Tag Archives: building ventilation

Site Slope, Gable End Exhaust Fans, and Setting Trusses

Today’s Ask the Guru blog answers reader questions about building on a slope, a recommendation for gable end exhaust fans, and setting trusses into columns.

DEAR POLE BARN GURU: I always see pole barns being built on level ground. is there any reason my concrete slab, perimeter board, and exterior grade can’t slope 1/4″ per foot from the back to the front of my pole barn garage? I realize the bottom of my metal siding would probably have to be cut at an angle to match the slope. DAVID in WESTFIELD

DEAR DAVID: Could and should are two different animals.

Everything is going to be far easier working from a level site, where grade can be sloped away from building at 5% or greater (a 5% slope is six inches in 10 feet). You don’t want to have water from any direction running towards your building.

International Residential Code (2021 IRC) Section R309.1 and International Building Code (2021 IBC) Section 406.2.4 address concrete floor slope: “The area of floor used for parking automobiles or other vehicles shall be sloped to facilitate the movement of liquids to a drain or toward the main vehicle entry doorway.” Floor slope by IBC only applies to “U” occupancy buildings of 1000 square feet or less. Actual slope requirement is unspecified however a generally accepted minimum is 1/8” per foot. Grade inside of building should be gradually built up to create this slope.

As far as cutting bottom edge of your steel siding to follow a slope, cutting each panel to precisely follow slope would be a difficult, if not impossible task. This also would leave a raw field cut edge in close proximity to ground and it will rust.


DEAR POLE BARN GURU: I have a 30×48 pole barn. 13′ at eaves/overhang and 19′ center height because of scissor truss. No windows. One man door and one garage door. It has closed cell insulation. Brown roof. Used for trailer/camper storage. Not a lot of work going on in this building. It stays a little cooler than the outside temperature but is humid with musty smell due to no ventilation. I would like to install a gable end louvered exhaust fan but everywhere i search i come up with different answers. I don’t need fast air movement, just replacement. i would like to know what size fan and air intakes are required to do this. Some of this information may be irrelevant but it was asked for among other inquiries. Thanks for your time and help. WILL in WINFIELD

DEAR WILL: You should plan upon three to six air exchanges per hour. With 23,040 cubic feet of interior space, you need 1152 to 2304 cubic feet per minute (cfm) as an exhaust. A professional HVAC provider can confirm amount of air intakes required, as well as set you up with an appropriately sized exhaust fan.


DEAR POLE BARN GURU: I have a question on installing trusses on a 16×40 pole barn. The bottom of all poles are square and measure what they should on the print. Knowing that the 6×6 poles are probably not straight, is there a trick to making sure that I install the trusses straight and square at the top? Always want to double check before Ii start lifting these 14 feet in the air. Thank you for any help. BOB in MOORCRAFT

DEAR BOB: This is one reason I like to build with my trusses directly attached to columns (using notches cut into columns to provide full bearing). After verifying trusses are exactly 16′ in length, trusses can be placed into notches, with outside of truss flush to outside of columns. This solves any width issues. Using recessed purlins, joist hung between trusses, purlins can be pre-cut to length of space between trusses, solving length issues. Once framing is completed, each roof plane can then be squared (read how here: https://www.hansenpolebuildings.com/2020/05/how-to-square-a-post-frame-building-roof/).

What is a Trickle Vent?

Apparently this is my week for learning brand new stuff, and as luck usually has it, when I get to learn – it is bushel baskets full!

A Hansen Pole Buildings client is constructing a post frame building home in Clallum County, Washington. In discussions with the local Building Officials, he was advised he would need to have “trickle vents” in his windows, if he did not have a forced air heating system.

One thing we do not profess to know much about (and we specifically exclude it from our scope of work) is HVAC (heating, ventilation, air conditioning). Having never heard of such a thing as a trickle vent, it was research time for me.

With an assist from a friendly young lady at the Clallum County Department of Community Development, I was directed to the International Mechanical Code, which states:

M1507.3.4.4 Outdoor air inlets. Outdoor air shall be distributed to each habitable space by individual outdoor air inlets. Where outdoor air supplies are separated from exhaust points by doors, provisions shall be made to ensure air flow by installation of distribution ducts, undercutting doors, installation of grilles, transoms, or similar means. Doors shall be undercut to a minimum of 1/2 inch above the surface of the finish floor covering.

Individual room outdoor air inlets shall:

  1. Have controllable and secure openings;
  2. Be sleeved or otherwise designed so as not to compromise the thermal properties of the wall or window in which they are placed;
  3. Provide not less than 4 square inches of net free area of opening for each habitable space. Any inlet or combination of inlets which provide 10 cfm at 10 Pascals are deemed equivalent to 4 square inches net free area.

Inlets shall be screened or otherwise protected from entry by leaves or other material. Outdoor air inlets shall be located so as not to take air from the following areas:

  1. Closer than 10 feet from an appliance vent outlet, unless such vent outlet is 3 feet above the outdoor air inlet.
  2. Where it will pick up objectionable odors, fumes or flammable vapors.
  3. A hazardous or unsanitary location.
  4. A room or space having any fuel-burning appliances therein.
  5. Closer than 10 feet from a vent opening of a plumbing drainage system unless the vent opening is at least 3 feet above the air inlet.
  6. Attic, crawl spaces, or garages.

So what actually is a trickle vent?

Trickle VentA trickle vent is a device usually fitted at the top of a window which allows fresh air to circulate naturally through a room, and allows polluted air out. They are controllable, to give the option of having them open or closed. When used correctly, trickle vents do not contribute excessively to heat loss. Trickle vents can also work in conjunction with mechanical extract fans when more immediate ventilation is required.

 And why use trickle vents?

 The Building Regulations state there should be adequate means of ventilation provided for people in a building because poor ventilation affects our health.

 Microscopic organisms, like house dust mites and fungi, thrive due to the moisture produced inside a home. Indoor air is also contaminated by chemicals discharged from the building itself and from the items we use within it, such as computers, carpets, furnishings, etc. In large quantities these pollutants can present a health concern and can cause or aggravate allergies, depression, and lung or heart conditions.

 In the past, adequate natural ventilation was provided by chimneys and gaps in the building structure, for example cracks around window and door frames. Modern living and improvements such as well sealed windows may increase indoor pollutant levels. To combat this, trickle ventilators are a safe and energy efficient way of providing fresh air.

 Trickle vents apparently are fairly widespread in the United Kingdom, and I found this commentary from an employee of a company which provides and installs them:

 “One of the major issues installations companies have with trickle vents is that when we have tried so hard to produce and install the best energy efficient windows possible, we find it completely contradictory to install trickle vents which badly affect the efficiency performance of the window. Once the windows with trickle vents are installed, the feedback from our customers is that they don’t use them. They find them ugly, unsightly, unnecessary, and that if they wanted ventilation, they would open a window. This is the second major issue, customers despise them. The problem here is that to a customer there is no obvious benefit.”

For years the push has been to make buildings tighter and tighter, so now new buildings have become too tight – with one possible solution being the trickle vent!