Originally written: Dec 2001
Most of the suggested details published by the various SIP manufacturers depict classic platform framing; that is, first floor walls erected as structure, second floor deck sitting directly on top of the first floor SIP top plate, second floor SIP walls on top of the second floor platform. This is commonly known as "platform" or "western" framing. If it continues skyward, the attic floor - or where codes permit - the "third" floor deck will sit on the second floor walls, and finally the roof will sit on this last highest platform. The most popular variation of this is to utilize top-mounted joisthangers sitting on the SIP top plate, set the joists and run the subfloor in over the first floor walls before setting the second floor walls. Same thing with the attic floor framing. Thus the construction sequence becomes SIPs, sticks, SIPs, sticks, SIP roof.
Now we know that one of the biggest hurdles to the homeowner specifying SIPs for his project is " Who can I get to put this thing together for me? My contractor doesn't want to bid or touch the stuff! So the industry responded by sending out its own factory trained crews. We have recently run into some examples where we learn that there is indeed a big difference between the professionally installed SIPs and those assembled by the novice. So it is a good thing to have the professional travelling SIP circus so as to prevent rotting SIP structures, but with the sequence described above, it is terribly inefficient for the SIP crew and expensive and awkward for the homeowner. Sometimes half or more of the professional SIP assembly crew's time is spent putting in conventional framing - both decks and walls. The alternative is to have the SIP crew go away after putting up the walls, have the local guys install the deck, call back the "SIPsperts" to put up the second floor walls, send 'em away again and have the local crew put in the second floor walls and next platform, and finally have the SIPsperts return to set the roof. See what I'm getting at?
Well, let's take an historical break and look for some help. It turns out that the first stick framing was "balloon" framing, where the second floor walls were of the same long continuous studs set for the first floor walls. The second floor was set on a ledger notched into the studs. This ledger would be anything between a 2 x 6 and a 5/4 x 4 so the studs would be notched somewhere between 1 7/8" (old lumber sizes) and 1" and the ledger nailed in. Obviously there were some problems with this system, so we don't see it around anymore. There were "conflicts" in shrinkage across the grain with the deck - which did shrink - and the wall height, which did not. More significantly, there were fire problems. When balloon framing burned, the continuous stud cavities acted like chimneys and encouraged the fire to burn more vigorously with the increased draft and also provided "highways" for the fire to spread to the second floor. Even with the installation of code-required firestopping blocking, fire spread was still a concern.
With SIPs these problems are eliminated. In fact, anecdotal evidence from the field shows that fire spreads much more slowly through SIP construction than through stick "chimney cavity" construction. Therefore we may safely re-introduce balloon construction as it helps us enormously with construction sequencing. The SIPsperts can storm in, raise the walls, post the roof and go on to the next job. No more regular stick framing for them. After they leave local labor steps in and frames the first floor walls, second floor deck, second floor walls, etc. If they really want to blaze through the job, a separate crew can set exterior doors and windows at the same time or the regular crew can weather-in the building first and then do all the interior framing within a water-tight, insulated envelope! - a great help in cold weather. Additionally, there is another opportunity?..
Isn't it so that one of the features touted by SIP advocates is its environmentally benign low impact on our forest resources? So let's see, we eliminate as much solid-sawn timber as we can from the envelope ?.and then fill it up with sticks? Wrong. How about considering light gage steel framing for interior walls and decks? We find it superior in many ways for several reasons, but that is a subject for another month. Let's finish up with a look at how we attach our deck to the wall and its effect on engineering.
Axial load tables available from the SIP industry show us that symmetrically loaded walls (walls loaded on the top plate so that loads go down both the exterior and interior skins) have an extremely high capacity - 4" nominal SIPs 8 foot high can handle loads of over 4000 pounds per linear foot, with 99 mph wind loading! If we look at the capacity of an 8 foot panel (same wind loading) loaded only on one skin, we see it drops to 1845 plf. This is still way above most requirements. For example:
A 32' wide building with a center-bearing girder or partition will throw 8 feet of floor load to the outside wall. At 60 psf total load ( live load plus dead load) this comes to 60 x 8 = 480 plf at the wall. We allow 100 pounds capacity for shear and withdrawal for a number 10 screw into 7/16" OSB. Five screws per foot would then be specified to fasten a ledger to the inner skin. Joist hangers fasten to the ledger and the job is done.
Now we know that one of the biggest hurdles to the homeowner specifying SIPs for his project is " Who can I get to put this thing together for me? My contractor doesn't want to bid or touch the stuff! So the industry responded by sending out its own factory trained crews. We have recently run into some examples where we learn that there is indeed a big difference between the professionally installed SIPs and those assembled by the novice. So it is a good thing to have the professional travelling SIP circus so as to prevent rotting SIP structures, but with the sequence described above, it is terribly inefficient for the SIP crew and expensive and awkward for the homeowner. Sometimes half or more of the professional SIP assembly crew's time is spent putting in conventional framing - both decks and walls. The alternative is to have the SIP crew go away after putting up the walls, have the local guys install the deck, call back the "SIPsperts" to put up the second floor walls, send 'em away again and have the local crew put in the second floor walls and next platform, and finally have the SIPsperts return to set the roof. See what I'm getting at?
Well, let's take an historical break and look for some help. It turns out that the first stick framing was "balloon" framing, where the second floor walls were of the same long continuous studs set for the first floor walls. The second floor was set on a ledger notched into the studs. This ledger would be anything between a 2 x 6 and a 5/4 x 4 so the studs would be notched somewhere between 1 7/8" (old lumber sizes) and 1" and the ledger nailed in. Obviously there were some problems with this system, so we don't see it around anymore. There were "conflicts" in shrinkage across the grain with the deck - which did shrink - and the wall height, which did not. More significantly, there were fire problems. When balloon framing burned, the continuous stud cavities acted like chimneys and encouraged the fire to burn more vigorously with the increased draft and also provided "highways" for the fire to spread to the second floor. Even with the installation of code-required firestopping blocking, fire spread was still a concern.
With SIPs these problems are eliminated. In fact, anecdotal evidence from the field shows that fire spreads much more slowly through SIP construction than through stick "chimney cavity" construction. Therefore we may safely re-introduce balloon construction as it helps us enormously with construction sequencing. The SIPsperts can storm in, raise the walls, post the roof and go on to the next job. No more regular stick framing for them. After they leave local labor steps in and frames the first floor walls, second floor deck, second floor walls, etc. If they really want to blaze through the job, a separate crew can set exterior doors and windows at the same time or the regular crew can weather-in the building first and then do all the interior framing within a water-tight, insulated envelope! - a great help in cold weather. Additionally, there is another opportunity?..
Isn't it so that one of the features touted by SIP advocates is its environmentally benign low impact on our forest resources? So let's see, we eliminate as much solid-sawn timber as we can from the envelope ?.and then fill it up with sticks? Wrong. How about considering light gage steel framing for interior walls and decks? We find it superior in many ways for several reasons, but that is a subject for another month. Let's finish up with a look at how we attach our deck to the wall and its effect on engineering.
Axial load tables available from the SIP industry show us that symmetrically loaded walls (walls loaded on the top plate so that loads go down both the exterior and interior skins) have an extremely high capacity - 4" nominal SIPs 8 foot high can handle loads of over 4000 pounds per linear foot, with 99 mph wind loading! If we look at the capacity of an 8 foot panel (same wind loading) loaded only on one skin, we see it drops to 1845 plf. This is still way above most requirements. For example:
A 32' wide building with a center-bearing girder or partition will throw 8 feet of floor load to the outside wall. At 60 psf total load ( live load plus dead load) this comes to 60 x 8 = 480 plf at the wall. We allow 100 pounds capacity for shear and withdrawal for a number 10 screw into 7/16" OSB. Five screws per foot would then be specified to fasten a ledger to the inner skin. Joist hangers fasten to the ledger and the job is done.
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