Archive for March, 2010

Important Design Criteria for Single Family Dwellings

March 21st, 2010

Before the first shovelful of dirt is moved, there are important code required design criteria which must be met for single family dwellings.  Chapter 3 of the International Residential Code addresses these and I will review them in this article.  I will save snow, wind, live and dead loads for another time.


Light and Ventilation

Habitable rooms (space for living, eating, sleeping or cooking) are required to have glazing not less than 8 % of the floor area of that room. That is; a 10’ X 10’ room needs 8 sq. ft. of windows.  Of those 8 square feet, 4(or half) have to open to the outside. Of course there are exceptions.  First, the windows don’t need to open if adequate mechanical ventilation is supplied.  You don’t need the windows at all if you have mechanical ventilation and artificial light.  Finally, sunrooms and enclosed patios are allowed to be used for natural ventilation if 40% or more of the exterior walls of the sunroom are screened.   You must take into account that emergency egress requirements  in bedrooms is maintained(This will be discussed later).

Bathrooms are required to have a minimum 3 sq ft of windows, half of which need to open unless you have the required ventilation.

Interior stairs have to have lighting which illuminates the entire stairway located near each landing.  The controls for the lighting must be at every floor level when the stairs have six or more risers. Exterior stairs are required to have lighting near the top landing and both the top and bottom landing if it leads to a basement.  The light switches are to be inside the dwelling.

Minimum room areas and ceiling heights.

One room per house need to be at least 120 sq ft and other habitable rooms can be no less than 70 sq ft.  One exception to this is the kitchen.  The minimum dimension of any habitable room is seven feet. Hallways are required to be a minimum of 3 feet. “Habitable rooms, hallways, bathrooms, laundry rooms and basements have to have ceiling no lower than 7 feet.  There are a number of exceptions for beams and girder, unhabitable basements, and bathrooms.


“Every dwelling unit” has to have a toilet, sink and tub or shower. They also need a kitchen with a sink, and water and sewage disposal.  Kitchen sinks, lavatories, bathtubs, showers, bidets, laundry tubs and washing machine outlets must have cold and hot water.

Shower and tub areas have to be made of a non-absorbent surface at least 6 feet from the floor. Water resistant drywall is no longer allowed to be used in tiled shower or tub areas (2006 IRC and IBC).


The requirements for glazing address hazardous locations and the requirements for safety glazing which must be used in these locations.  For an excellent article which covers this subject in depth and gives you a historical and safety perspective about glazing go to the Code Check Web Site.  There are 11 areas considered by the IRC as hazardous locations.  There are also 10 exceptions to these locations.  Most importantly are doors, pool/sauna, hot tub, shower and tub enclosures/walls and fences,  large windows which are close to the ground, guardrails and glazing adjacent to stairways and ramps.  Section 308.4 of the 2006 IRC covers these locations along with the exceptions.  Skylights and greenhouse glass requirements are also covered.


A separation between the dwelling and a garage is a subject full of misconceptions.  “It’s a firewall….it has to have two layers of 5/8 gyp board on the garage side…., the door has to be a fire door with a closer are popular fallacies.   I can only speak for the last 20 years of CABO and BOCA requirements for separations,  but none of these have ever been required.  Here is what is required by the IRC 2006:

  1. You can’t have an opening into a garage from a bedroom.
  2. If you have duct work penetrating into the garage, it has to be made of a minimum of 26 ga sheet metal and cannot open into the garage.
  3. Other penetrations of the separation wall must be fireblocked.
  4. The garage shall be separated from the dwelling (including its attic) by ½ inch gyp board on the garage side (you could theoretically have open studs on the dwelling side).
  5. If there is a habitable room above the garage, the separation must be 5/8 gyp board (type X).
  6. The supporting structure of item 5 shall be protected by ½ gyp board.
  7. If there is a door in the separation wall, it must be either 1 3/8 inch solid wood, solid or honeycomb steel minimum 1 3/8 inch thick, or a 20-minute fire-rated door. (Wouldn’t a fire-rated door require a closer and positive latching?)

Emergency Escape and Rescue/Egress

A requirement for bedrooms in basement has now become a requirement for all basements with the exception of those under 200 square feet and used only for mechanical purposes.  An emergency window or door must be installed.  For basements with all sides partially or completely below grade, the emergency window is usually installed.  The sill height of such window cannot be more than 44 inches from the finished floor.  The minimum opening area has to be 5.7 square feet (820.8 square inches), with a minimum height of 24 inches and a minimum width of 20 inches. The well outside the window has to be at least 36 inches wide and 9 square feet in area. If a casement window is used, the well must accommodate it to fully open.  If the depth of the well is greater than 44 inches, a ladder or steps shall be installed.  The ladder or steps can impede into the required area of the well as long as the window can be fully opened. The rungs have to be 3 inches from the wall, 12 inches wide and not more than 18 inches

apart. Bars or grates can be placed over these openings but like the window itself-it must be operational from the inside without the use of a key or special knowledge.

Bedrooms must also have an opening, either a door or window meeting the above requirements.  If the bedroom or basement is at the grade level, the minimum opening is reduced to 5 square feet.

One exit door is required from the dwelling unit.  It has to be a minimum of 3’ wide and 80” high and should be side-hinged. No keyed locks are allowed on this door. There should be a landing (the width of the door and 36” in length) outside all exterior doors (there is an exception for non-egress doors).

Stairs, Handrails, Guard

Stairways and hallways shall be 36” wide minimum, with 80” of headroom at the stairs.  When a stair is enclosed, any walls, sofitts and the underside of the stairs have to be covered with ½ drywall.

Maximum stair rise is 7 ¾” and minimum tread depth is 10 inches.  These dimensions tend to vary by jurisdiction.  In 2003, The State of Michigan amended these to 8 ¼ and 9 inches.  The maximum deviation from the biggest rise to the smallest can be no more than 3/8 of an inch.  Landings are required at the top and bottom of each stairway (some exceptions) and a stair cannot exceed 12 feet in vertical rise.

Handrails are required on stairs exceeding three risers. Their height should be 34 to 38 inches measured perpendicular from the nosing to the top of the handrail.  They need to be terminated into the wall or into a newel post and a 1 ½ inch space (minimum) must be provided between the wall and the handrails.  Handrail grip size can either be circular between 1 ¼ and 2 inches in diameter, 4 to 6 ¼ inches perimeter dimension for non-circular(with a maximum cross section of 2 ¼ inches or on those with a perimeter dimension more than 6 ¼ inches it requires a finger recess on both sides of the profile.  This profile must meet certain dimensional requirements (Section R311.5.6.3).

Finally guards are required where a raised floor surface is more than 30” above the floor/grade below. They have to be 36” high (minimum).  Open stairs with a total vertical rise over 30” would also require a guard which would have to be 34” high (minimum).  Opening limitations for these guards are 4” and 4 3/8” for stair guards.  Please note that the forbidden “ladder effect’ was taken out of the IRC in 2003.

Smoke Alarms

Hard wired, interconnected smoke alarms are required in all sleeping rooms, outside the sleeping rooms, and on each story of the dwelling including the basement.  Exceptions are allowed for additions and renovations.

In Conclusion

I’ve only begun to scratch the surface of material covered in Chapter 3 of the International Residential Code. For those of you new to the Code, this is a great place to start and familiarize yourself with its requirements.  Those of you have been in the business for a while; this is a great chapter to frequently review because of the amount of information covered in it.  Unless otherwise noted, I have referenced the International Residential Code, 2006.

Articles, Residential Code