Originally Published as: Post-Frame Buildings that Last for Generations
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Post-frame construction has humble beginnings, but the industry as a whole has grown into something more as processes and products have become more sophisticated. This is a survey of some of the best practices to ensure long-lived buildings as many people look for longer-lasting investments. Consider whether your projects encompass all of these practices. If not, there is no need to change your process on a dime, but you could start offering upsells on items like metal panels and perhaps slowly move on to some other improvements.
How To Build Longer Lasting Structures?
The best way to ensure a building’s durability is to start with quality components, then combine quality craftsmanship with the best processes currently available. We will look at select improvements in processes and products that can create long-lasting buildings.
Let’s begin with some of the components that are most important to the structural integrity of the building.
Trusses and Post Connections
Among some of the early improvements in post-frame construction were pre-fabricated trusses, which are pre-engineered to meet load requirements. According to the National Association of Home Builders, about 75% of residential home builders use prefabricated roof trusses. The reasons many builders have moved to prefabricated trusses include time and cost savings.
When choosing pre-fabricated trusses, ensure that the plant they come from is certified by a third-party inspection service and that they are specifically engineered for the building and location.
Further, truss plates were formed to strengthen the trusses and help with stability. Heavy duty truss plates are preferred to distribute the load to the poles, increasing stability over joints that were simply nailed or screwed. The truss to post connection needs to be sufficient to help transfer the load to the posts; in general it is best to use heavy bolts to ensure secure assembly of the two components. Consult local codes and engineering guidelines because regional practices may vary.
Site and Foundation
If a building is to last many years, site preparation and the foundation are very important. For example if the site is level in a wet climate, the ground right by the building should be sloped away from the building so water doesn’t pool there. Drainage tile can be installed to help with drainage.
Post-frame foundations are often concrete slabs, either floating or monolithic. The monolithic slab is generally more solid and long-lasting than a floating slab, as concrete is continuously poured to include the concrete floor and concrete footers which extend below the frost line to transfer the load to more stable ground. The posts and walls are installed atop the footings. Monolithic slabs are great in rocky areas, but they are risky in areas that are susceptible to the frost-thaw cycle. When you make a floating slab, the footers and walls are not actually attached to the slab/floor. Overall slabs can last up to 50 or 100 years or more, but they may have problems in areas where flooding is an issue because they are so low to the ground. All footings should be packed with a granular backfill so that moisture will drain away.
Stem walls are another option that is often adopted in areas with uneven ground, especially where soil erosion or shifting ground is an issue including significant frost heave, and the slab will be elevated which helps in a flood situation. Footings are sunk in the ground, then blocks, usually concrete, are placed and built up to form the stem wall. Then the slab is poured over the stem wall and the posts are anchored to the slab. A builder can opt to pour the stem wall using wood as a form, then the anchor can be embedded in the stem wall. The stem wall can be reinforced with rebar for stability. Some builders opt to leave the slab unattached to the stem wall; it floats inside the walls which can help with soil heave. Like the previous slab options, this foundation can last from 50 years to well over one hundred.
A pier and beam foundation elevates the building so that a crawl space is created that has benefits like greater access to plumbing and electrical lines, but more to the point for our discussion, it creates greater stability on a sloped site, and due to the building’s elevation, there is less likelihood of flooding. The pier and beam foundation is more resistant to seismic activity, so if it is common in the area, this is a choice to consider. However, in general, pier and beam foundations may last only 15 years, unless maintenance is kept up, such as moisture mitigation and repair if needed, checking for pests, and pest removal. If carefully maintained, this type of foundation can last up to 50 years.
A simple post foundation is used for some structures, for example some ag buildings. In that case, they should be built on a solidly compacted “soil slab.”
In summary, when you are looking for a long-lasting foundation, you must take several things into consideration: The climate, the groundwater table for moisture considerations, the bearing capacity of the soil, the use of the building, and previous uses of the site (are there chemicals or other lingering effects which the structure will have to contend with?). While you are probably acquainted with the foundation types that work well in the area you build, check the specific site you are working with before you decide.
Posts
The load bearing posts are, of course, an important piece of post-frame construction. Initially they were timbers squared or rounded to fit into holes in the ground, then braced with horizontal beams.
Over time, builders and building owners began to see rotting of the post sections that were placed directly in the ground. As chemicals were found that could help to protect the posts, builders began using posts that were treated to avoid decay and pest abuse. The treatments used helped to keep the posts in good shape much longer. Still, wood placed directly in the ground is not the best choice if you are looking to extend the longevity of the building.
Innovative individuals in the industry began looking for solutions to this problem, and many have been developed. There are sleeves that can be placed over the posts to create barriers that keep moisture away from the section of the post in the ground. There are products that are baked until they bond with the wood, creating a surface that is virtually impermeable. There are also concrete columns that are sunk in the ground to create a pedestal for the post to connect to the wood pole above the surface. Whichever solution you choose, the standard today is that while you should use pressure treated wood, you should not put the treated wood in the ground without one of these forms of added protection.
Post Spacing
Another way to help ensure longevity is to place the posts close enough to ensure that the structure is not overstressed and can transfer the load to the ground. This can be established in a couple different ways. If the building has engineering plans, the spacing will have been specified. If the building has no engineering plans, the builder will need to consult established framing tables.
Fasteners
It is important to ensure that you are using the correct, quality fasteners. Are the materials compatible with the type of fastener you are using? If the fastener will be exposed to the weather, use fasteners with corrosion-resistant coatings, galvanized or stainless-steel fasteners.
Avoid cross-threading by ensuring that you have the right nut and bolt so that the threads align. Also take care to put the bolt in straight. If the building is in an area that experiences seismic activity, locking fasteners or thread locking compound should be applied.
Another concern is proper fastener placement. A consistent screw pattern can prevent leaks, and it will allow the metal to expand and contract, rendering the metal’s thermal expansion and contraction harmless. This will make the metal roof more durable and weather resistant.
Fastening metal panels in the flat or pan instead of through the ribs provides a more secure attachment.
Roofing and Siding
You know that all metal is not the same. Metal gauge, or thickness, should be considered when choosing roofing or siding panels. 29-gauge is the standard for many post-frame construction projects. A thicker gauge can lend greater durability if accompanied by a high level of tensile strength, which is not always the case.
Surprisingly, some thinner panels can have greater tensile strength. Check the example below:
Engineering Calculations -Tensile Strength
psi = lbs. per square inch
- Grade 80 = 80,000 psi
- Grade 50 = 65,000 psi
- Grade 40 = 55,000 psi
- Grade 37 = 52,000 psi
- Grade 33 = 45,000 psi
Commercial Quality = 30,000 psi
Panel Strength Calculations
Grade 80 Panel- 29 gauge
Strength per 36” width of panel
36”x.015 (29 ga) x 80,000 lbs. =43,200 lbs. inch2
Commercial Grade Panel – 26 gauge 36”x.019 (26 ga.) x 30,000 lbs.=20,520 lbs. inch2
The Difference:
43,200 lbs. = 2.105 = 110.5%
20,520 lbs.
In this example the thinner panel in a higher grade is 110% greater tensile strength. Therefore, it is important to consider the tensile strength along with gauge when choosing metal.
Another note on metal is that it is a better first line of defense against fire than many other choices. If the client wants a different type of roofing or siding, it is best to check out its fire rating along with expected longevity and inform the client of your findings so they can make an informed decision.
Metal Coatings
The metal coating protects the metal while providing an aesthetic. Three of the most common metal coatings include polyester, siliconized modified polyester (SMP) and PVDF (polyvinylidene fluoride). As the industry strove to find the solution to fading, chalking, and weatherization, these coatings were developed. Each is considered to be an improvement upon the last with price tags to match. However, that is not the whole story.
All coatings are composed of three things, pigment, resin, and additives. Resin uses the additives to adhere to the pigment, thereby encasing and protecting the pigment. Polyester provides a hard surface and protection from weatherization and is considered to be a good choice for a metal coating that can last up to twenty years. SMP adds silicone to the mix which inhibits chalking and creates a better choice that can include a warranty up to 30 years. PVDF contains alternating carbon-hydrogen and carbon-fluorine bonds giving this coating great resistance to UV exposure, extreme temperatures, humidity, and oxidation, and it ensures the color remains bright and sharp, losing only five Hunter units over 35 years. This is superior fade resistance.
SMP creates a harder finish, so it is the stronger candidate in the impact resistance category. However, if the focus is on color retention, the PVDF coating is better.
Insulation and Vents
Investing in quality insulation that makes sense for the climate can be an important factor in the life of post-frame buildings; it keeps the temperature more stable, avoiding the freeze thaw cycle and helps prevent moisture which can lead to mold and mildew forming. Some insulation will end up settling and flattening out. Rigid board or spray foam are considered good choices as they can help prevent moisture problems. Reflective insulation can assist in keeping excessive heat out.
In addition to the insulation, try to ensure that all wall spaces around doors and windows, anywhere that air could get through, are caulked or weather stripped. When it comes to the roof, however, air flow is important.
As in the case of insulation, vents in the ridgeline, at the eaves, or perhaps at the gables, should be installed to avoid moisture problems which can lead to decaying walls. Since hot air rises, it will carry excess moisture out through the vents in the ridgeline and cooler air will be pulled in through the eave vents.
If you are using the building for certain activities, you will need more ventilation than the passive system outlined above. These include:
• Frequent washing of equipment
• Storing items that off-gas such as chemicals, fertilizer or other substances that off-gas.
• Chemical Usage, including spray paint, high-strength cleaning materials, or other strong chemicals.
• Housing animals; they will breathe, sweat, and create waste, all of which raise the moisture level.
• Living space requires extra ventilation. Building codes will specify what is required.
• A person or people spending a lot of time in the building requires further venting.
• Curing new concrete.
If you need more ventilation, a larger passive ventilation system can be used or a powered ventilation option. There are open ridge vents, vented cupolas, or RV-35 and RV-100 ridge vents. Powered options include attic fans or ventilators, power vented cupolas, exhaust fans, or air exchangers.
More and More People Want Buildings That Will Last
The material tips for longevity that we have considered are largely dependent on the proposed use of the building, as well as the climate, and in some cases the soil type and condition.
It is likely that you can find more ways to increase the expected longevity of your buildings. Doing what you can to improve longevity as well as offering upgrades can improve your status with customers who are not looking for “temporary buildings”.
