7.1 Roof Types, Framing Terminology & Structural Loads
Key Takeaways
Under MRC R802.3, the ridge board must be at least 1-inch nominal thick and no shallower than the cut end of the rafters, and hip and valley rafters at least 2-inch nominal.
Under R802.3.1, rafter ties are at least 2x4 nominal. Collar ties or ridge straps that resist wind uplift go in the upper third of the attic space, at least 1x4 and no more than 4 feet on center.
Rafters need at least 1 1/2 inches of bearing on wood or metal and 3 inches on masonry or concrete (R802.6).
Truss members may not be cut, notched, drilled, spliced, or otherwise altered without the approval of a registered design professional (R802.10.4).
Michigan ground snow loads range from 20 psf to 100 psf by county and township, and above 70 psf the structure must be engineered rather than taken from the code tables.
Roof Shapes and Terms
| Term | Meaning |
|---|---|
| Gable | Two slopes meeting at a ridge, with triangular end walls. The sloped edges are rakes |
| Hip | Slopes on all sides meeting at hips and a ridge. No gable ends |
| Gambrel | Two slopes per side, steep lower and shallow upper |
| Mansard | Four-sided gambrel. The lower slopes are nearly vertical |
| Shed | A single slope |
| Dormer | A framed projection through a roof slope, which creates valleys and sidewalls |
| Eave | The lower edge of a roof slope |
| Valley / hip | Internal / external intersection of two roof planes |
| Cricket (saddle) | A small roof that diverts water around a chimney or curb |
| Square | 100 square feet of roof surface |
| Slope | Rise in inches per 12 inches of horizontal run, such as 6:12 |
Under Subpart M, low-slope means 4:12 or less and steep means more than 4:12. The code sets separate minimum slopes for each roof covering (Chapter 9).
Conventional Framing Rules (2015 MRC)
| Item | Rule |
|---|---|
| Ridge (R802.3) | Rafters frame opposite each other, or offset no more than 1 1/2 inches, to a ridge board at least 1-inch nominal thick and not shallower than the cut end of the rafter |
| Hips and valleys (R802.3) | A hip or valley rafter at least 2-inch nominal and not shallower than the cut end of the rafters, braced at the ridge to a bearing partition or designed to carry the load. Below 3:12, ridges, hips, and valleys supporting rafters must be designed as beams |
| Rafter ties (R802.3.1) | Where ceiling joists are not parallel to rafters, or not connected at the plate, install rafter ties at least 2x4 nominal, connected per Table R802.5.1(9). Without ceiling joists or rafter ties, the ridge must be supported by a wall or engineered beam |
| Collar ties or ridge straps (R802.3.1) | Resist wind uplift. Located in the upper third of the attic space, at least 1x4 nominal, spaced no more than 4 feet on center |
| Bearing (R802.6) | At least 1 1/2 inches on wood or metal, 3 inches on masonry or concrete |
| Rafter notches (R802.7.1) | Cuts in rafters follow the joist rules. Notches in cantilevered rafter tails are allowed if at least 3 1/2 inches of rafter remains and the cantilever is no more than 24 inches |
| Trusses (R802.10.4) | No cutting, notching, drilling, splicing, or other alteration without approval of a registered design professional. Adding load beyond the truss design, such as HVAC equipment, requires verification. In Michigan, a registered design professional is licensed under the Occupational Code (R 408.30513) |
Why rafter ties matter: Loaded rafters push the top of the walls outward. Ceiling joists or rafter ties low in the attic hold the rafter feet together. A roofer who finds sagging ridges and bowed walls on an older home with a vaulted ceiling and no ties has found a structural problem. It needs a builder or engineer before reroofing.
Michigan amended R802.10.1 in two ways. Truss design drawings must be provided to the building official and approved before installation, and they must be delivered to the jobsite with the trusses. At permit application, Michigan's truss design data sheet may be submitted instead of the drawings (R106.1.4). Trusses also need temporary and permanent bracing as their design drawings require.
Loads a Roofer Must Evaluate
Dead Load
Dead load is the permanent weight: framing, sheathing, underlayment, roofing, and fixed equipment such as solar arrays. Typical approximate weights:
| Material | Approximate weight |
|---|---|
| 7/16-inch OSB or 1/2-inch plywood sheathing | About 1.5 psf |
| Asphalt 3-tab shingles | About 2 to 2.5 psf |
| Laminated (architectural) shingles | About 3 to 4.5 psf |
| Metal panels | About 1 to 1.5 psf |
| Slate, clay, or concrete tile | Roughly 8 to 15+ psf |
These are ballpark values. Use the manufacturer's weight for each product.
Live Load
Roof live load is temporary weight from workers, equipment, and stored materials. Under MRC Table R301.6, the minimum roof live load for a member with up to 200 square feet of tributary area on a roof flatter than 4:12 is 20 psf. It drops to 16 psf for slopes from 4:12 to less than 12:12, and to 12 psf at 12:12 and steeper. Snow loads usually control Michigan roofs.
Snow Load
Michigan replaces the IRC snow map with its own Table R301.2(5), listing ground snow loads by county, with exceptions for specific townships and cities:
| Location (examples) | Ground snow load |
|---|---|
| Wayne County (most), Hillsdale County (most) | 20 psf |
| Barry County (most) | 35 psf |
| Allegan County (most), Berrien, Alpena | 50 psf |
| Benzie, Manistee, Mason | 60 psf |
| Alger, Baraga, Marquette (most) | 70 psf |
| Houghton (most) | 80 psf |
| Keweenaw (most) | 90 psf; 100 psf in Eagle Harbor, Grant, and Houghton townships |
Under R301.2.3, buildings in areas with a ground snow load over 70 psf must be designed by accepted engineering practice, not the prescriptive rafter tables. Drifting at valleys, dormers, and lower roofs increases local loads. Adding weight, such as a second roof layer, tile, or a solar array, requires confirming the structure can carry it (R908.2, R909.2).
Wind
Michigan's design table lists a 90 mph basic wind speed, with site-specific exposure (Section 2.1). Wind creates uplift. That is why roof covering fastening, sheathing nailing, rafter-to-plate connections (Table R602.3(1)), and collar ties or ridge straps all matter.
Evaluating an Existing Roof Structure
Before bidding, check the roof from the attic and from the roof:
- Sagging ridges or rafters, or a wavy deck between rafters.
- Split or cracked rafters and truss chords, and site-cut truss members.
- Missing rafter ties with walls pushed outward.
- Water stains, rot, or mold on sheathing and framing.
- Existing layers of roofing, which add dead load. More than one existing layer also rules out a recover (Section 8.2).
If you find damaged trusses, an unusual load such as a heavy tile change or snow above 70 psf, or signs of structural movement, tell the owner in writing and require a design professional's repair before proceeding.
Where must collar ties or ridge straps used to resist wind uplift be placed under 2015 MRC R802.3.1?
At the top wall plate
In the lower third of the attic space
In the upper third of the attic space, at least 1x4 nominal and no more than 4 feet on center
Anywhere, as long as they are 2x6 or larger
During a reroof, a roofer finds that an HVAC installer cut through a truss web to route a duct. What does MRC R802.10.4 require?
Nail a 2x4 alongside the cut web and continue roofing
Install metal mending plates on both faces
Nothing, if the adjacent trusses show no sag
The alteration needs the approval of a registered design professional, so obtain an engineered repair before continuing
A home in a township with a Michigan Table R301.2(5) ground snow load of 80 psf needs a roof structure. What does the MRC require?
Use the prescriptive rafter span tables for the 70 psf column
Use the 20 psf roof live load
Use the national snow map in the base IRC
Design by accepted engineering practice, because the ground snow load exceeds 70 psf
Sections you finish are checked off in the contents.