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Thread: need help w saltbox roof trusses and metal roofing

  1. #1
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    need help w saltbox roof trusses and metal roofing

    My daughter has asked me to build her an 8x24 saltbox shed for storage as step one and then to add a lean to workshop to back side next year.

    I am thinking of using a pole barn approach for this storage shed which will have 3 open bays each about 8 feet wide for mowers, bikes and potting area. No floor just gravel. Poles will be 4x4 cemented in ground. It will be built in CA where snow or frost are non issues I have never built a saltbox and am looking for advice as to how to design roof trusses and possibly a metal roof which is also new to me. Lowes tells me to give them the exact dimensions of roof and they can special order metal roofing pre cut at a very reasonable price.............$2.50 a linear foot 36 inches wide.

    I plan on having the front height about 8 feet above ground and the back height about 7 feet above ground............having that height should allow me to build a lean to workshop on skids on backside next year with a minimum of 76" headroom. Planning on T&G 3/4 ply floor on 2x4 joists 12" on center. Double doors on ends of lean to workshop to facilitate cutting long boards.

    Any tips or suggestions appreciated.............don't think " lean to" is the proper term but you get the idea?

    thanks in advance. I am leaving lakeside cottage near Bangor Maine today, driving to Florida and then flying to CA to play foreman w 3 grandsons on this shed project. This is going to be fun!

  2. #2
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    No idea about trusses but in California you may want to think about putting foam board insulation below the metal roofing due to the potential condensation that may form on the underside of the metal and drip on items below. Think about using a light color for the metal as well or it will get very hot inside of the building. I like pole buildings and have a 30x40x10 pole barn that has served me well.
    David B

  3. #3
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    I built a woodshed a bit like that (in CA) a couple years ago. I didn't think of it as a saltbox, just a shed with a roof tilted enough to ensure rainwater run-off. I poured piers in the ground, with Simpson post connectors cast in. 4x4 posts socket into those. The posts are basically on a 8' by 8' grid. My woodshed is 8x16, but could be one bay longer to be 8x24. Stud-and-plywood walls hang between the posts on the back and sides, and provide wrack resistance. No wood touches the ground, so there's air circulation under the walls, and so there's little termite threat. Beams (okay 2x8s) run the 8' direction from post to post. The roof rafters are 2x4, running the long direction in the roof. The roof beams are tilted so the roof is about a foot higher in front than in back, and there's a gutter at the back. The roofing is corrugated steel, screwed to the rafters. The corrugated steel panels are standard 10' panels stocked at Home Depot and Lowes -- no custom cutting involved. The screws for the panels have washers to seal around the screw holes. At the front, there are diagonal braces from the posts to the front rafter. They start at about head level, and go up to the rafter at 45 degrees. They give wrack resistance across the front of the shed, but don't interfere much with access. Two rolling barn doors cover the front of the shed. Each one is a bit more than 8' wide. They're plywood front and back with a 2x2 frame in between. The whole thing is built with doug fir construction lumber and T111 plywood. No wood goes into concrete or touches dirt, so the wood is not pressure-treated. The "floor" is gravel. The shed sits at 2400 feet in the Sierras, so it sees snow in the winter and 105 degree temps in the summer. It is still going strong six years later.
    Last edited by Jamie Buxton; 10-19-2011 at 12:38 PM.

  4. #4
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    I can't find a pic, but here's a drawing done in the planning phase.
    Attached Images Attached Images

  5. #5
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    First, a salt box roof would look like a capital "A" with one leg shorter than the other, not a single slope or shed roof, like in Jamie's picture. Typically saltboxes evolved out of additions on to a house leaving one end of the roof a full story (or more) lower than the other end. I'm not sure how that would work with only a one foot height difference. Also a 1ft drop in an 8 ft span (1.5/12) is not enough pitch for a screw down metal roof. Probably more like 3/12 or 4/12 is what the manufacturer would recommend. And I agree that post connectors are they way to go. Keep wood off the ground whenever possible. I would rethink the 76" ceiling height, that's is only 6'4". A typical door is 6'8", I would consider 7' a minimum for ceilings and in a shop probably 8' is a minimum when considering materials and moving them around the shop.

  6. #6
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    [QUOTE=Rob Fisher;1793325]... Also a 1ft drop in an 8 ft span (1.5/12) is not enough pitch for a screw down metal roof. ..QUOTE]

    True if you're building habitable space. But mine is a shed for firewood, and the OP's is for mowers, bikes and potting. If there's an occasional leak, that's not a problem. As it happens, the only leak I've ever noticed in mine is from a pilot hole for a screw, where I missed the rafter.

  7. #7
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    Quote Originally Posted by Jamie Buxton View Post
    True if you're building habitable space. But mine is a shed for firewood, and the OP's is for mowers, bikes and potting. If there's an occasional leak, that's not a problem. As it happens, the only leak I've ever noticed in mine is from a pilot hole for a screw, where I missed the rafter.
    Very true. Still with a slope that low water will tend stay on the roof longer and cause material failure more quickly. It will also tend to collect leaves and other debris more easily. All of this is probably a mute point with your climate, no snow or freeze thaw really makes things easier. The architect in me would still want a minimum 3/12.

  8. #8
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    Quote Originally Posted by Rob Fisher View Post
    ...Still with a slope that low water will tend stay on the roof longer and cause material failure more quickly...
    Really? Water will not just roll downhill when faced with a pitch of one foot in eight?

    Actually, I think a more germane issue with corrugated metal roofing is where the screws go through the roofing. All the manufacturers instructions I read suggest putting the screws in the valleys of the corrugations, rather than on the ridges of the corrugations. That strikes me as begging for trouble. If the neoprene washer doesn't seal properly, or fails over time, then all the water running down the roof is going to run over a potential leak. For this wood shed, I found some wood (sold by the same supplier) that has a zig-zag face that matches the corrugations of the roofing panel. These zig-zag sleepers go on top of the roof rafters in the shed. The roofing screws go through the ridges of the corrugations, through the sleeper, and into the rafters.

  9. #9
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    Quote Originally Posted by Jamie Buxton View Post
    Really? Water will not just roll downhill when faced with a pitch of one foot in eight?...
    The simple answer is yes, it just takes signifigantly longer at a slope of less than 3/12. It will tend to stand longer with the a greater potential for damage the longer it sticks around. Any roof slope less than 3/12 is typically considered a low slope. One foot in eight is a 1.5/12 slope. Low slope roofs usually require a membrane type roof, EPDM, PVC or TPO, among others. Metal roofs can be used on low slopes with soldered seams. And asphalt shingles can be used down to about 2/12 with full ice and water shield underlayment. Roofs at a 3/12 and above are designed to shed water. Roofs below 3/12 are typically designed to hold water like a bowl, and then funnel it to a drain. Usually the goal of a roof is too keep water out so one normally tries to move water away in a quick fashion.

  10. #10
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    This is a picture of a salt-box shed design from a company in NC. with only an eight foot width and span (assuming partitions every bay) I would go with plain rafters tied together with plywood gussets at the peak, then 1x3 horizontal stringers @ 2ft spacing to attach the metal roofing to.
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    Retired - when every day is Saturday (unless it's Sunday).

  11. #11
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    Quote Originally Posted by Rob Fisher View Post
    The simple answer is yes, it just takes signifigantly longer at a slope of less than 3/12. It will tend to stand longer with the a greater potential for damage the longer it sticks around. Any roof slope less than 3/12 is typically considered a low slope. One foot in eight is a 1.5/12 slope. Low slope roofs usually require a membrane type roof, EPDM, PVC or TPO, among others. Metal roofs can be used on low slopes with soldered seams. And asphalt shingles can be used down to about 2/12 with full ice and water shield underlayment. Roofs at a 3/12 and above are designed to shed water. Roofs below 3/12 are typically designed to hold water like a bowl, and then funnel it to a drain. Usually the goal of a roof is too keep water out so one normally tries to move water away in a quick fashion.
    Hmmm.. My understanding is that water flows downhill unless there's something that helps it move uphill. (..Isn't there a rude plumber's rule about that?...) For instance, underneath shingles there is surface tension, and there can be ice dams, to help the water wick back uphill. That's why low-slope roofs are not made with shingles, but with continuous membrane roofing. On this shed roof, each roof panel runs the entire height of the roof. There's no overlapping which might aid in water moving uphill.

  12. #12
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    Quote Originally Posted by Jamie Buxton View Post
    Hmmm.. My understanding is that water flows downhill unless there's something that helps it move uphill. (..Isn't there a rude plumber's rule about that?...) For instance, underneath shingles there is surface tension, and there can be ice dams, to help the water wick back uphill. That's why low-slope roofs are not made with shingles, but with continuous membrane roofing. On this shed roof, each roof panel runs the entire height of the roof. There's no overlapping which might aid in water moving uphill.
    Yes, water flows down hill, I'm not sure where you thought I said it flows uphill. And water does not need to flow uphil for a roof to leak. In fact I would guess most times roofs leak at joints in material with the water flowing from above. In your metal roof example there are still side to side joints that are not water tight but rely on the fact that the water is moving too fast to get stuck in the overlap. There are also all of the screw holes, which is the typical leak point in screw down metal roof. The longer water stays on a roof, any roof, the greater a chance for a leak. The lower the slope the slower water will move. At any rate the whole point of this was to point out that a 1.5/12 slope for a screw down metal roof is generally considered to be too low of a slope for such a roof. It also probably voids the manufacturers warrenty.

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