New Member, First Question

[Hondarider]

Hello, first post here. I am working on a plan to build a 16x30 cabin on Helical peirs in Homer, AK to rent out. Here is a picture of what I have drawn so far, open to any opinions or ideas or corrections anyone has to offer.




My first question (probably of many) is about the perimeter beams (probably glulam) resting on the piers and the porch. I drew it as continuing through from the house foundation to the posts on the porch, but I'm wondering if it would be a better plan to have completely separate framing for the porch, with a layer of house wrap separating, in the case that in the future potential rot from unfinished decking could migrate to my beam, which would be a much bigger problem if it is the same one supporting the house. Thanks for any input and this is all just preliminary, trying to get my thinking right about my design.

EDIT: Use the photobucket IMG tag link. It is the 4th, or bottom link usually. Just paste that into the message, leave a line space before and after and the image will publish as above.  MD

[Don_P]

Welcome to the forum. Yup, cantilevering a beam outside is a great way to bring water in. As a rental it will probably be scrutinized by insurance and inspections a little harder. Have those conversations during this design phase.

Follow your load path on that roof, I'm not seeing support for the ridgebeam out at the porch.

[Hondarider]

Thanks for the reply. I'm guessing you're talking about the connection transferring the load at the top of my porch posts? Here is a more closeup shot of what I have so far: the stack of 2x4s acting as a post from the ridge beam to some kind beam spanning the two porch posts (currently drawn as doubled up floor joists sandwiching a plywood flitch plate), and upping the plywood sheathing on this wall to 3/4" for added shear strength to stiffen this "beam" even more, and some kind of steel knee brace.



It looks to me like it might be better to run my rim joist "beam" to the outside of my post and butt the headers into that instead. Maybe then knee braces wouldn't be needed? What do you think?

[ChugiakTinkerer]

Welcome to the forum Hondarider!  Are you going to have Techno Metal Post put in your piers?  I've long been fascinated with the technology, especially their small equipment for driving the piers.

It looks like you're well dialed in on using Sketchup and its plugins for testing out design plans.  If your plans are for a long-term rental you may want to consider opening up the top floor with a dormer or two.  Windows on either end make for a dark tunnel on the upper floor and if you'll have any interior walls up there it will be even darker.

From a brief look at the official web sites, neither the city nor borough have building code enforcement, so you're free to build however you wish.  Depending on your future plans, it may be worthwhile to look into having your building inspected during construction in order to get an energy efficiency rating.  This may be useful come the day when you want to sell the place or if you're contemplating a mortgage.

I've been in the design stage for a while now on our cabin project at Lake Louise.  What I've slowly come to realize is that you want to start at the roof to get an estimate of the total load the building will be subject to, then work your way up from the foundation in ensuring that load is distributed and supported from the ridge line all the way down to the piers.  To dig a little more into Don_P's question, are you designing a structural ridge?  A snow-laden roof of this size in Homer could have a load upwards of 30,000 lbs.  A structural ridge would be a hefty beam running the length of your roof and supported to carry half that snow load.  With a structural ridge you would have 15,000 lbs bearing on the ridge beam and 7,500 lbs bearing on the walls that support the rafters.

There are other ways to build a roof, and your classic rafters with a ridge board is one way.  All of the weight of the roof is supported by the walls.  The downward force on the roof then exerts an outward thrust that will cause the building to collapse if not dealt with.  The easiest way to keep the walls from spreading apart is to tie them together with rafter ties.  These rafter ties form the ceiling joists in a typical frame construction.  This works great if you want a flat ceiling, but not so well if you want a cathedral ceiling.  Another way to deal with the spreading force is to uses trusses engineered to withstand it.  Scissor trusses can fit the bill nicely in your cabin plan and can probably be purchased for close to what you would pay for the solid rafters.

Here's one version of my cabin design with scissor trusses.  They are turning out be a logistical problem for me because I am 10 miles off the road system,  They could be a sweet solution for you.

[flyingvan]

   Here's what I think of as a scissor truss

[Hondarider]

Thanks for the replies. I redrew it as I described with the "beam" extending to outside the posts.



Chugiak, I hadn't considered scissor trusses much, I'll have to look into them more. The thing I like about my plan so far of using deep TJIs or else scabbing on a double ceiling to dimensional lumber, is that I would get plenty of room to blow cellulose with the cavity extending all the way out over my walls instead of creating a cold tight spot at the transition where the trusses sit. I'm sure there are ways to get around that though.

My original plan is a structural ridge beam, supported by the walls at both ends, and with one structural interior wall to break the beam span. Thanks for the numbers, I hadn't looked those up yet, maybe that will make it easier to see if a beam will be too big to be worth it.

[Hondarider]

Ok, let me know if I'm thinking about this right: Assuming the previously mentioned 30,000lb snow load as an example, 7500 to each sidewall and 15,000 goes to the ridge which is a 29.4 ft. span, so ~510lbs/linear foot. The span is broken by the structural interior wall into 12.7ft. and 16.7ft. spans with loads of 6377lbs (12.7*510) and 8517lbs (16.7*510)respectively. Each of these spans delivers half of its weight to an exterior wall and half to the structural interior wall. So 3238 to the front wall, 7476 (3238+4258) to the interior wall, and 4248 to the back wall.


                                    15,000 lbs snow load at ridge
Front wall<-------------------------------29.4 ft.----------------------------------->Back wall
                     6377lbs                                                     8517lbs
Front wall<---12.7ft.---->structural interior wall<------------16.7ft.------------>Back wall
3238lbs                             3238lbs+4258lbs                                                 4258lbs             

[Hondarider]

If your plans are for a long-term rental you may want to consider opening up the top floor with a dormer or two.  Windows on either end make for a dark tunnel on the upper floor and if you'll have any interior walls up there it will be even darker.

Here's another view of what I have so far, I'm hoping that having the "Catwalk" between the loft and the bedroom will let more light make it's way upstairs. I'm planning on situating the building broadside so that the gable end walls face East and West, no North windows, and the largest windows facing South for the view and the solar gain. What do you think? You've got me thinking that the bedroom will still be pretty dark. Maybe some interior windows in the bedroom wall to let in more of the light from downstairs?

A pair of dormers would look perfect, add lots of light since they'd be south facing, and make slightly more useable space too. I haven't added them yet because I'm wanting to keep it simple and easy to get dried in quickly, and have as few complications as possible when it comes to the insulation, vented cavities, and roofing details, but I know it's all possible, and it may be the best bet in the end.

[ChugiakTinkerer]

Yep, you've got the right idea about distributing the load.  The number I threw out yesterday was a bit of a WAG, so you may want to take a moment confirm I'm in the right ballpark.  Snow and wind load calculations for all seem to originate from building code version of the Dead Sea scrolls, now called Minimum Design Loads for Buildings and Other Structures.  It is published by the American Society of Civil Engineers and is abbreviated as ASCE 7.  You can buy the latest revision in print or e-book from the ASCE, or you may be lucky like I was and find a PDF file of an older version on the web with enough searching.

Snow load calc start with the amount of snow that falls in your area.  For Homer that number is 40 lbs per sq ft and is called the ground snow load.  The ASCE 7 provides a formula for the expected roof load that is essentially:

  RoofDesignLoad = 0.7 * GroundSnowLoad * a bunch of other factors

The other factors take into account roof pitch, thermal regime, roofing material, etc and result in a number that is usually less than 1.0 but could even be higher.  For now let's assume it is 1.0.  The roof area is the ridge length multiplied by the width from eave to eave.  I assumed 24" overhang all around, resulting in a roof area of 20x34 = 680 sf.  The design for snow is 28 lbs per sq ft, and the dead load, the weight of the roof itself, is usually estimated as 10 lbs. per sf.  So your roof should be designed to support a live load of 28 psf and a dead load of 10 psf.  680 sf * 38 psf = 25,840 lbs.  Just to keep things simple for now let's stick with 30,000 lbs.  I suspect if you work your way through the ASCE 7 you'll determine your roof load will be much less, as a steeply  pitched metal roof sheds snow pretty well.  When it comes to sizing your ridge beam you may need to work out that number more precisely.

One point in going through this process is to figure out how much load will be bearing on the beams you have supporting the interior wall and the front wall.  The column under the ridge beam at that end is supporting 4258 lbs.  The beam needs to support that, as well as a few more lbs for the weight of the wall framing, window and siding, etc.  You can determine what size beam you need by doing some calculations using this web page: http://www.timbertoolbox.com/Calcs/beamclc_ctrptld.htm

Mechanical properties for Fb, E, and Fv can be found on the web.  Using a value of 4500 lbs for the point load, you'll find that it is very difficult to support that load with any beam made of sawn lumber.  It took a triple LVL 12" deep to support that load.  That is for the front wall.  In your interior bearing wall, it appears you are designing for the floor joist to support the wall above.  It may be doable, but I would encourage you to have a load bearing wall on the first floor to carry this load straight to the piers.  Here's why - the interior load bearing wall is supporting 7500 lbs from the roof.  The beam below has to support all that as well as some of the floor load.  If you want to support that with a beam rather than a post or bearing wall below, then you need to calculate what beam would be required.  It probably can be done with exact engineering calculations with two separate load points, but let's treat it like a single load at midpoint.  Again going back to the beam calculator it would take five LVLs at 11" deep to support that load.

I just looked a little more closely at your stairs rendering and I see that you have perhaps a partial wall on the first floor to support that load.  This helps immensely and I encourage you to think about ways to distribute the load up and down as much as possible rather than using horizontal members.  For a long span it makes the size and cost explode dramatically.  Alternatively, this is where the trusses become your best friend.  If you use trusses all of the load from the roof is put on the north and south walls.  This frees you up from having to worry about supporting the roof with the interior walls.

[Edit to add] Whoops, I grabbed the back wall column load and not the front wall column load.  So the beam over the porch would just barely pass with two LVLs at 1.75" x 11.825" and 16' long.  A good demonstration of the importance of working through all the load calculations meticulously.  A good lumber yard should have someone who can calculate the loads and tell you what you would need for LVL as a ridge beam and/or what will work for trusses.  They have some fancy-pants software that the wood manufacturers provide for speccing out the various products.

Ok, let me know if I'm thinking about this right: Assuming the previously mentioned 30,000lb snow load as an example, 7500 to each sidewall and 15,000 goes to the ridge which is a 29.4 ft. span, so ~510lbs/linear foot. The span is broken by the structural interior wall into 12.7ft. and 16.7ft. spans with loads of 6377lbs (12.7*510) and 8517lbs (16.7*510)respectively. Each of these spans delivers half of its weight to an exterior wall and half to the structural interior wall. So 3238 to the front wall, 7476 (3238+4258) to the interior wall, and 4248 to the back wall.


                                    15,000 lbs snow load at ridge
Front wall<-------------------------------29.4 ft.----------------------------------->Back wall
                     6377lbs                                                     8517lbs
Front wall<---12.7ft.---->structural interior wall<------------16.7ft.------------>Back wall
3238lbs                             3238lbs+4258lbs                                                 4258lbs             

[ChugiakTinkerer]

Just to pile on a little more from my previous post.  The ridge beam will have to be pretty comparable to what I just calculated for the interior wall support.  It has a design load from the roof of 8500 lbs with a span of 16.7 feet.  That means you will need something comparable to five LVLs at 1.75 x 11.875 inches.  It's very doable, but I think this is where trusses will cost out less than LVLs.   I got a quote for scissor trusses last year that came out to about $1700 for my 16x28 floor plan.  LVLs from Spenard Builders run about $6.50 a lineal foot for 11-7/8".  If you did a ridge beam in LVL it alone would cost around $1,100.  You would then add the cost of your TJI or sawn lumber rafters and I'm pretty sure trusses win out cost-wise.

Another thing, if you go with trusses then your east and west walls won't need a load-bearing column down the middle.  This allows you to put in a bigger window, letting in more light.

[akwoodchuck]

For the money that Techno Post will charge ( plus the added hassle of skirting and weatherizing the perimeter), you could DIY a very nice crawlspace foundation (concrete footer, treated stem wall, bituthene, perimeter drain).... I've used the screw piles on numerous builds and never been that impressed. They would work fine for your porch posts, though...

[ChugiakTinkerer]

If you want to work with trusses in Sketchup, there is the Medeeek Truss Plugin, available at http://design.medeek.com/resources/medeektrussplugin.html

The plugin author posts updates on CountryPlans forum at http://countryplans.com/smf/index.php?topic=12940.msg190679#new

The plugin makes truss mockups easy to produce, so you can play with various design ideas to your heart's content.

[Hondarider]

Just to pile on a little more from my previous post.  The ridge beam will have to be pretty comparable to what I just calculated for the interior wall support.  It has a design load from the roof of 8500 lbs with a span of 16.7 feet.  That means you will need something comparable to five LVLs at 1.75 x 11.875 inches.  It's very doable, but I think this is where trusses will cost out less than LVLs.   I got a quote for scissor trusses last year that came out to about $1700 for my 16x28 floor plan.  LVLs from Spenard Builders run about $6.50 a lineal foot for 11-7/8".  If you did a ridge beam in LVL it alone would cost around $1,100.  You would then add the cost of your TJI or sawn lumber rafters and I'm pretty sure trusses win out cost-wise.

Another thing, if you go with trusses then your east and west walls won't need a load-bearing column down the middle.  This allows you to put in a bigger window, letting in more light.

Thanks for all the thought put into your replies, it's really helpful to talk with someone from Alaska who knows Alaska prices and building quirks. I'm in Central China teaching English for the winter, and with a 14 hour time difference and slow internet, a quick phone call to SBS for prices, etc. isn't so easy.

Using this chart: http://www.aitc-glulam.org/pdf/Capacity/DF_26.PDF
I think with a ridge load of 510/ft I should be able to use 3 1/8 x 10.5 glulam (625lbs/lf) for the 12.7 span and 3 1/8 x 13.5 glulam (604lbs/lf) for the 16.7 span. I can't find prices for these exact beams, but from what I found on northland's website 3 1/8 x 12 is $13.50/lf, so I think I'd be around ~$500 for the two beams, Correct?

(spreading the rafters to 24 OC) 26 16ft 11 7/8" TjIs @$2.40/ft =$998

So: ~$1500 for ridge+rafters,
plus the two extra beams in the floor and the extra framing in the walls to support the ridge. (You included the additional LVLs needed in the floor in your $1100 number I think)

The reason I originally drew the beam under the doorway instead transferring the weight to the bathroom wall below, is that I had drawn the first level floor with only perimeter beams, with floor joists spanning the full 16ft, so I thought it would be the same difference since I have to get the weight to the perimeter foundation beams bearing on the piers, weather I use a beam in the first floor or the second. Thoughts on this? Is one way better than the other? Or will it even work to have only perimeter foundation beams and piers?

So yes, trusses would be cheaper. The reasons I am still hung up on to the structural ridge are the Ceiling height-- since scissors can't parallel the ceiling and roof pitches, I would have to raise the sidewalls more to regain head clearance in the upstairs space-- and insulation. Theres no easy way I can find to do cellulose in a steep scissor truss, even if it were designed with a raised heal to get adequate depth over the wall area, leaving me with nasty fiberglass batts, or expensive spray foam.

Thanks for the input, keep it coming!

[Hondarider]

For the money that Techno Post will charge ( plus the added hassle of skirting and weatherizing the perimeter), you could DIY a very nice crawlspace foundation (concrete footer, treated stem wall, bituthene, perimeter drain).... I've used the screw piles on numerous builds and never been that impressed. They would work fine for your porch posts, though...

Did you work with Techno Metal Post specifically? What kept you from being impressed? So far I'm really impressed with David's customer service and the fact that he is the one answering peoples questions about TMP online, even in other parts of the country. He at least seems knowledgable about his product. I've read what I can about them online, which is not much when it comes to full foundations, most of what I find has to do with repairs or additions. Maybe you can share some experience. I'm really into the idea of a quick and painless foundation, but maybe it's too good to be true. My original plan is to buy property this spring, spend the summer commercial fishing, then start construction in September, so anything to speed up the time to getting dried in and working indoors is a big bonus.

[ChugiakTinkerer]

I can see how the long distance communications can make things rather difficult.  I think my $1100 estimate was based on using 1-3/4" x 11-7/8" LVL beams in the ridge and supporting the porch.  But it was a quick and dirty estimate meant only to point out that the costs of trusses might be comparable to the costs of a ridge beam and rafters.  I don't know what building supply vendor you will use, but the FOB Anchorage prices for SBS can be found through their rural catalog here: http://store.sbsalaska.com/.  Tht's where I got the LVL prices.  Your price for 3-1/8 " x 12" LVL per linear foot is around the price of two 1-3/4" x 12" LVL so it seems a reasonable place to start for cost estimating.

I've never worked with TJI rafters, so have no experience with them.  When a deep rafter is desired I believe they can be less expensive than sawn lumber.  But I also believe their installation requires specific hardware that is an additional cost and requires more time for installation than trusses.  I'm just saying don't rule out trusses yet.  You can get parallel chord trusses at whatever depth you want, so you can get the interior roof height you desire as well as the depth for insulation.  Where trusses are flat out a superior choice is in the time it takes to install.  If you have access to a crew, or your supplier can provide a boom truck, you can get your roof framed in half a day.

You're at a great stage right now to think about where you want to be on the construction management triangle: fast, cheap, or good - pick two.  If you want to get dried in by October, trusses may help you get there.  It's certainly worth it in my mind to get a few quotes on trusses and on TJI rafters and then weigh which one meets your needs better.

[Edit to add] I guess I need more coffee, I was conflating a glu-lam with an LVL.  Anyhoo, I think it's worth working through a total cost estimate for the loft floor and roof with the two design options.  I'd do it if I were you, but then I do have a tendency to over-analyze. 

[akwoodchuck]

Did you work with Techno Metal Post specifically? What kept you from being impressed? So far I'm really impressed with David's customer service and the fact that he is the one answering peoples questions about TMP online, even in other parts of the country. He at least seems knowledgable about his product. I've read what I can about them online, which is not much when it comes to full foundations, most of what I find has to do with repairs or additions. Maybe you can share some experience. I'm really into the idea of a quick and painless foundation, but maybe it's too good to be true. My original plan is to buy property this spring, spend the summer commercial fishing, then start construction in September, so anything to speed up the time to getting dried in and working indoors is a big bonus.

Yeah worked with the TMP product on a bunch of projects, from simple porch/deck support to a full couple thousand sq. ft. house....I do like em for attached decks, they're quick easy and dont heave. Too shaky for a house in my opinion...but they have made sonotubes basically obsolete in my life. Dont want to be telling tales out of school, but I've had serious problems with getting them installed on layout...like a half a foot off in some cases....but that was quite a while ago.

[ChugiakTinkerer]

I can see there being a chance for layout problems even with the best operator.  Your ground conditions will determine if you can even auger the pile in, much less keep it oriented and positioned precisely.  Too many cobbles and you won't even be able to use helical piles.  Have you done some digging on the property to see what lies underneath?

[Hondarider]

Yeah worked with the TMP product on a bunch of projects, from simple porch/deck support to a full couple thousand sq. ft. house....I do like em for attached decks, they're quick easy and dont heave. Too shaky for a house in my opinion...but they have made sonotubes basically obsolete in my life. Dont want to be telling tales out of school, but I've had serious problems with getting them installed on layout...like a half a foot off in some cases....but that was quite a while ago.

Hmm...Thanks. I guess my best bet is to try to see a building they've done in person when I get to Homer so I can see if the amount of movement is worth it to me. I've read that how you choose the bracket to attach the post to the beam makes a big difference in how much it can move, with a big long bracket acting like more of a brace. How did you deal with them being that far out of line? Can they make up for some of it when welding on the bracket? I sent him a picture of my drawing and he said the engineer would recommend 10 p3 piers and no bracing if they are less than two feet out of the ground.

I can see there being a chance for layout problems even with the best operator.  Your ground conditions will determine if you can even auger the pile in, much less keep it oriented and positioned precisely.  Too many cobbles and you won't even be able to use helical piles.  Have you done some digging on the property to see what lies underneath?

Not yet... I don't even own property yet haha. But I will check it out when I do.

[Hondarider]

New question: Am I out of line with only 2x4 studs for my structural wall with the snow and seismic activity in the area? I understood that most buildings are 2x6 now, but mostly for added insulation space. I'm considering either a double stud 2x4 wall, with the interior non load bearding, or 2x4 with foam board added outside. I know the final answer should come from someone with good knowledge of engineering and/or standard prescriptive building codes, but I'm still in the early stages.

[ChugiakTinkerer]

The IRC allows 2x4 studs up to 10' on 24" center for single story walls and 16" o.c. for two story buildings.  You might want to look into earthquake and wind forces at your property site to see if there is a need for 2x6.  Otherwise it probably comes down to what you want to optimize for.  Back to that triangle... do you want to optimize for lowest outlay costs, lowest utility cost, or lowest time to construct?  For example, rigid foam on the outside is probably going to be the most expensive insulation option, but also the fastest to install.  For me, it would depend on who is paying the utility bill. 

By the way, the International Residential Code is online here: http://codes.iccsafe.org/app/book/toc/2015/I-Codes/2015%20IRC%20HTML/index.html

[flyingvan]

The difference in cost in framing with 2x6 instead of 2x4 is negligible, unless you are building 10,000 houses.  The advantages are many---you mentioned insulation already.  Pipes and wires can be run out of reach of siding nails and drywall screws.  Notch outs for various things are easier.  2x6's tend to be straighter than 2x4's and even small warps show up in your drywall or wall finish. 

[Hondarider]

The IRC allows 2x4 studs up to 10' on 24" center for single story walls and 16" o.c. for two story buildings.  You might want to look into earthquake and wind forces at your property site to see if there is a need for 2x6.  Otherwise it probably comes down to what you want to optimize for.  Back to that triangle... do you want to optimize for lowest outlay costs, lowest utility cost, or lowest time to construct?  For example, rigid foam on the outside is probably going to be the most expensive insulation option, but also the fastest to install.  For me, it would depend on who is paying the utility bill. 

By the way, the International Residential Code is online here: http://codes.iccsafe.org/app/book/toc/2015/I-Codes/2015%20IRC%20HTML/index.html

Thanks, that will be helpful. I've been spending a ton of time reading about insulation and air and vapor barriers at the green building advisor blog. I'm interested in doing something with a thermal break over the studs, which have an R of 1 per inch. Double wall or exterior foam both work, but since exterior foam is a vapor barrier it stops moisture that is traveling through the wall at the sheathing, which is only a problem if the sheathing is cold enough to make it condensate. For this in zone 7, the minimum R value for a level of exterior foam that keeps the sheathing above the dew point is R10 for 2x4 and R15 for 2x6. R10 is doable with a single 2 inch layer, but R15 bumps it to two layers, which I don't think I want to mess with or pay for.

The difference in cost in framing with 2x6 instead of 2x4 is negligible, unless you are building 10,000 houses.  The advantages are many---you mentioned insulation already.  Pipes and wires can be run out of reach of siding nails and drywall screws.  Notch outs for various things are easier.  2x6's tend to be straighter than 2x4's and even small warps show up in your drywall or wall finish. 

Thanks, I forgot about protecting plumbing and electrical from screws.