covering, and it is set by the engineer of record, not by the roofing material alone. A typical Maryland design is governed by the county’s ground snow load and the minimum roof live load, which is 30 PSF in Baltimore and Prince George’s Counties. If you are changing roof covering, confirm the existing structure meets the required load.
How do I calculate snow load for a Maryland roof?
Start with the jurisdiction’s ground snow load, then apply ASCE 7 exposure, thermal, importance and slope factors to get the roof load. Add drift surcharge where snow can accumulate against walls or on lower roofs. An engineer should confirm the result, particularly for complex roofs.
Do I need a permit to replace a roof in Maryland?
It depends on the county. Baltimore County requires one when 50 percent or more of the sheathing or deck is replaced, while Harford County treats in-kind replacement as an ordinary repair. Call the building department before you commit to a schedule.
Is the ASCE 7 map enough for Maryland?
Not always. The national standard is the base, but a county may adopt its own ground snow load or amendments, as Prince George’s County does. The local requirement controls, so check it first.
Sources
- What is the design snow load for Prince George’s County?
- Prince George’s County minimum plan submission requirements
- ASCE 7-22, American Society of Civil Engineers
- Maryland winter forecast and NOAA normals, MyWild
- Baltimore County roofing and siding permit
- Harford County residential roof replacement guidelines
- Talbot County permit requirements
- City of Rockville residential permit submittal requirements