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Homeowner Forum / Construction / August 2005Removal of ceiling joists in garage truss
I have a friend who is trying to gain additional head clearance in his garage. The garage is 20' x 38'. The roof pitch is approximately a 7/12. Roof is constructed of 2x4 rafters 2' on center with a 1x6 ridge beam. Every other rafter (every 4') has a joist running from exterior wall to exterior wall which sits on the top plate of the wall and is tied to the rafter. He would like to remove these joists and replace with collar ties approximately 1'-2' up the rafter to gain ceiling height. Anyone know whether this sounds feasible? He could collar tie every rafter rather than every other as is done now. With 2X4 rafters at 2' centers, that roof is almost ready to collapse, already. This is no job for an amateur. If you start cutting out the ceiling joist, you will have the garage in a plie on the ground with you under it. Those ceiling joist that are 4' on center are the only thing holding the walls together. If the joist are cut, the walls will move apart and the roof will come down. I am not in favor of getting an engineer for every little project but this is one where you need someone who knows what they are doing.  SignatureJerryD(upstateNY) I have a friend who is trying to gain additional head clearance in his garage. The garage is 20' x 38'. The roof pitch is approximately a 7/12. Roof is constructed of 2x4 rafters 2' on center with a 1x6 ridge beam. Every other rafter (every 4') has a joist running from exterior wall to exterior wall which sits on the top plate of the wall and is tied to the rafter. He would like to remove these joists and replace with collar ties approximately 1'-2' up the rafter to gain ceiling height. Anyone know whether this sounds feasible? He could collar tie every rafter rather than every other as is done now. Thanks for the advice. Obviously if we were able to go this route we would attach the collar ties before removing the joists. But after I posted I started looking at span tables and given a 30psf snow load and 10 psf live load span for 2x4s on 24" centers was very small (~6ft) so obviously the garage roof is like you said woefully underdesigned. But there is actually no visible sagging. Its amazing how inadequately, by todays standards, things are often built, but they seem to last. Anyway I guess the new question is how to fix the garage roof so that it is properly supported. I guess the two options are two either increase rafter size (which is difficult but not impossible without removing the roof) or create trusses. Any suggestions? In a previous post wrote... > Anyway I guess the new question is how to fix the garage roof so that > it is properly supported. I guess the two options are two either > increase rafter size (which is difficult but not impossible without > removing the roof) or create trusses. Any suggestions? Jerry's advice was sound and should be followed. Hire a local engineer who can inspect the situation and give advise as the proper course of action. This roof is already very weak. Don't compound the problem by seeking advice from the members of a newsgroup that cannot observe the problem first hand. This problem can be solved (I have designed solutions for similar situations), but not in this newsgroup. It requires first hand observation. SignatureBob Morrison, PE, SE R L Morrison Engineering Co Structural & Civil Engineering Poulsbo WA On 8/24/2005 7:54 PM US(ET), Joe took fingers to keys, and typed the following: >With 2X4 rafters at 2' centers, that roof is almost ready to collapse, >already. > He said "garage truss" in the header which is a little stronger than just 2x4 rafters. >This is no job for an amateur. >If you start cutting out the ceiling joist, you will have the garage in a [quoted text clipped - 5 lines] >is one where you need someone who knows what they are doing. > SignatureBill > On 8/24/2005 7:54 PM US(ET), Joe took fingers to keys, and typed the > following: [quoted text clipped - 4 lines] > He said "garage truss" in the header which is a little stronger than > just 2x4 rafters. You should read more than the subject line. These parts indicate roof rafters: Roof is constructed of 2x4 rafters 2' on center with a 1x6 ridge beam. Every other rafter (every 4') has a joist running from exterior wall to exterior wall which sits on the top plate of the wall and is tied to the rafter. If it is a truss he should definitely not mess with it. R Unfortunately he has little money to hire an engineer. I think I'll try to get him to have the city engineer look at it. I think he may just be inclined to completely leave it alone. Bob, while I understand how difficult it is to provide advice without first hand viewing, it is a very basic structure. I appreciate any advice anyone can give and realize that everything from a newsgroup is caveat lector. I was just talking with him and he indicated that he had talked with a carpenter who wanted to make the rafters into scissor trusses. Any thoughts? That's the first thing that makes any sense. I think you have been telling about a "cut" roof. The rafters come together at the top, usually fastened to a ridge board. The bottoms of the rafters are fastened to the top plate of the walls. The bottom connections are locked horizontally by the ceiling joists. This forms a huge triangle. If you cut the bottom of the triangle, it will push out the side walls and/or fall down. Collar ties placed higher on the rafters cannot control this load. Trusses are special designs to deal with these loads, usually with lighter lumber and less lumber. Scissor trusses can be designed to allow more headroom, usually used to create "cathedral" ceilings. You can also deal with the lateral thrust by creating a structural ridge beam. Either approach screams for a structural engineer or an extremely knowledgeable carpenter/builder. (top posted for your convenience) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Keep the whole world singing . . . . DanG (remove the sevens) dgriff237@7cox.net >I was just talking with him and he indicated that he had talked >with a > carpenter who wanted to make the rafters into scissor trusses. > Any > thoughts? J.C., sounds like a cracker box. Recently built a workshop with garage door 20' by 20'. 2X6 rafters and joists 16" OC. 2X8 ridge board. 2X12 doubled up w/glued plywood as center joist attachment beam via metal joist hangers. 2X6 supports from 2X12s to ridge board with scabs on side of the supports. Yellow pine 2X12s and 2X8 ridge and fascsia, PSF remainder. No collar ties. No purling. Roof pitch is 5 on 12. Ignored the naysayers about the ceiling height should not be standard ceiling height in such a building. I did create a raised ceiling area for the garage door hardware in the ceiling. Its raised 5 1/2", worked out great. Ran some big eye bolts in the 2X12s, use it to anchor a chain hoist. Used it to pull an engine from a car recently. No visible sagging was noticed. Did make a protesting creaking noise though. Sounds like you'll have to basically rebuild your roof/ceiling structure to accomodate a raising ceiling. It also sounds like this is open framing, no sheetrock in the ceiling at the moment. > I have a friend who is trying to gain additional head clearance in his > garage. The garage is 20' x 38'. The roof pitch is approximately a [quoted text clipped - 5 lines] > height. Anyone know whether this sounds feasible? He could collar tie > every rafter rather than every other as is done now. Actually I finally went and looked and the garage structure is slightly different than he told me. There are 2x4's running to roof peak connected to a 1x6. Every other set of rafters has a joist running across, and then 1x4 strapping is used to create a W truss (all nailed to the outside of the 2x material). He decided not to raise the ceiling and is going to make a W truss on the 2x's that are not trussed yet. He is going to use 3/4" plywood gussets with 3" shanked, concrete coated nails. This should increase the strength significantly and allow him to hang dry wall to the ceiling. |
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