inferior built shed - want to shore up

huangyhg

inferior built shed - want to shore up
okay, so i inherited this 14 x 20 outbuilding that seems to have been built without regard to framing guidelines. i want to shore it up and finish the inside to make it my office/guest house. it's not very old -- couple of years -- and seems to be holding its own for now, except door sags even after rehanging, but feel time and wind will change that.
here are the main problems (i think):
single top plate along 20' sides
- top plate along 14' side is single plate with two side-by-side 2x4's on top
wall studs are 16" o.c.
rafters are 24" o.c. and 2x4's
- roof is approx. an 8/12 pitch and there are 2x4 collar tie about 4' from peak
- rafters end at top plate (do not extend past wall)
no ridge board
exterior is lap siding only (no plywood underneath)
no window/door headers (single 2x4 only)
(the foundation of this building three 4x6 skids seems acceptable)
rafter ties are 2x4, 24" o.c. of course, that span the 14' without any other bracing (seem very flimsy to finsih out!)
interior finish will be relatively light-weight beadboard-look paneling on ceiling and walls. the building is not wide enough to add a center bearing wall to cross brace the rafters to which was my first thought. then i'd essentially have two 7x20 sheds. ugh. i looked in a few books regarding shoring up old buildings experiencing structural problems, and one solution just for wall integrity seemed to be cross bracing between the wall studs (which would make insulating the walls fun). but that also does not address the under-sized rafters and flimsy top plate. any ideas?
thanking you in advance, paige
paige,
there are several things you can do.  first, add a 2x6 turned up on the inside of the single top plate, with the bottom of the 2x6 flush with the bottom of the top plate. notch for rafter clearance as necessary.  fasten with appropriate nails (12d) at 6 inches center to center.  fasten to rafters with hurricane clips.
next, add headers for all doors and windows.  next, tie the system all the way from the rafters to the foundation with a continuous tensile tie.  this takes the form of strapping the rafters to the top plate with steel straps, tieing the top plate to the studs with a strap, and tieing the stud to the sill (sole) plate with a strap, and then tieing the sill to the foundation.  all of this should be done at about 48 inches on center (check local codes...it might be different where you are).
lastly, you will need to provide a "let-in" brace at all corners.  this is a 1x4 placed diagonally starting at the top of the corner and extending down and at a 30 to 45 degree angle to the bottom.  each intersecting stud will have to be routed to accept the 1x4, as will the top and bottom plates.
good luck.
ron
thank you for the fix ron.
if the fix wasn't within my scope of capabilities (which include time/tool/physical constraints  and it looks like it isn't, the labor pool in my area which is affordable is limited by communication & tools (green-card immigrants). they are hard-working and have a sound knowledge of basic, new construciton, but are limited when it comes to power tools and anthing outside of straight cuts like routing.
in light of that, would this work as well?:
1) add wall studs under the 24" o.c. rafters/rafter ties (ceiling joists), since of course only about three 24" o.c. rafters match up to the 16" o.c. wall studs below. to my (ignorant chemisty major) mind this would somewhat make up for the lack of top plate;
2) brace the undersized rafters by retrofitting with a "w" truss configuration, secured with metal plates;
3) instead of 1x4 "let in" bracing, diagonal metal straps along the same path;
4) use metal connecters for rafter/top plate connection;
5) add headers as suggested.
anything more complicated and i think it might be easier for my labor pool to just tear the roof off, add the top plate and go to 2x6 rafters with an actual ridge board, add the headers, etc. a little more expensive, but i'm already talking with a group about building another (smaller) outbuilding to serve as a storage building since this building will be converted to living space. perhaps while putting together the new building they can do this new roof at little extra labor costs.
maybe a third option would be to have a foundation type company come jack up the roof in tact, add a top plate, and ??? add a new 2x4 rafter for (effectively) 12" o.c. rafters?
what seems strikingly underbuilt to a novice are the rafters and the rafter ties (or ceiling joists). the rafter ties seem so flimsy spanning the 14' unreinforced that i would not even want to nail the ceiling paneling to them. and i just can't imagine the 2x4- 24" o.c. holding the roof up adeqately. i see sagging in my future . probably the only positive with the rafters is that the roof has a pitch that, perhaps, puts enough of the load on the walls that it will not sag terribly? however, since there is only one top plate, that's where the load is going? i have little clue what the ramifications of only one top plate are though.
what do you think of the other options? are these fixes that won't work well enough?
thanks so much,
paige
paige,
let's get to work before the sagging starts !.
the double top plate does several things.  it provides for lateral diaphragm stiffness and more consistent load transfer (up and down) for the roof/wind loads.  further, the top plates are to be overlapped at corners so as to create more strength there.  next...they are required by most codes.
you can use a continuous header in lieu of a double top plate.  that was my attempt in offering the 2x6 on the side.
the let-in brace is not hard to do.  you can do all the cuts with a circular or reciprocating saw and use a chisel for the notches.  i prefer a router because it is cleaner and neater, but we're not building a watch, so other methods will work!
another simple way to get the corner bracing in lieu of the let-in bracing would be to use a full sheet of 1/4-inch structural underlayment such as thermaply structural grade starting in each corner.  put lots of nails in it.  then cover with 3/8-inch gypsum board.  finish rest of interior wall with 5/8-inch gypsum board and it will all match up, though eventually you might get a fine crack along the joint at this location.   3/8-inch plywood could be used in lieu of the thermaply, but you will have to "mud-in" a 1/8-inch transition to the other dry wall.
don't use metal straps for the let-in brace.  they have to work in tension and compression.
for the rafters, you might want to use a "k" configuration, for bracing.  imagine a "k" on its side, with straight part being the bottom chord of a truss, the other pieces being diagonals.  this will effectively "truss" your rafters and replace the collar beam. (assuming 14-foot span)
good luck,
ron