Tag Archives: deflection

Insulating a Steel Truss Building

Insulating a Steel Truss Building

Reader JONATHAN in MISSISSIPPI has been planning a building using steel trusses and has insulating questions. He writes:

“I have recently found your blog and I have to say I am on good information overload.  I’ve read your posts on insulation and air barrier more than twice maybe more.  I live in Mississippi so hot and so humid.

My plan is to build a 32×60 using steel trusses 10′ on center and 2×6 purlins and at the 28′ mark I am wanting to put up a wall to cut the space in two, half wood shop half living area. My biggest question is about insulating the roof for both areas the same, which would be a closed/unvented roof (no attic). I am going to put sheeting over the whole building (walls and roof) and use closed cell spray foam for insulation on the roof, filling the entire cavity of the 2×6’s.  On the underside of the 2×6’s I am going to install some seasoned metal for the ceiling. 

My question is, what if anything do I need to install between the metal roofing and the sheeting? Tyvek? 30# roof felt? or would this work https://www.lowes.com/pd/48-in-x-250-ft-1000-sq-ft-Synthetic-Roof-Underlayment/3151833? Does a unvented/closed roof need to breathe any? Because if it doesn’t I really like the synthetic roof underlayment. Or do you have any suggestions?

On the walls I am going to stud vertically between the posts with 2×6’s with sheeting on the outside, cover it with Tyvek, and metal over that. What suggestions do you have on insulating the walls? Do I need an additional vapor barrier on the inside of the walls? I was thinking maybe a thin layer of closed cell foam on the inside and going with mineral wool insulation batts between the studs.

I had a lot more questions than I thought I did, whew! I just want to make sure I am doing it right, without any problems down the road and I am ok with a little overkill and cost to do it. Just wish I could afford/justify SIP panels for the roof.  

Any and all information and guidance is appreciated.”

Mike the Pole Barn Guru writes:
I will first express my concern for your desire to use steel trusses. Unless your provider can furnish engineer sealed drawings showing adequate load carrying capacity for your particular circumstances (you have added dead loads beyond what they are typically designed for, as well as an appropriate wind load) I’d be running away from them. They also should be designed to minimize deflection. I’d want some written proof of these trusses having been third party inspected for quality as well. You are going to be making a significant investment into your new building – no reason to have it fall down around you.

Moving forward. Between roof sheathing and steel roofing you do need to have something. A minimum of 30# felt should be used, although synthetic underlayment would be just fine. You may want to investigate a system including a ventilated roof mat, as it will reduce thermal heat transmission. A weather resistant barrier such as Tyvek would be an absolute wrong product.

For walls, you should create a thermal break between studs and interior. I’d glue two inch closed cell foam boards to stud inside face and then glue 5/8″ gypsum wallboard to foam board inside face. I’d probably fill wall cavity with BIBs insulation rather than closed cell foam and mineral wool batts. This will more fully fill cavity without creating voids.

I have yet to see SIPs as being economically practical. They appear to be expensive enough so as to preclude ever being able to recoup investment costs.

 

 

What Thickness OSB to use Under Shingles

What Thickness OSB to Use Under Shingles

Reader JOSH in POST FALLS writes:

“My pole building is going to have asphalt shingles. I know how much you dislike shingles vs a metal roof, but the garage needs to match the house. My question is what thickness of OSB should I use? I saw 7/16″ in Appendix VI in your guide, but wasn’t sure if that was adequate in my case. My trusses are 10′ OC and purlins are 2′ OC with a 50# snow load. I’ve seen a lot of random chit chat on forums about roof sag and such, but none of the posters seems to have any substance to back up their “guesses” about what is really adequate. Looking forward to hearing your expertise on the subject.”

Mike the Pole Barn Guru writes:


Deflection is the dictate here – the IBC (International Building Code) allows for maximum deflection of a shingled roof to be l/180 for live plus dead loads or l/240 for live loads only. Keep in mind, these are the maximum allowable deflections – which means under a full design load you can expect to see a deflection (sag) between the purlins of over 1/8″. Of course these loads will only be experienced with a roof covered with snow, which means you are not able to see the sag.

The tables in the Code itself do not cover your live load, so a trip to the TECO® OSB Design and Application Guide tables is necessary. You need to find a sheathing with a span rating of at least 32/16, which means either 15/32″ or 1/2″ thick OSB. With this thickness any deflection under dead loads only (the weight of materials only) should prove to be imperceptible.

For the curious – 7/16” OSB has a span rating of 24/16 and with supports every 24 inches is good for a roof live load of 40 psf (pounds per square foot) with a 10 psf dead load. The thickness required by Josh’s circumstances are good for roof snow loads of up to 70 psf (again spanning 24”). For heavier loads – a 40/20 rated panel (19/32” or 5/8”) will support a 130 psf live load and 48/24 rated panels (23/32” or ¾”) are good to175 psf!

If deflection (sag) is a concern, there is really only one time to decide to err on the side of conservatism and go with a thicker panel – before you make the investment!

 

Drywall Idea, Bolt Counts? and Don’t D-I-Y This!

DEAR POLE BARN GURU: Will I have problem with moisture in the wall if I nail drywall to the gerts and leave the 6×6 poles exposed? I may put a stove for heat in it while I am in it occasionally. I have insulted the roof. Concrete floor. JAMES in NEW ALBANY

DEAR JAMES: Provided you have a good building wrap between the siding and the wall girts (read more about building wraps here: https://www.hansenpolebuildings.com/2012/11/house-wrap/), as well as a well-sealed vapor barrier between the girts and the drywall, you should be able to minimize the effects of moisture in the wall.

Now your bad news. I will take a wild guess and surmise your post frame building has girts nailed flat on the outside of the columns. If so, and you attempt to drywall to the inside face of the girts, be prepared for infinite issues with the drywall joints cracking due to excess deflection.

If there is no building wrap, a quick and easy fix is to have an inch or more of closed cell foam insulation sprayed on the inside of the siding.

I’d most probably either build a vertical stud wall between the columns, or place another set of horizontal girts on the inside of the columns. Either of these would afford an insulation cavity with enough depth to make a difference. This would allow BIBs insulation to be blown into the wall with a minimal number of heat transfer points.

DEAR POLE BARN GURU: How many lag bolts should be used in a 4 x 6? This is for the truss supports. COREY in PAW PAW.

nailing trussesDEAR COREY: My educated wild guess is your post frame building has trusses placed on top of a truss carrier (basically a header from column to column).

You can find the size and number of required fasteners by looking at the data prepared by the engineer who designed your building, as this information will be on the sealed plans.

Numerous factors would be involved in the determination of adequate fastening. If the carriers are notched into the columns, far fewer fasteners will be required, as they will only be needed to resist wind loads.

If the carriers are placed on the sides of the columns, then the roof load is typically the governing factor. The fasteners then have to resist the live loads (snow and any attic bonus or storage space) plus the dead loads (weight of roof system and covering, as well as any ceiling.

The spacing of the columns and span of the truss impact the number of fasteners as well.

If for some reason this information is lost or missing from your plans, a competent local RDP (Registered Design Professional) should be engaged to provide a connection design for you, as this is hugely critical to prevent unexpected failures which could result in bodily harm or death. DO NOT GUESS.

 

DEAR POLE BARN GURU: How can I build a strong 30 foot truss that won’t sag. LARRY in TYLER

DEAR LARRY: I hate to just throw out the obvious, but in your case I will – DO NOT BUILD YOUR OWN TRUSS.

Prefabricated metal connector plated wood trusses are nothing short of an engineered miracle. You can have them designed to support any load which you can conceive of, have them delivered to your site and engineer sealed drawings are provided to confirm the required load conditions are met.

A quick Google search of “Tyler Texas Wood Roof Trusses” will give you several possibilities to discuss your needs.

 

 

Thoughts on a Floor

Thoughts on a Floor:  

Brought to you by reader ANDREW in LEBANON:

“Hi! I am looking at purchasing a post frame building to use as a new home. We are well on our way with being under contract for the land and one of your recommended builders is meeting me at the site this week to make sure the land is good/flat enough.

I will be hiring the construction of the exterior and then build the interior myself.

With that said, here is my question (I will do my best to describe it by typing.) Instead of pouring a huge concrete slab (building will be 60×96), I want to do a typical crawl space to be easier to run plumbing and such, plus make changes as needed. Also, concrete slabs are expensive, especially for 5,000+ sqft. What are your thoughts? I will run 2×10 side by side (doubled up) the entire 96′ length supported every 12′ by concrete footers and building columns. This will be roughly 24″ from the ground (haven’t fully decided on the height yet). Along with that, going to 60′ width, I will use 2×8, 16″ OC. I forgot to mention, along the inside perimeter of the posts, I will be running 2x10s attached to the posts. The ends will have the 2×10 laying on top (along with concrete/building posts every 12′), and the joist ends resting on the eave sides.

With all that said (hopefully legible and not rambling), what do you think? I think it is a pretty solid plan and will not only save a lot of money by not doing a slab, I will effectively have a crawlspace. Yes, I know this will raise the entry points so the door looks like it will be off the ground 3+ feet, but I will be putting a decent sized deck on the front as well as a smaller one on the rear point of egress. A quick reply would be greatly appreciated so I can hopefully discuss more with the builder as well as for my own personal planning purposes. Thanks a lot!”

DEAR ANDREW:  I am a fan of living on wood instead of concrete, so crawl space makes total sense to me.

The right way to do this is to have your floor incorporated into the original engineered plans for your building. This will assure you of several things – the footings will be designed with an adequate diameter to resist settling (last thing you want is to have a post or posts sink. It also will make sure the size of the members will be adequate to support the loads both from a weight bearing standpoint as well as deflection. Your doubled 2×10 idea for supporting the floor joists is hugely under designed and it is very possible it would create a failure condition, not something you want to have occur in your new home.

Floor deflection is an under discussed realm (you can read more here: https://www.hansenpolebuildings.com/2015/12/wood-floors-deflection-and-vibration/). 2×8 #2 at 16 inches on center and 2×10 #2 at 24 inches on center are going to have virtually the same spanning abilities as floor joists, however the 2×10 floor will meet L/480 requirements for deflection, while the 2×8 joists just barely meet the code minimum of L/360. The added plus – the 2×10 joisted floor takes 16% less board feet of lumber and is less expensive to build!