Tag Archives: vented soffit

Post Frame Purlin Blocking

Every time I begin to rest on my laurels and think I have covered all post frame (pole barn) building basics up jumps yet another one to bite me where I deserve to be bitten due to my overlooking it.

Our independent drafting team at Hansen Pole Buildings (thanks Kristie) came up with this question recently.

“As we are building our building, a question came up: what is the reason for purlin blocking? Why do we need it? What’s the important purpose for it? We will be doing this step tomorrow and actually considered skipping it (sorry, bad of us I know). Is this all explained in the CM, because I have looked and couldn’t find the why’s. I bet ALOT of people skip this step and just wanted to see why we have it.”

Well, our 500 page Construction Assembly Manual covers lots of “how tos” and very few “whys”. Biggest reason is we would hate to make it into a 700 or 800 page manual. We try to cover it all and continually add to it and improve it, so every time we get a question not covered by it, we add more information. Even though these subjects do not make a dime for Hansen Pole Buildings, we have recently expanded sections on Site Preparation and Concrete Floors. It is all part of us delivering “The Ultimate Post Frame Building Experience™” https://www.hansenpolebuildings.com/2019/05/the-ultimate-post-frame-building-experience-2/

Back on task – I will preface this by letting you readers know Kristie and her husband are currently erecting their own Hansen Pole Building.

There exist two types of purlin blocking:

At endwalls (this is Kristie’s case) – Building Codes require airflow from vented soffit on gables overhangs be blocked off. Ventilation for dead attic spaces must be accomplished by either a combination of eave and ridge vents or by gable vents. Venting through end overhangs will disrupt airflow for a properly ventilated attic space.

Structurally a solid load path must be provided in any building to transfer wind shear loads from roofing to ground. Purlins overhanging an end truss and attaching with a hanger such as a Simpson H-1 do not accomplish this. Brackets will not prevent purlin rotation under extreme loads. Properly placed, endwall overhang soffit panels can be attached to these same blocks, as they serve a plethora of duties.

Purlin blocking can also be “mid-span” – when a 2×10 or larger member (girt, purlin, floor joist, etc.) is 2×10 or greater mid-span blocking is required if a member is unsupported for more than eight feet.

There you have it and if you win on Jeopardy thanks to this, I will work for a percentage.

Pole Building Ventilation

Ridge vent without soffit vent

One of the most overlooked areas of pole building construction is proper ventilation. Lack of proper pole building ventilation becomes even a greater issue when an enclosed attic space is present. The International Building Codes require any dead attic space to be ventilated. Without adequate ventilation, moisture from condensation will begin to accumulate on top of the ceiling. Mold and mildew can form on the underside of the roof sheathing and on the roof trusses.

One of the least expensive options for a new pole barn, especially with steel roofing, is to have a vented ridge. Very easily installed at time of construction, if there is ever a possibility of a flat, level ceiling being installed in the building, a vented ridge is a must.

A ridge vent without a soffit vent doesn’t work, and here’s why. By virtue of their design and location on the roof, ridge vents are predominantly exhaust devices. Warm moist air from inside the building rises, passes through the ceiling material and attic insulation and out through the highest point – the ridge.

The attic space will get makeup air to replace the air the ridge vent has exhausted along the path of least resistance. If there is plenty of soffit venting and if you have a relatively tight ceiling, then the makeup air will come from outside, which is desirable, summer and winter. However, without soffit vents, the makeup air comes from indoors, a situation which is not desirable in any season.

So what to do if you have a building with an attic space, and little or no ventilation?

attic ventilationIf mold is already a problem, scrub the affected areas with a diluted bleach and soap solution. Once clean and dry, a mold resistant paint can be applied.

I’ve heard others suggest a roof design without ventilation, an issue that is volatile and multifaceted. Basically – doing away with the dead air space in the attic. As I see it, the choice to go or not to go with attic ventilation does not in itself ensure good performance. The bottom line with attic assemblies, whether vented or not, is that they be done properly.

Filling the attic space with cellulose insulation may be an option, although expensive and is not a 100% guarantee to solve the problem. Cellulose insulation is dense, blocks airflow and contain salts which inhibit mold growth. In my opinion, many of the innovative uses I’ve seen for cellulose are experimental, but those experiments seem to be working well. If conditions permit, you could find a way to blow in cellulose at the gable ends of the trusses. Better yet, you could fill the truss cavity from the ridge. In any case, with cellulose insulation as a “total fill”, I recommend the use of a vapor barrier, such as plastic sheeting on the underside of the trusses and/or ceiling joists or a vapor-barrier paint applied over the drywall.

Only without a dead air space should a ceiling vapor barrier be utilized.

Maintaining low indoor humidity may be also effective, but it may require wintertime humidity below 25%, which could be uncomfortable, as well as near impossible to achieve.

Add Pole Building Ventilation.

If your pole building does not have enclosed vented overhangs, it may be possible to add ventilation along the top of the building sidewalls. However, each individual case should be examined, as drilling holes or cutting into a structural member could compromise the building’s integrity.

If a continuous ridge vent is not present, install one. Make certain there is a clear air flow from the attic space through the ridge – which may entail the removal of any sheathing (oriented strand board – OSB or plywood), vapor barriers, or other insulations directly beneath the ridge.

Although the newer versions of the Code prohibit the combination of gable vents and ridge vents, gable vents may be installed in each end of the building – to provide an intake for outside air.

The real solution – is to build it right in the beginning. Prudent design with vented soffit overhangs and a vented ridge may involve some initial investment, but prove to be an insignificant cost over the life of the building. And trust me, no one I’ve ever talked to had a “fun” time dealing with mold.