How to Splice a Joist or Rafter

Submitted by Ray Thornburg on Thu, 11/09/2017 - 19:14

How to Properly Splice a Joist or Rafter

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About Splicing Wood

     We'll begin our discussion by reminding everyone that carpenters have been splicing wood together for thousands of years. It's one of the things carpenters do best but still there are those who say that this is the purvue of an engineer and no one else. Headers, jacks, sills, beams, girders rafters, are all spliced together in one way or another. The fact is that there are many things carpenters do that are allowed because we all know they work. This is called prescriptive solutions. Of course we're talking about splicing a joist or rafter as part of a repair technique for a rotted, termite damaged or over spanned joist. Sometime we just need a longer piece of wood (like a hip king rafter).


      Whatever the reason the secret to a good splice is the connection method and good workmanship. The connection method has to be so good that the separate units will act as one; unless a whole unit is substituted or “sistered up”. This discussion is not intended to give credence to any particular splicing project and can not substitute for proper engineering analysis but is only intended to help guide carpenters as to the proper concepts and best practices for proper splicing procedures. Every project is different and has their own particular load bearing criteria which may necessitate different methods, additional support or even engineering analysis. Comments to this blog is encouraged.




inadequate splicing methods


At left is a diagram of some inappropriate repair techniques. Crawlspaces can be tight and uncomfortable to work in so the temptation for shortcuts and substandard workmanship is common. Lack of experience plays a part. Supervisors are often reluctant to review their employees work. Whatever the reason substandard repairs can be structurally deficient. Most common causes is lack of proper nailing technique and splicing to compromised wood.



The most common way joists are repaired is to “sister” it up. Essentially sliding a whole member beside it and nail the heck out of it from both sides with a lot of nails). If the new sister bears on wood at least 1.5” on each end then you're good to go. The most common “mistakes” in this method would be not nailing the sisters together good enough, inadequate bearing on one side or another,




gangnail on truss example  The truss companies use a “gang-nail” to splice wood together (pictured at left). A gang-nail is basically a thin piece of metal stamped out so that many metal tangs stick out into the wood about a 1/2” or so. It is stamped into the wood at the factory with a hydrolic press.





Plywood can be used to accomplish the same thing and for many years many carpenters made their own job built trusses using plywood as the gusset This practice fell by the wayside mostly because manufactured trusses came down in price. Also contractors could not be sure their workers were experienced enough to do this properly and so opted for engineered wood for liability reasons.




splicing a rafter with plywoodPictured at left is a 2x6 rafter spliced together using 3/4” plywood. Very strong connection. In this case the rafter as a whole is actually stronger than it would have been because the splice is more than adequate and the plywood adds to the strength. In this case the framer just needed a longer rafter. Use your common sense when deciding how many fasteners are needed. Splitting the wood does no good. In this case the framer used 9- 8d nails on each side of splice which is adequate in this case.





splicing with plywood example Pictured at left is a diagram of some recommend prescriptive repair techniques for floor joists. Some allowances can be made on the length and thickness of the plywood gussets to suit your particular needs. While it is prudent to “over engineer” your repair; it may be acceptable to use 1/2” plywood or a somewhat shorter gusset depending on your circumstances. The strength of the connection is dependent on the shear strength of the nails so use plenty (driven straight and not slanted). Do not use screws because most are not rated for structual applications. If the repair was engineered then the repair technique would be drawn out on paper, stamped with the engineers approval stamp and it would list the number and type of fasteners needed.








Splicing method for rotted joist Pictured is joist repair method for end rotted joist using plywood and some suggestions for best practice. Click the image to enlarge. Splicing with plywood can have some advantages over splicing with a solid joist in many situations. Use common sense when deciding how many fasteners to use. Use an adequate amount however it does no good to split the wood with too many.















So when is splicing only on one side acceptable?  For example in most of the examples above a short joist is filled in and spliced on both sides with sound wood. This helps relieve compression and tension forces resulting from loads imposed. Sometimes it's ok only to splice on one side for example when adding rafter tails to rafters that already bear on the top plate. Another example that could be ok is adding to the top of a rafter that is too short because the contractor ordered lumber which is 8" too short. In these case the load imposed are not very much for the amount of fasteners typically needed to make such a splice work so would be ok in most circumstances.

Floor loads however are greater so we would discourage the temptation to only splice on one side. This should be evaluated on a case by case basis but if it absolutely has to be done then nail it with plenty of nails from both sides (not slanted) and with plenty of overlap.

In the field I noticed a lot of workmen cut a sister short on one side or another to get it to fit between the 2x2 ribbons. It would be better to remove the ribbon, install the sisters and replace with new ribbons so the sister has adequate bearing on both sides. It might actually be easier this way. Resist the temptation to reuse the old ribbon as it is probably too chopped up to do any good by the time it's removed.









Fixing a Sagging Subfloor


Easy way to fix sagging subfloor    Often we're presented with a situation where the subfloor has gotten wet from a leak or has a weak spot. When replacing the subfloor is not practical like when cabinets are in the way etc. then adding blocking from underneath can help. Picture at left shows how to easily fix a sagging subfloor. You're want to cut some 1x2 cleats, measured down the appropriate distance attach it, slide the block in, jack it up with a floor jack or car jack, install the other cleat, add fasteners into blocking as needed. Repeat as needed. This method is suggested because trying to push the subfloor up with one hand and nailing it with another is almost impossible especially if you want your floor flush.



Replacing A Girder

Up till now we've only talked about repairing a joist or a rafter. How about when we need to repair a girder or sill. For the most part you cannot sister a sill or a girder because you cannot sister to compromised wood. This is also because loads are resting directly on the girder. Girders and Sills should be replaced when they rot or have termite damage. Some people will try to hide the rotted board by placing a nice treated piece in front of it or sometimes even under it. Then they will run joists to it like the rotted girder doesn't exist. This is poor practice at the very least. Whenever possible rotted girders should be replaced with new wood. I think where people run into problems then they're not thinking outside the box as far as how to replace the damaged pieces. Girders do not have to be dimentional lumber. They can be engineered lumber or smaller members glued or nailed together to form larger component.










Now let's see what we find out in the field....




truss gusset not nailed     In this picture the framer did a good job making the job built trusses but failed miserably on nailing the gussets. Seems he only nailed the gussets on one side thinking the nails were long enough to penetrate the other piece or maybe he just got lazy. As you can see they are coming apart and so the truss could fail. The gussets should be face nailed from both sides.





improper splice on floor joist picture

 Here is a pathetic attempt to splice some floor joists under the bathroom. The long view makes the overlap seem longer but I believe the nearest one is less than a foot with very few nails. Drooping and joist sag can be seen in the photo. Believe it or not this kind of poor workmanship is not that uncommon. It would have been easy to do it right, but they chose not to.





Written by Ray Thornburg.


Comments to this blog are welcome.

Submitted by Ray Thornburg on Thu, 11/09/2017 - 19:14


Anonymous | Sun, 06/06/2021 - 10:13

I recently had multiple water damaged or sagging TJI joists replaced with LVLs. Problem is that many of the new LVLs seem to have inadequate bearing on the sill plates (many have less than 1.5 inches of bearing and some essentially 0 inches. Furthermore they were sistered to the adjacent old TJIs via screws through the flanges rather than web with sandwich board.) Is there any way to salvage this situation? Is it permissible to splice the new LVLs to increase their bearing length? I'm worried that all the screws they placed horizontally through the old TJI flanges compromised their integrity as well. Thank you.

Ray Thornburg | Tue, 06/15/2021 - 18:24

Thank you for your question. Properly repairing a TJI joist would require a written detail from the truss company about how it should be done. This is because you are repairing their product. So their engineer should specify exactly how the repair should be made and how many and the type of fasteners to be used. So to recap...their product is an engineered product so it will take an engineered solution to fix it. Could it be fixed through prescriptive means....maybe....Typically we do like to see bearing on both sides of at least an inch and a half. Maybe a ledger could be installed.  But really there are too many variables involved to make a determination sight unseen. You do have some valid concerns however.....Good luck to you...

Anonymous | Wed, 04/21/2021 - 09:04

Excellent article, thanks! I'm looking to cover a 24' span with 12' laminated joists. Would splicing them such that there are two 12' lengths end-to-end on one side, two half-length joists at each end with a full length in the centre on the other side, with plywood sandwiched in between, be sufficient? Something like this: ___________________________________________________________________________ |____________________________________|_____________________________________| /_________/________________/___________________/__________________/_________/ |_________________|____________________________________|____________________| <---------------------------------------24'-----------------------------------------> Thanks! Peter

Ray Thornburg | Thu, 05/13/2021 - 13:06

Thanks for your question. If I understand correctly and I'm not sure from the wording of the question, but it sounds like you want to make a long joist out of shorter pieces or either you're trying to make a laminated beam to support a load. In the old days carpenters would make their own laminated beams with plywood and glue nailed together. This practice fell to the wayside over legal concerns because in reality this aspect would fall under engineering analysis. Even when there is an engineered drawing showing how it is to be built carpenters often do not follow the instruction to the T. The same would be true if you wanted longer joists to cover a certain span. Many lumber companies and Truss manufactures do offer engineered girders etc. and will calculated the beam design to support certain design loads for you. Then they're sell you the correct lam beam or truss for your needs. Anyway I hope this helps.....

Anonymous | Fri, 03/19/2021 - 10:50

Ray, terrific article. Thank you for informing us. I need to replace about six feet of inside rotten rim joist under a sliding glass door that exits to a wood deck. I will be replacing the door, but would like to keep the siding and deck ledger in place as I do this project because the aluminum siding is no longer available and will not come off intact. I intend to temporarily support the deck and then back out some ledger screws, and also temporarily support the inside joists about four feet from the wall. The basement is unfinished. Then I will remove the door, cut out the damaged portion of rim joist, and drop in a new rim joist in two sections so I can slide the first piece under an inside wall. I intend to make my cuts midway between the inside joist ends. Does this sound like a good plan to you? The downside is that there will be no direct access for fastening to the outer side of the new rim joist, though the ledger screws will anchor into it. I will use joist hangers inside and out. Instead of sistering 1/2-inch plywood to the new joints between the inside joists, would plates on one side and straps on the top using structural #9 wood screws serve the same purpose? If so, what size plates to use? If not, will sistering the plywood between the joist ends provide enough overlap length? Finally, do you have a favorite flashing product that I would use to prevent this from ever happening again? Thank you.

Ray Thornburg | Thu, 03/25/2021 - 08:38

Thank you for your kind words and for your inquiry. It sounds like you have a good grasp on what's going on. I couln't be sure about your last question however. I actually like the pvc flashing as long as it's going to be covered up or painted. PVC is resistant to salts which are often found in treated wood. These salts can eat up galvanized and metal flashing. PVC flashing can be bent on a metal break if needed.

Anonymous | Fri, 01/29/2021 - 12:33

Regarding splices, what would it take to make the following shed roof design structurally sound? I’d like to add a mud room onto my log cabin. Due to access constraints I could only haul in 8’ lumber. The roof span is 16’ and the ledger will be supported by a existing horizontal structural beam on the cabin roof structure. I intend to splice (2) 2x8x8’ boards which will be supported with a knee wall at about 4’ down the roof span from the ledger. This leaves a 12’ span from the knee wall to the new shed roof wall of the mud room. Is it acceptable to splice these (2) 2x8x8’ beams together with overlapping plywood splice on either side or by sistering them with a 3rd 2x8x8’ beam which will be supported by the knee wall?

Ray Thornburg | Mon, 02/08/2021 - 07:01

So essentially you just need longer members and you want to know if you can splice the wood together with plywood to form a longer can successfully splice 2x8  rafters with 3/4 plywood strips 32" long nailed and glued on both sides in most cases. To me plywood is preferable than sistering with a similar member because plywood is extremely strong in the vertical position and won't crack like real wood. As long as you're not exceeding the original span rating for the lumber you're using you should be fine. Hope this helps....Ray

Anonymous | Sun, 01/03/2021 - 15:46

I have a 14' joist only rotted on the end of about 8" which rests on a brick pocket with no room to support an additional joist. My plan is to 1) cut out the rotted end piece and replace with a 24" piece butted to the good portion of the joist 2) then splice two 24" 3/4" exterior plywood pieces on each side of old joist and new joist insert on the end. Note the brick insert pocket only has room for a single 2x6" piece. 3) splicing the two 24" pieces to joist snd new butted piece, I will use construction adhesive and carriage bolts to hold them together . I've seen carriage bolts used in lieu of nails or screws. If carriage bolts are ok, what size and how many do you recommend? Thanks Ray

Ray Thornburg | Mon, 01/11/2021 - 20:19

Thank you for your question....I don't believe there would be any advantage to using carrage bolts. Don't use normal deck or sheetrock screws as they are not rated for structural applications. They now make some structural screws (make sure they are listed and labeled for that use). Typically normal 8D sinkers are good enough. Use a palm nailer if space is limited. Depending on the load might be better off with 32" pieces of plywood. The glue is a good idea.  Hope this helps....Ray

Anonymous | Tue, 12/29/2020 - 22:45

I love the suggestions you've mentioned! I have definitely been needing something like this, my only concern is what kind of plywood to use? I only see at the typical big-box sores standard C grade plywood. Is that acceptable or do I need to get structural grade plywood with a higher grade?

Ray Thornburg | Thu, 12/31/2020 - 10:24

Thank you for reading and that is a good question. I prefer to use plywood rather than OSB because in my tests plywood has proven superior for this use...though my tests are not in any way scientific. As far as plywood grades is concerned any plywood graded for use on roofs  (typically 1/2") or floor (typically 3/4") will work fine. These types of plywood is generally already on the jobsite (at least they used to be before OSB came along). and are easy to work with and relatively cheap. They also have some exterior glue in them which will make them more suitable and durable than interior grades. I would avoid interior grades of plywood.

Anonymous | Sun, 07/05/2020 - 13:45

If one were to replace the full length of a 14' joist bearing on brick in pockets either end, with no room to maneuver in continuous new joist and no room to fit any more lumber (ie splice material) over brick could it make sense to remove existing, install 7' on either side to meet middle, then splice full span or close to to it up to the brick, on both sides if necessary? The splice material not bearing seems it might conflict with your guide, but length of splice would seem to make the joist "act as one."

Ray Thornburg | Wed, 07/08/2020 - 16:48

Thank you for your sounds like you're asking if you can take two pieces of lumber and splice them to act as one without losing strength. The answer is yes in most cases if you use real plywood to splice the two pieces on both sides and nail appropriately. Avoid making the joint in the center 1/3 of the span.  Make sure your joist is straight because once you splice it this way it cannot be repositioned. The splicing material would not have to have bearing if the replacement is the same size as the original lumber. Hope this helps....

Anonymous | Wed, 07/01/2020 - 07:25

On the last example “pathetic attempt” was the method correct to splicing the Joist to make it reach the end but the nailing was the problem? Or was it the whole thing?

Ray Thornburg | Wed, 07/01/2020 - 11:19

Thank you for your question.....I see two problems insufficient nailing and not nearly enough overlap. It should have enough overlap to make the unit act as one.  If the break is in the center of the span it's going to need a lot of overlap to keep it from sagging later. Hope this helps....Ray

Anonymous | Sat, 07/04/2020 - 16:35

Yes lots of help thank you !

Anonymous | Thu, 04/23/2020 - 11:53

This was exactly what I was looking for, and it turns out I have more pieces of plywood than lumber, so my splices, as long as I nail the heck out of them from both sides, and straight, should work out just fine!