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Building Code - Interpreting Building Code

Star Here is my interpretation of the relevant bits of building code that I need to take note of.

These snippets come from the IRC 2009 rules, but note that building codes vary in different local areas so you will need to do your own checking up on codes in your area.  Also, this is just a few of the building codes that caught my eye and appeared relevant to my case.  It is not a full set, so you should consult a full up to date code book to make sure you meet all the codes.

For plumbing codes see the Plumbing techniques page and for electrical codes see the Electrical techniques page.



Code says that footings must bear on undisturbed ground.  They are required to extend down below the frost line.

In my area the frost line is only 12 inches down, but I am choosing to go down more than 5 feet to get to very solid ground and to avoid lateral movement.

In my foundation design, the bottom part of the footing is made of polystyrene and it is the polystyrene part of the footings that bears on the undisturbed soil.  It is necessary to show calculations to your local building department to prove that the polystyrene can handle the pounds per square inch without creep exerted by the weight of the house over the footing area .

Soil tests based on existing soil maps, test data records, or other documentation with quantifiable data that are based on accepted geotechnical methodologies.  Code directs the building official to make a determination of whether to require soil testing or not.  If you don't do a soil test then you will be required to assume the worst possible soil.  In my case, even though the soil is very good and solid, I chose to assume in the structural engineering that the soil is the worst possible.  This meant the foundations were designed to be much stronger than they actually need to be.  I decided to spend the money that the soil test would have cost to buy extra rebar and concrete for the footings.

By way of demonstrating how good my soil actually is, the following picture shows a less than 1' wide strip of my soil happily supporting about 3 feet of differential water level.

Soil Supporting Water 

Typically bad soil may be 1500 psf and good soil 3000 psf.  IRC give a worst case of 1500 psf (10psi) soil with 3 story house built using 8 inch thick concrete walls requiring a footing width of 42 inches.  If the soil is 3000 psf (21psi) then the corresponding width is 21 inches.  In my case I made the width of the concrete at the bottom of the footings 49" (7" wider than IRC requires for worst case soil).

Stepping of foundations on sloped sites is allowed by code.  I do not recommend using stepped foundations.  Not only is it more complicated to do but I worry about what would happen in an earthquake.  I prefer to excavate the house site totally level to make the foundations easier (and also provide a daylight basement).

IRC requires footing concrete to be reinforced with rebar in seismic areas of D0, D1, D2, and E.  In A, B, C areas you are not required to have it.  Personally I think it's always best to use reinforcement.

For a pad footing on a post (eg supporting a deck) you calculate the weight that the post needs to support, and you know the weight baring capability of the soil, and that tells you the area of the pad of concrete that is required.

In seismic D areas there may be a limit on the height of backfill of earth behind the basement wall.  In practice your licensed structural engineer will do the calculations and specify the height of the backfill that has been allowed for.

The IRC seismic provisions specify prescriptive limits on the weight of the upper parts of the house such as roof, ceiling, walls, and floors, and also limit the number of stories.  In practice these prescriptive limits do not apply if the house has been structurally engineered.

When designing for earthquake regions it is best to stick with a regular rectangular shaped house so the weight on all the footings is fairly uniform. This means that the house will tend to subside uniformly if the ground liquefies in a big earthquake.  Uniform subsidence is not too much of a problem.

Slab on grade implementations must have insulation under the slab that is at least R-10.  EPS can conservatively be regarded as R-4 per inch, so R-10 equates to 2.5" of EPS.

The ground should be sloped away from the foundations to direct rain water away.  Where it is not feasible to provide the prescribed fall of 6 in. within the first 10 ft. away from a foundation, the code includes new performance language requiring drainage away from the foundation without prescribing a slope.

Gravel or crushed stone drains are required to extend at least 1 foot beyond the outside edge of the footing and 6" above the top of the footing, and be covered with an approved filter membrane material.



Stair width must be more than a clean 36" measured from the drywall and any skirting boards.

Stairs must be uniform so there are no discontinuities in the stride for the user.

Minimum tread width is 10".  On turns it is possible for the inside edge to have down to a 6" tread provided a 10" tread width is achieved within 12" of the narrow side.  My choice is to use an 11" tread width for additional safety.

Maximum rise is 7-3/4".  The actual height is determined by the distance between finished floors divided by the chosen number of stairs (as long as the answer is less than 7-3/4).  For nice stairs, choose the number of stairs such that the rise comes out just less than 7.5".  Note that the number of stair risers is one more than the number of drawn steps on the plan.

The stair nose must be between 3/4" and 1-1/4".  It is best to make it 1-1/4" so that it can be refinished in later years to repair any wear.  When the house is under construction you can even make the nose 1-1/2" and then trim it to 1-1/4" after you're done with the heavy construction traffic (but prior to the final building inspection).

Minimum headroom above the stair nose is 6'8".

Spiral staircases have their own set of rules that allow them to have a tighter turn and a higher rise.

A landing is required at both the top and bottom of stairs.  The landing width must not be less than the stairs and must have a length in the direction of travel of more than 36".

Doors can be at the top of an interior stairway (eg to a basement) as long as it does not open over the stairs, ie must open outwards.

Hand rails on stairs must be between 34" and 38" above the tread nosing.  My choice is to make it 37".

The hand rail must be up to or beyond the first and last stair nose.

The diameter of the hand rail must be between 1-1/4" and 2" if round and other equivalent dimensions for other shaped cross-sections.

Need 1/2" drywall on the underside of stairs as a fire precaution.

A flight of stairs must not have a vertical rise of more than 12 feet.  This is an important consideration when doing even your initial floor plans as it may require the stairs to be L shaped with a landing part way up.  I made my stairs L shaped from the basement and to the bedroom level in order to meet this requirement.

Here's a good diagram from Steven Randel that summarizes many aspects of stairway building code.  His complete article is here .

Stairway Building Code Diagram 



Railings on the landing around the stair floor opening must be 42" above finished floor height.

On horizontal surfaces (eg the edge of floors that form a balcony), a 4" sphere must not be able to pass through.  (The 4" sphere equates to a baby's head.)  On stairs, a 4-3/8" sphere must not be able to pass through (slightly bigger baby).

Rails on stairs must be continuous.

Porches, balconies, ramps or raised floor surfaces located more than 30 in. above the floor or grade below shall have guards not less than 42 inches in height.  Open sides of stairs with a total rise of more than 30 inches above the floor or grade below shall have guards not less than 34 in. in height.  In practice, if you have any raised floor areas then keep these less than 30" to avoid the need for railings.


External doors

The external main door must swing into the house.

The step down to a landing outside an external door must be less than 7-3/4".

The doorway lip board must be less than 1-1/2".

Outside an external main door there must be a landing area with a width greater than 3 feet.

If an external door is not the main door (sometimes known as the front door) then you can have steps down (without a landing), but you can only have 2 steps if there is no landing.

The landing must be at least the door width and have a length in the direction of travel of more than 3 feet.

A dwelling unit must have an external door that goes to the outside without requiring you go through the garage.


Windows (and other glazing)

The window sill height of an opening window must be at least 24" off the floor if the window opening is more than 72" (6 feet) off the external ground.  The 24" is measured from the floor to the actual opening of the window.  If the window is non-opening (ie a picture window) then the requirement does not apply.

It is worth adding an extra inch to the 24" to allow for someone adding thick wood flooring later that would raise the floor height.

Windows in bedrooms and basements need to function as an escape route.  The maximum sill height allowed is 44" (3 feet 8 inches).  The opening size must be more than 5.7 square feet.  The opening width must be more than 20 inches.  The height of the opening must not be less than 24 inches.

A window size of 24" high is not ok because the opening in a 24" high window is something like 22" after allowing for the frame.  A horizontal slider window that is 3 foot wide is not ok because the window opening is less than half the window width.  If however the sliding part of the window was designed to be easily removable then it probably would be judged to be ok.  A horizontal slider window that is 5' wide and 3' high is a good minimum to use.  This will give an opening of something like 6.5 sqft, which is comfortably more than the required 5.7 sqft.  A 3' wide 3' high casement window meets the requirement.

In my basement design, where the window openings start at 4'6" above what would be the finished floor, it would be necessary to implement a 1 foot high platform floor to meet the 44" requirement if it were not for the fact that I have two doors from the basement.

Safety glazing is required on glass doors and on windows adjacent to doors if within 24" of the door opening.  Safety glazing means tempered or laminated glass that has passed the appropriate tests.

It is important to check that French doors have safety glass.

Also need to assess the full height picture windows in relation to doors to see which ones need to be safety glass.

Safety glass is also required in windows that meet all of the following criteria.
    1) Larger than 9 sqft.
    2) Bottom less than 18" above floor.
    3) Top edge greater than 36" above floor.
    4) Walking surface within 36" of window.

To avoid limits on the percentage of glazing, the U-factor of windows must be better than 0.30.  This equates to R-3.33.


Smoke and CO alarms

Smoke alarms must be interconnected.

Must be a smoke alarm in all sleeping rooms.

Must be a smoke alarm outside each sleeping area.

Must be one in the basement.

Must be one in the attic (if the attic is habitable).

Primary power for smoke alarms must be from the main AC source and there must also be battery backup.

The smoke detector system shall be monitored by an approved supervising station and be maintained in accordance with NFPA 72.

Carbon monoxide detectors are required.

It is best to buy smoke alarms that have built in CO detection capability.


Fire sprinklers

Fire sprinkler systems are required by international building code in all houses after Jan 1st 2011, but local jurisdictions will likely soften the requirement in their particular area.

In my case I needed them anyway because my road is officially too steep for fire engine access.  In practice they will come ok as they are very helpful guys, but those are the official rules.

Sprinkler systems must be installed in accordance with IRC Section P2904 or NFPA 13D.

Homeowners are allowed to install their own systems (rather than needing a licensed contractor), but a licensed designer is likely to be required.

The system needs a minimum water discharge of 10 minutes.

It is ok to feed the sprinkler system from the regular house water system as long as it can meet the 10 minute operation requirement.

Sprinkler heads are needed in all rooms except for:  Closets less than 24 sqft, Bathrooms less than 55 sqft, Open porches, Garages, Attics.

In practice it is best to put a sprinkler head in every room even if a room is listed as not needing it.  In large rooms it is best to have two.

It is not necessary to have a fire truck connection point, but they are allowed.  If you do include one it is required to be tested at 200psi for 2 hours.

I have decided not to have a fire truck connection point because the house plumbing and the sprinkler system water supplies are connected so the pressure test would wreck the house plumbing and fixtures.



In an attached garage (or in my case part of the basement that also serves as a garage) you need to use 5/8" Type X drywall or equivalent.  This is even more important where there is a living space above the garage.

In my case half of the "basement" will be used as a garage and half will be unfinished space, but there is a fireproof wall between the garage area and the unfinished space area.  The drywall needs to be properly sealed, but it does not need to look pretty.

The door from a garage to a living space is required to have a fire rating of 20 minutes.  This is not a particularly hard requirement to meet.  It just means buying a pre-hung door with the necessary spec.

A bedroom must not be connected to a garage space by a door.

Pipes from a garage into a living area require sealing around them using an approved material to limit the free passage of fire and smoke.

In my case with the whole house being above the garage (basement) there are lots of pipes that need to all be thoroughly sealed.  I will use fire resistant foam that has the necessary sealing and fire resistant spec.

Ducts between the garage and the house living area must be 26 gauge steel or other approved material.

Garage floor surfaces must be of an approved non-combustible material.  A raised floor using uncovered 2x4s would not meet this requirement.

The area of floor used for parking vehicles is required to be sloped to facilitate the movement of liquids to a drain or towards the main vehicle entry doorway.  It may be necessary to implement a thin sloped concrete layer on the floor to provide a drainage slope if the building inspector requires.



Habitable attics are not considered a story, but must meet minimum room size & ceiling height requirements, require a smoke detector, emergency escape and rescue opening and means of egress complying with R311.  Ceiling height in accordance with R305.

Occupiable attic floor area must be at least 70 square feet.  Occupiable space is enclosed by the roof assembly above, knee walls on the sides and the floor/ceiling assembly below.

For rooms with sloped ceilings, at least 50% of the required floor area of the room must have a ceiling height of 7 ft with no ceiling height less than 5 ft.

R-38 insulation is required in cathedral ceilings in an attic that is a warm zone.

EPS can conservatively be regarded as R-4 per inch, so R-38 equates to 9.5" of EPS.



Walls need to be insulated to R-24 (if you want to not incur window limits).

EPS can conservatively be regarded as R-4 per inch, so R-24 equates to 6" of EPS.

Foam (eg the polystyrene of ICF blocks) on interior walls must be covered with 1/2" gypsum board.  This means you cannot just have wood paneling such as wainscoting over ICF polystyrene.  You need to have drywall under the wood paneling.

In wet areas such as bathrooms, regular drywall is not allowed as a substrate for tiles.  Even green board cannot be used as a direct substrate for tiles.  Using a waterproof membrane prior to tiling allows pretty much any substrate to be used, but it is still better to use something other than drywall.

For both walls and ceilings, drywall baton support can be 24 inches on center if perpendicular.

The minimum width of a hallway is 3 feet.


Ceiling height

The minimum ceiling height according to the 2012 International Residential Code is 7 feet.   There are exceptions such as basements and sloped ceilings.  The information below is based on page 54 of the IRC.  All dimensions are based on finished measurements.
Habitable spaces, including hallways and laundry rooms, must have a ceiling height of 7 feet.

Rooms with sloped ceilings must be no lower than 5′ and have no less than 50% of the ceiling less than 7′.  The knee walls can be no lower than 5′, the space beyond this does not count.

A bathtub with a shower head must have a ceiling of no less than 6′ 8″.

Unfinished basements must be 6′ 8″ with the exception of beams, duct work, or other obstructions which must be no lower than 6′ 4″.


Decks and flat roofs

Decks and flat roofs require drainage, eg an overall slope or sloped to a drain or sloped grooves.


Exterior Insulation Finish System (EIFS)

Code for EIFS requires water resistive barrier, then rigid polystyrene insulation, then a reinforced base coat, then a trowel applied textured finish.  Code requires a means to drain any moisture trapped behind the EIFS to the exterior.  It also according to code must stop 6 inches from grade.  If you use proper well documented building science then it is likely that the building inspector will be happy with your design.

My concrete wall design with 8" of EPS polystyrene over the concrete may well be classed as EIFS.  In my case, stopping 6 inches from grade makes no sense as the foundations are also EPS lined.  Also in my wall design there is deliberately no air gap.



Bathrooms must have either an operable window or an extractor fan unit to the outside.

Even if you do have a window it is still sensible to have an extractor.

A pipe to an ERV or HRV counts the same as having a simple extractor fan.

Showers need to be bigger than 30" x 30" or 25" with an area of 1300 square inches (which is 25"x52").

A clearance of 21" minimum is required in front of a toilet, sink, or bidet.

A 15" minimum distance measured from the center of a toilet to a wall or other obstruction is required.

Backer board of cement, fiber-cement, or glass mat gypsum conforming to ASTM C 1288, C 1325, or C 1178 must be used.


Fireplaces and chimneys

Use a manufactured fireplace insert because meeting IRC rules on a masonry fireplace is too complicated and will not work well anyway.

The fire insert must meet the latest IRC requirements.  It must be tested to UL127.

Fireplace inserts must be installed as per manufacturer specifications.

Chimneys must be at least 3 feet above the roof and at least 2 feet higher than any part of the building that's within 10 feet.



If the outside design temperature (the official figure from the government in your area) is less than 60 degrees F then a heating system capable of maintaining an inside temperature of 68 degrees F is required.

Sizing of the heating system must be properly calculated using an approved calculation method.

Furnaces or other appliances with a flame or electrical switching that are in a garage must be installed at least 18" off the ground to avoid them igniting any heavy vapors on the floor from cars.

Heating appliances (eg furnaces) in garages must be protected from accidental impact from cars.  This can be achieved using a curb or bollards or by mounting sufficiently high.

Heating appliances (eg furnaces) must have a 30" x 30" minimum area in front of the controls.

Doors to the heating appliance (eg furnace) room must be at least 24" wide and large enough to remove the appliance.

A room housing a heating appliance (eg a furnace) must be at least 12" wider than the appliance and there must be at least 3" of clearance on all sides and at the back.  Therefore it follows that if you are mounting the furnace on the wall, it must be spaced 3" off from the wall.

The gas pipe to the building must enter the building above ground.



A 5 person house (ie 4 bedrooms) needs 15 x 5 cfm (cubic feet per minute) air exchange, ie 75cfm.

Foundations should provide venting of the foundations (although this requirement varies depending on where you live).  A radon vent pipe should be 3" or 4" PVC.  Given that there is no radon in my area I have decided to route the radon vent pipe under the footing rather than have it come through the slab so that having a perforation is the slab membrane is avoided.

It is sensible to make sure you have good radon venting even if the radon levels are below the allowed limits.  In a high radon area, use a 4" pipe.  It should not be the same pipe as used for the DWV stack.  If it comes out through the roof then the pipe must clear the roof by 12".

If radon gas levels are found to be above the allowed level even with a 4" pipe from the foundations then a fan unit needs to be installed in the pipe to turn the vent from a passive vent to an active vent.


Extraction vents

A clothes dryer exhaust must be 4" diameter and made of smooth walled rigid metal with no screws that might catch lint.

"Dryer exhaust systems shall be independent of all other systems and shall convey the moisture to the outdoors.  The maximum length of the duct is 25 feet."  This requirement does not make sense in the context of an ERV or HRV.

If you send the hot exhaust air directly to the outside you will be wasting a lot of heat.  A better bet is to duct it to an ERV or HRV unit that can do heat exchanging with fresh (cold) air brought from outside.

A clothes dryer must be exhausted to the outside.

The exhaust from a dryer must be at least 3 feet from an open-able window or door.

The exhaust from an ERV or HRV has a similar requirement.

The exhaust from a dryer must have a backdraft damper rather than a screen (because lint gets caught in a screen.

It follows that an ERV or HRV exhaust also must meet this requirement.

Cooker tops must have a hood with an exhaust to the outside made of smooth metal (not aluminum).  There must be a damper door on the outside end.

To avoid heat loss from the house it is best to route the cooker extractor exhaust to an ERV or HRV unit.  Care should be taken in the duct design and placement to avoid sucking up grease into the ERV/HRV (a high ceiling makes this easier).

A cooker vent must be able to draw 100 cfm (cubic feet per minute).

The ERV or HRV will need to meet this requirement while also sucking air from all the other rooms.  You may be able to design a system that restricts or closes off vents in other rooms when the cooker exhaust is required.

In a well sealed house (energy efficient houses need to be well sealed) it is not desirable to have too much air sucked out by a cooker extraction fan because that will require makeup air to come from somewhere and the ERV or HRV will not be able to keep up warming the incoming air.  You should aim to do the minimum to meet the building code requirement.