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RalphA
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# Posted: 13 Nov 2015 23:06
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Hello. First time for me posting on this site. Lots of great information here! . However, I am in desperate need of how to figure a weight load limit, and wind stress limit, on my Gambrel style roof. I have searched the internet and very little information is out there on weight loads on this style of building. My building in a 14 x 20, 2x4 construction 16oc with 8 foot walls. The structure of the Gambrel roof is as follows, (Please refer to the diagram attached below). The span or width at the base of the roof is 14 feet (A). The second width point is 10 feet (B). The inside height is 6.5 feet (C). And each section of the roof line is 5 feet (D & E).
I took this building down and am now reconstructing it on my property. It was originally constructed with the metal roof panels directly attached to the rafters, or Gambrel tresses. It stood for 15 years with no problems that I know of. However, West Texas gets a lot of wind in the Spring. Not to mention heavy thunder storms. I want to deck the roof with OSB to make a tighter structure. Each of the 4 x 8 sheets of OSB weights approximately 47 lbs. Each of the four roof sections will require just over three 4 x 8 sheets, approximately 150 lbs, for a total of 600 lbs for the OSB alone. Then each of the four roof sections will have 7, R-panels. Each panel weights approximately 14 lbs, so each section about 100 lbs. Therefore that total weight on this roof is about 1,000 lbs. These tresses are 16oc.
So, is this structure able to withstand that type of weight load? I do not really want to add support inside of the tresses, unless it is absolutely necessary. I love the space that this style of roof creates for the loft. Thank you so much for your help and any assistance, as this is the first time I have attempted this level of construction.
Ralph
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Steve_S
Member
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# Posted: 14 Nov 2015 07:10
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Welcome Ralph, hope you enjoy this site.
Gambrel Roofs can be quite tough and take a lot. Without a Ridge Board or Purlins it's weaker and may be subject to racking. Weight of 3/4" OSB is about 75 Lbs, 5/8" about 65 Lbs each sheet. 7/16-1/2" isn't for a roof.
I would be concerned with the existing 2x4's having been previously used and weakened with the holes that are likely 1-1/2" to 2" deep.
Have a look at this site: Blocklayer.com Gambrel Roof Geometry Calculator and you can tinker with it to see the differences.
Upgrading to 2x6 you only lose about an 1-1/2" but that can be compensated for in the geometry and if you add Purlins & Ridge , you get a really tough roof.
Here's an example from Blocklayer with Ridge, Purlins and gussets with 2x6. That would certainly handle sheathing +. I would still go with Tin Roof with a venting airspace between Sheathing layer & tin to keep you cooler in summer.
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Don_P
Member
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# Posted: 14 Nov 2015 07:47
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I'm not seeing any load info on that Steve, is that site creating anything more than a pretty picture?
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Steve_S
Member
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# Posted: 14 Nov 2015 09:07
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Don, check it out, the link is there. No it doesn't do the loading calc's but sure enough someone here will have those numbers off the top of his head...
I've looked (searched) for loading data on gambrel roofs and didn't find anything that was clear and I don't want to muddy those waters.
I put that there so Ralph could see the internal dimensional changes if he upgraded to 2x6 vs recycling the previous 2x4's. And if he wanted to do so, the dimensions, cuts etc are all shown which is a good bit of help when trying to figure stuff out.
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MtnDon
Member
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# Posted: 14 Nov 2015 12:48 - Edited by: MtnDon
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I don't see any fields for loads, such as snow load, on that blocklayer page,either. The following statement appears on the blocklayer site.... "Geometric calculator only. Check local building regulations for sizes, spans angles etc. " The second sentence really means to check with an engineer because the code books do not have a prescriptive solution to rafter sizes for a gambrel roof.
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RalphA
Member
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# Posted: 14 Nov 2015 15:19
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Thank you so much Steve for your reply! I agree, A 2x6 would be much better than what I currently have. However, I believe I am gonna be limited to three choices for several reasons. 1. Reassemble it the way it was with just the R Panels. 2. Assemble it with the OSB and listen to most people that I have spoke with in the construction field, stating "That is a strong type of roof so you will be fine." (not to sure about that statement) Or 3. Assemble it with the OSB and R Panel and reinforce it with the ridge board and purlins.
Steve, I was concerned about reusing the old tresses because of the holes as you mentioned. My solution to this was as follows. I used new 2x4 on my double top plate. Old nails were not removed from the 2x4 but cut from the old top plate, to avoid splitting and cracking. Some of the nails had to be punched out. In the reassembly I used 3 1/2 screws for a better and tighter fit. I also used a metal strap (plumbers strapping) that ran up the 2x4 about 8" under the top plate and back up the other side, to aid in wind sheer loads. Also, all but three of the 16 tresses are screwed to a 2x6 joist that is the support for the loft. (see attached)
In researching this I did find and look at the site you mentioned Steve, blocklayer. A great site! If the inspectors are like everyone else around here they will not know the difference. However, I trying to build this the best I can with what I have. I would like it to stay in place. I have seen what wind and storms can do. Amazing! Funny that load tables on this type of structures are so hard to find. Thank you everyone for your help! Still not to sure of what I will do. Thanks for all the help and advise!!
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RalphA
Member
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# Posted: 14 Nov 2015 15:21
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Thank you so much Steve for your reply! I agree, A 2x6 would be much better than what I currently have. However, I believe I am gonna be limited to three choices for several reasons. 1. Reassemble it the way it was with just the R Panels. 2. Assemble it with the OSB and listen to most people that I have spoke with in the construction field, stating "That is a strong type of roof so you will be fine." (not to sure about that statement) Or 3. Assemble it with the OSB and R Panel and reinforce it with the ridge board and purlins.
Steve, I was concerned about reusing the old tresses because of the holes as you mentioned. My solution to this was as follows. I used new 2x4 on my double top plate. Old nails were not removed from the 2x4 but cut from the old top plate, to avoid splitting and cracking. Some of the nails had to be punched out. In the reassembly I used 3 1/2 screws for a better and tighter fit. I also used a metal strap (plumbers strapping) that ran up the 2x4 about 8" under the top plate and back up the other side, to aid in wind sheer loads. Also, all but three of the 16 tresses are screwed to a 2x6 joist that is the support for the loft. (see attached)
In researching this I did find and look at the site you mentioned Steve, blocklayer. A great site! If the inspectors are like everyone else around here they will not know the difference. However, I trying to build this the best I can with what I have. I would like it to stay in place. I have seen what wind and storms can do. Amazing! Funny that load tables on this type of structures are so hard to find. Thank you everyone for your help! Still not to sure of what I will do. Thanks for all the help and advise!!
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MtnDon
Member
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# Posted: 14 Nov 2015 15:33
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What type of screws were used as shown in the photo? Unless they are structural rated screws they may not be safe. The very common "deck" screws found in most stores are hardened to make the drive socket head more resistant to tear out when driven with a power tool. This treatment also makes the shank brittle. Common constructions nails are not hardened. Nails will bend when a shear force is applied to them. Failure can be slow over many years. Non structural screws will more likely snap suddenly. Screws that are approved for structural use come with a paper pedigree, papers that prove testing was done and what sort of loads those screws are suitable for. Screws may have greater pull out resistance than nails but that's about all.
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adakseabee
Member
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# Posted: 14 Nov 2015 16:27
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Quoting: MtnDon Screws may have greater pull out resistance than nails but that's about all.
Agree! That is why I use both screws to resist tension tending to pulls things apart and nails to resist shear forces along with construction adhesive, gusset plates and Simpson Strong-Ties wherever appropriate.
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RalphA
Member
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# Posted: 14 Nov 2015 17:16
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Well.... I did not know that. Why is it I always learn something after I build it? They are just exterior 3 1/2'' screws. However I did use both nails and screws in the tresses.
Thanks guys!!
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Steve_S
Member
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# Posted: 14 Nov 2015 19:04
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@MtnDon
Quoting: Steve_S No it doesn't do the loading calc's but sure enough someone here will have those numbers off the top of his head...
I looked all over and could not find anything for loading on Gambrel. I had a similar stressor when I was considering Gambrel for my build. I guess there is just so many variables in that type of roof... I dunno.
SECOND ON Simpson Strong-Ties ! They got something for everything and I use them quite a bit... sure simplifies a lot of things and building inspectors seem to like them as well... A Happy Building Inspector makes life so much simpler.
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RalphA
Member
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# Posted: 14 Nov 2015 19:25
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Quoting: Steve_S I looked all over and could not find anything for loading on Gambrel. I had a similar stressor when I was considering Gambrel for my build. I guess there is just so many variables in that type of roof... I dunno.
That's what I ran into as well. I looked for many hours and came up with no real figures. But, I did find this site and I am enjoying a lot of what it has to offer!
Thank you Steve for looking, I do appreciate it!
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MtnDon
Member
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# Posted: 14 Nov 2015 20:24
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Some things simply need an engineer.
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RalphA
Member
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# Posted: 14 Nov 2015 22:31
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MtnDon
Quoting: MtnDon Some things simply need an engineer.
Very true, which I am not even close to in any way. Thanks for your help Don!!
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Julie2Oregon
Member
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# Posted: 15 Nov 2015 04:10
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Hi RalphA!
Are you planning to live in this as a small cabin? You're very right about the wind in West Texas. I live in Abilene (I'm moving to Oregon and building a cabin there next year!). The wind is fierce, frequent tornado threats, terrible thunderstorms often with huge hail and destructive straight-line winds. You do need to build accordingly.
I've had a new roof and two repairs on my home's roof in 8 years. It's not the design or the quality -- it's the crazy weather events. You DO need to ensure that the roof can handle up to baseball-sized hail and up to 60 mph winds on a regular basis. I'm not exaggerating.
Also, what sort of foundation are you planning on? My house is on piers which is actually fairly common here and a selling point for me. Because this area goes through exceptional drought, interrupted by occasional years of record rainfall (like this year), the clay soil expands and contracts bigtime. Concrete foundation problems are endemic. When I went house-hunting, every house I looked at that was on a slab had a variety of issues. Piers are easy to adjust, and I reskirted with a new frame and metal.
I just wonder what sort of foundation would best support your gambrel building with the reinforced roof, with the weather beating up the structure?
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adakseabee
Member
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# Posted: 16 Nov 2015 20:20
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[quote=Julie2Oregon]I just wonder what sort of foundation would best support your gambrel building with the reinforced roof, with the weather beating up the structure?[/quote
Julie, I'd use the time-tested method used by many farmers and ranchers with livestock and machinery to get under cover - pole barn construction where poles extend into the soil several feet and extend up the walls to support the framing for the roof. http://www.pole-barn.info/how-to-build-pole-barn.html
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RalphA
Member
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# Posted: 16 Nov 2015 21:01
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Hey Julie2Oregon, I wish I was moving to Oregon. This place has such violent weather. My building is nearly complete at this point. My foundation is concert (5 sack) 24' deep and 24' wide for the footer and 5 1/2 in the center slab. And of course reinforced with rebar. I will not be living in it but I want it to stay put to the best of my ability. I too like pier and beam, but not on this. Most people that have looked at my roof tell me it is over kill, but better safe than sorry. Each 2x4 tress is nailed with two nails on each side, and one screw on the end. I also used a metal strap on every other tress. Now I believe I will add a Simpson Strong Tie just in case. Only thing is that I have to use them on the inside due to the high degree angel of the tress. All pictures and diagrams I've seen show them on the outside wall. Not sure if that will mater or not. Thanks!
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Don_P
Member
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# Posted: 16 Nov 2015 21:21 - Edited by: Don_P
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Outside is correct but either will hold this together there.It's an uplift strap, in that situation a piece of flat metal strap down the rafter and onto the wall accomplishes that. For the load there, I wouldn't care if it was band strapping.
The max rafter span is 5', a 2x4 is plenty adequate for any normal loading.
Look at your top sketch. Imagine a load on the upper roof pushing down. If the 10' dimension can spread under the toggling action of the upper rafters, then the peak will drop. That is the way these sheds fail, they swayback, the ends stay high and the center drops.
This is not a truss. If the upper roof is trussed then that failure cannot occur. A 10'2x4 tied from gusset plate to gusset plate forms a triangle... a truss. Triangles cannot change shape, as long as the 10' tie remains connected it holds the 10' dimension and the peak cannot drop. So trussing it would work.
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OutdoorFanatic
Member
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# Posted: 16 Nov 2015 21:26
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Quoting: Steve_S Upgrading to 2x6 you only lose about an 1-1/2" but that can be compensated for in the geometry and if you add Purlins & Ridge , you get a really tough roof. Here's an example from Blocklayer with Ridge, Purlins and gussets with 2x6. That would certainly handle sheathing +. I would still go with Tin Roof with a venting airspace between Sheathing layer & tin to keep you cooler in summer.
Yeah, I would either double up on the 2x4's or switch over to 2x6.
Would help to build a couple of knee walls too.
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RalphA
Member
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# Posted: 16 Nov 2015 23:11
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Thanks Don for the clarification on terms. Seems like there are many terms used interchangeably. Also good idea on the 10' 2x4!
Thank you!
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