Calculating Roof Loads

Tricks of the Trade

Calculate roof loads by understanding the difference between live loads and dead loads and you will be able to safely and confidently order the correct size roof beams, structural ridge beams and rafters for your log home or cabin roof. Get it wrong and you may make a disastrous mistake that could cost you big time.

This shows the enormous additional roof load imposed by snowfall.

Load Path for Roof Loads

It’s very important that you size the roof framing members correctly to carry the projected roof loads and to provide a continuous load path. The first step is determining what your projected roof loads will be. This is determined by your location and the expected wind and snow loads.

Check with your local building inspectors for these figures. Where we build cabins in western North Carolina we provide for a 90 mph maximum wind zone and projected snow roof loads of 20 lbs/sq.ft.

You must plan your log cabin design around carrying these roof loads down to the foundation of your cabin. This is called the load path and consists of the entire structure that carries the load.

The load path starts with the snow load transmitted to the roofing, then to the rafters, the ridge beam, the ridge beam support columns and down to the foundation in one continuous, connected string.

snow-roof-load

Dead Weight and Live Weight

Another distinction to make is between dead weight and live weight. Dead weight or material load refers to the permanent weight of the structure itself, basically all the lumber, logs, fasteners, shingles, etc. that make up the cabin component, whether it is the roof or floor or wall.

These figures can be obtained by literally adding the weights of the wood. See below table for typical material dead load or dead weights.

The live weight refers to the transitory added weight that the structure will be required to support for a short period, such as people walking in the loft area, or snow contributing to the roof loads. It can also include furniture normally added to a room such as an upstairs room.

You can design a loft area to only be a storage area, for example, and not suitable for furniture and people on a full-time basis. Minimum design live loads are prescribed by code, and are available in the code book.

Check with your local building department or local library for copies of the code book.

2015 International Residential Code

IRC Quick Card

2015 International Building Code

IBC Quick Card

Deflection

There is also the issue of deflection. You can design how much deflection you will get in beams, joists and rafters from the combined roof loads.

The amount of deflection allowed is specified in the building code. You will often see figures such as L/360 or L/240 which are the deflection factors.

For example, a 10'-0" span = 120" so using a deflection factor of L360 you would divide 120” by 360” to get .33" or 1/3 of an inch of allowed deflection.


Sample Dead Load Material Weights
For Roof Loads

Pounds per square foot (PSF) for roof loads

Sheathing and Decking

  • 15⁄32” plywood . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5 (PSF)
  • 23⁄32” plywood . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3
  • 1 1⁄8” plywood . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4
  • 3⁄8” OSB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4
  • 7⁄16” OSB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5
  • 1⁄2” OSB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.7
  • 19⁄32” OSB . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.0
  • 1” decking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3
  • 2” decking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3


Ceilings

  • 1⁄2” gypsum board . . . . . . . . . . . . . . . . . . . . . . 2.2 (PSF)
  • 5⁄8” gypsum board . . . . . . . . . . . . . . . . . . . . . . 2.8
  • 1” plaster with lath . . . . . . . . . . . . . . . . . . . . . . 8.0


Roofing

  • 2-15 lb. and 1-90 lb. rolled. . . . . . . . . . . . . . . . . . 1.7 (PSF)
  • Asphalt shingles . . . . . . . . . . . . . . . . . . . . . . . . . 2.5
  • Wood shingles. . . . . . . . . . . . . . . .. . . . . . . . . . . 3.0
  • Asbestos-cement shingles  . . . . . . . . . . . . . . . . . . 4.0
  • Spanish tile. . . . . . . . . . . . . . . . . . . . . . .. . . . . .19.0


2” Framing (12” on center)

  • 2” x 4” (for 16” o.c. divide by 1.33) . . . . . . . . . . . 1.4 (PSF)
  • 2” x 6” (for 16” o.c. divide by 1.33) . . . . . . . . . . . 2.2
  • 2” x 8” (for 16” o.c. divide by 1.33) . . . . . . . . . . . 2.9
  • 2” x 10” (for 16” o.c. divide by 1.33) . . . . . . . . . . 3.7
  • 2” x 12” (for 16” o.c. divide by 1.33) . . . . . . . . . . 4.4


Note: Wall weights are per square foot of wall -- multiply weight times wall height for plf.

Insulation (per 1” thickness)

  • Rigid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5 (PSF)
  • Batts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5



See our page on structural ridge beams for an example of how to use roof loads for correctly ordering the rafters and beams for your log cabin roof.

Return from Roof Loads page to Log Cabin Home Design page

Return from Roof Loads page to Log Cabin Connection home

Enjoyed this page? Please LIKE and SHARE it on Facebook:

New! Comments

Have your say about what you just read! Leave me a comment in the box below.

Home | About Us | Contact Us | Privacy Policy | Disclaimer

FAQs | About This Site | Advertise | Site Map | Glossary | Virtual Library


©2008-2017 Log Cabin Connection
All rights reserved

LIKE and SHARE Log Cabin Connection:

NEW! Check out our Facebook page:


Ebates Coupons and Cash Back

Log Cabin Design

Log Cabin Home Design
Aging In Place and Universal Design Tips
Log Cabin Design Tips
Log Styles: Making Sense of the Choices
Save Money with Careful Cabin Design
Tricks of the Trade: Structural Ridge Beam
Tricks of the Trade: Calculating Roof Loads
Tricks of the Trade: Wiring Log Cabins
Tricks of the Trade: Efficient Cabin Windows
Tricks of the Trade: Install Log Siding
The Wood Handbook
Integrating Log Home Systems
Loft and Deck Railing Ideas and Designs
Building with Milled Logs
Log Home Chinking Blocks Air & Moisture Infiltration
Log Standards for Log Structures
REScheck for Energy Code Compliance

Recent Articles

  1. Outdoor Rooms -- Transition to Nature

    Outdoor Rooms bring the outside in and the inside out. They are a graceful transition to your own natural retreat.

    Read More

  2. Using Milled Logs to Build your Log Cabin

    Using milled logs from a log supplier makes building your log home easier, more energy efficient and saves money over log cabin kits.

    Read More

  3. Install Log Siding: Tricks of the Trade

    Learn how to install log siding to provide a rustic log cabin look to your home.

    Read More