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 | Sat, 05/14/2022 - 13:40

So I'm building a deck and the span is 6 ft. but I only have 4ft 2x6 boards. All I need to do is go to the store and buy some 14ft 2x6, cut them in half so I have 7ft 2x6. However, I have so much extra wood my wife, with the whip, is not allowing me to buy any more so I have to use what I have. This is causing me to have to join the 4ft boards together. ps don't let women be the boss of what you're doing.

Ray Thornburg | Mon, 05/16/2022 - 16:01

LOL....marriage counseling is beyond the scope of a standard home inspection....however....I think what you need is a board stretcher......look on EBAY....they have everything....

Anonymous | Thu, 05/12/2022 - 19:48

Great article Ray! My contractor's "plumber" cut a 6" long supernotch almost all the way through two joists in order to route a new toilet drain line perpendicular to the joists. When I discovered this, we all agreed to reroute the toilet drain a more sensible path through the joist bay, parallel to the joists, and put the two cut joists back together. How to fix the joists? Each joist has a 6" notch cut from above that runs down through 95% of the joist. The joists are about 15 feet long, and each end sits on a structural masonry exterior load bearing wall (it's a narrow 19th century stone twin). The cut/notch happens to be right smack in the middle of the joist span. The joists are true 2x8 or thereabouts. The joists are above the second floor (the new toilet is on the third floor). Most of the joist span is pretty accessible, but I'd rather not cut new joist pockets in the masonry walls nor wrangle a 15' piece of lumber. Following the logic of this article, I am thinking I could just fill the notch with a 6" piece of true 2x8, glue it to the old joist on three sides, then glue up and nail 8 foot pieces of 3/4" plywood on either side of thew newly-reconstituted joist. Does that sound right? Or, do I need to entirely sever the joist, widen the notch to 24", add a 24" block of lumber, and then glue/nail up the plywood pieces?

Ray Thornburg | Fri, 05/13/2022 - 16:32

Yep.....You're on the right track.....fill it in and put plywood on both sides...

Anonymous | Tue, 03/22/2022 - 23:19

Hi ray, my current attic has 2x4 (actual) and I’d like to sister 2x6 to them to increase the strength to add boards and use for storage. Half the attic I was able to fit the 2x6 span sill to sill. However for the remaining area, I no longer have adequate space to squeeze in the 2x6. The span is lass than 14’ and I was thinking of splicing the 2x6 either in the middle or offset to the existing joist. Would this be sufficient, as it would sit sill to sill and sistered to existing joist? Thanks.

Ray Thornburg | Fri, 03/25/2022 - 17:51

Thank you for your question. It sounds like your roof is trusses judging from the 2x4 ceiling joist spanning 14'. Trusses are designed for a little bit of light storage and many people just throw a piece of plywood up there so they would have a place for their Christmas tree etc. and be done with it.  If the area is insulated it is not uncommon to build up the floor (ceiling joist of truss) so as not to compress the insulation. Yes adding and properly nailing wood together does strengthen it and splicing that 2x6 would probably be ok. Of course since I can't see your project its had to say for sure what would be acceptable but if you're not storing engine blocks, water heaters or lumber up there you're probably be fine. Hope this helps.....

Anonymous | Thu, 03/10/2022 - 11:08

Ray - Thank you for this article, very informative and educational. I have an area above the open 20' x32' garage where the previous owners put attic stars, and sistered some 2x4s to the trusses above the gang nail/pressed plate for subfloor support of a makeshift attic storage space. Trusses are 2x4 Fink style, 24" on center. I know the static load of this scenario is like 10psf on the bottom chord, and I'm looking to make leave the center open, with storage out near the bearing walls. My pan is to put (3) 2x8x20' joists in parallel to the trusses, fully on the top plate, but joined by 3/4" x 7" x 4' plywood gussets on both sides. Then use 2x4 lumber perpendicular to the existing 2x4 trusses to the each end bearing wall on the ends, and between in the middle, with 1/2" plywood to keep the weight down. With a 1/4" gap, this creates a 'floating floor' that when it deflects will load across multiple the trusses at the same time. Again, no heavy storage, and only storage within 4' of the exterior bearing walls. Thoughts?

Ray Thornburg | Tue, 03/22/2022 - 12:34

Thank you for your you know most trusses allow for a little bit of light storage anyway....if you're planning on more then you're have to beef it up so to speak which is what it sounds like you have in mind. My thought is that single ply 2x8's won't span 20'. So if you're splicing two pieces together with plywood it will make a longer piece but the span rating will not be longer than a 2x8 would be rated for normally. A 2x8 spanning 20 foot will sag under it's own weight over time. Alterations to a trusses is generally not allowed but you can nail lumber to the side of trusses already in place. It probably won't improve it's designed rating because that would require an engineer, however it won't make it weaker. Hope this helps....

Anonymous | Mon, 03/07/2022 - 07:37

This is exactly what I was looking for. I am remodeling a second floor bathroom and replacing the old fiberglass tub enclosure with a 1930's cast iron built in. The second floor was built using truss's on 24 inch centers. I am concerned with the weight, but I do not want to get my PE involved as I am doing it on the QT as in California they would not let me use the disassembled 1934 bathroom that I have. Not low flow, lead in the fixtures and tile glaze and one and one... So, I want to add a floor joist in the existing bay. But there is no way to a long member in there to span the 17 feet from the outside load bearing wall to the house center load bearing wall. (I have the truss plans but nothing else on this house). The bottom chord is a 2x12 so my thinking is to use two 10 footer 2x12 cut down to make the exact span, about 9 feet each and then use 3/4 plywood on each side (2 feet in each direction from the seam) with your recommended nailing schedule. The bay is open in the attic and in the bathroom I am opening the floor for that bay for plumbing. My plan is to place this joist directly under the lower 1/3 of the tub which is where the most water would be. My question is should one use a water proof glue or construction adhesive before the nails? I will cross block this to the truss's on either side in 3 or 4 places then nail the new plywood floor down to seal the bay. Then we will add another layer of plywood over to being it all up from 3/4 inch to 1-1/4 inch deck. Does this sound like a plan?

Ray Thornburg | Tue, 03/22/2022 - 11:54

Thank you for your question. It sounds like you're asking if you can build a beam or joist in place with shorter pieces spliced with plywood.  In most cases yes but remember the built up single member piece will probably not be stonger than a single member full length piece. Probably the best glue to use would be waterproof type elmers wood glue that they sell in one gallon jugs at the hardware store. If you don't have that then construction adhesive will work too. Hope this helps....

Anonymous | Sat, 03/05/2022 - 07:24

Thank you for putting this article together! I am considering building an aframe cabin and having trouble finding lumber that is long enough for the rafters. What are your thoughts on splicing the rafter just above the "rafter tie" or loft area? ^ / \ /// \\\ / - - -\ / \ / \

Ray Thornburg | Sat, 03/05/2022 - 19:11

I have sucessfully spliced rafters using plywood on both sides glued and nailed many times. If done correctly it does not matter where the splice most cases....If you're going to sister a rafter to make it longer it's probably better to sister it at the top. Of course this is not a blanket statement as there are too many variables involved like the projected load or other bracing for instance. Hope this helps.....Ray

Anonymous | Thu, 02/10/2022 - 11:35

Thank you so much for spending your time to put this together. Well explained. For blocking to correct sagging floor, is it 2 by lumber you recommend or plywood? Can you also pls comment on plywood splicing vs using steel face plates or steel tie for wood to wood connection? steel plates, unless custom, usually not very lengthy, and do not cover your recommended 48 inch span coverage with plywood. Thanks a lot

Ray Thornburg | Wed, 02/16/2022 - 17:52

Yes....examples given are typically regular 2x dimential lumber. I'm not sure if steel plates would work, every case is different and there would be too many variables to comment on (thickness of the metal for instance).  They do make a variety of metal connective hardware and you would have to look at the listing on the hardware to see if it is approved for a particular use. Hope this helps...

Anonymous | Thu, 02/03/2022 - 03:44

This article is right on point with the situation that I am facing. Thanks for posting. My situation is eerily similar to your last picture..."Here is a pathetic attempt to splice some floor joists under the bathroom." The previous owner scabbed back 1' (12 of them), installed a 2-ply 2x8 beam on top of 3 6x6 posts (it has held for over 10 years, but is poorly done and I want to improve). My plan is to splice 4ft 2x12's (will give me 3' overlap to existing) to get back to the original 3 ply support beam, which rests on a concrete ledge in the crawlspace. What is the best connection method in this situation? Thanks in advance for your response!

Ray Thornburg | Tue, 02/08/2022 - 06:30

Thank you for your kind words...In answer to your is heard to give advice on any particular situation without seeing it because what I imagine you're talking about and what you have planned may be two totally different things. In general however regualr framing nails are ok to use like 16D sinkers or 8D sinkers. If using a hanger try using hanger nails. Deck screws are not recommended because they will snap in half under a load. Same is true of rang shank nails. Structural screws are ok (check the listing). Screws can be used to "pull" the wood together and then you can finish it off with nails.....if space is limited you can use a palm nailer. Drive the nails straight (no slanted fasteners). If you're building up a beam from fresh wood three nails on seams and stagger 8 inches on center should be adequate for most situations. If you're sistering damaged wood don't be stingy on the nailing pattern....hope this helps....Ray

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!