Subscribe or Renew Membership Subscribe Renew

How to reinforce 2x4 attic floor joists?

Senna's picture

Just installed an attic ladder in 90 year old house. Plan is to use attic for storage not for living. Currently the joists are 2x4’s ( real 2x4’s not 1 ½ x 3’s). What should I do to reinforce them? I was thinking of taking 2x4’s and screwing them , on their sides , to the old joists and covering with ½ plywood or OSB.

Any problems with that plan?

Spans are not huge, nothing bigger than ten feet unsupported, I think.

(post #95332, reply #1 of 30)

What's your current joist OC spacing? 16 inch or 24 inch or something else? This spacing makes a difference.

If 16 inch OC, then for light storage, doubling up on the 2x4s to make them 4xs should be OK.  When installing flooring, screw it down rather than will give you better support and help stiffen your joists more.

If current joist spacing is 24 inch or larger,then besides sistering joists,  you may want to add more 2X4  joists in between to "knock down" the spacing distance...or you could sister in larger size joists ( 2X6S)  to your existing. of course, the 2X6s will cause you to raise your floor height.


(post #95332, reply #2 of 30)

Spacing is 16 inch.

I was thinking of predrilling holes and screwing the 2x4's together on edge or on top of the old 2x4's ( rather than side by side ) to make 2x8's. This way I would have space for insulation.

(post #95332, reply #3 of 30)

how about  installing strongbacks?

(post #95332, reply #6 of 30) man!!! Exactly what I was thinking....great minds and all I guess.

J. D. Reynolds

Home Improvements





(post #95332, reply #8 of 30)

What's are strongback's?

(post #95332, reply #9 of 30)

Here`s a quick sketch showing how either 2 x 4s, or in your instance I might suggest going to 2 x 6, are fastened together and then situated running perpendicular to attic floor joists. You`ll want to choose a location that is both out of your way in regards to storage, yet strategically placed so that they might strengthen areas unsupported from below.

Perhaps I`ll attach a second for clarities sake.

J. D. Reynolds

Home Improvements





(post #95332, reply #10 of 30)

A lot depends on how much height you have available in this attic.  If you have room to stand up, then the strongbacks look like a good idea.  But if you only have 3-4 ft. to the ridge, simple sistering with 2x6's should be strong enough for the amount of weight you'd ever be likely to put up there.  My attic's small like that, and I went with sistering. 


-- J.S.




-- J.S.


(post #95332, reply #13 of 30)

More questions.....

How many strong backs do you suggest? One every 16 inches or just a few? I want to have flat storage space and insulate between. Cover with 1/2 inch ply?

Two more pictures attached.

There is on old 3 or 4 inch galvanized pipe with runs down the middle of the attic then does a 90 degree turn and runs in to the chimney. What would that be used for? I don't think it is still in use but I'm not sure. I don't think it was a plumbing vent, maybe an old stove?

(post #95332, reply #14 of 30)

As I both mentioned and sketched....the strongbacks run perpendicular to the attic joists. While one, run staright down the middle of the attic, might suffice, I would prefer to divide the attic into thirds and run two, spaced equally, from end to end.

Couldn`t tell you what that pipe was...coulda been any of a number of things....I`d make damn sure it wasn`t still live before attempting to remove it.

J. D. Reynolds

Home Improvements





(post #95332, reply #15 of 30)

asennad the strong backs support the ceiling by spanning them and terminating at a bearing point like a partition wall or gable end wall. you will need to install squash blocking at all of these points. I wish I could give you a drawing of this but I don't know how to do that maybe jaybird will if your real nice to him. it looks like you have a 12/12 pitch roof so no need for phony wall. it looks like the vertical 2 bys running down from the ridge are to support long spans or the ceiling joists . how long are the spans there?

(post #95332, reply #18 of 30)

"it looks like the vertical 2 bys running down from the ridge are to support long spans or the ceiling joists . how long are the spans there?"

I don't think these vertical boards support anything and were left over from the build.

The attic itself is 15 wide by 22 long. With the 2x4 joists ( 16 inch centers) running the 15 foot width.

I'm going to take the good advice presented to me on this board and

install 2x6 strong backs running the length ( 22 feet ) of the attic.

The only other issue I have left to resolve is what type of flooring material to use. I don't want to put 1/2 inch ply on the 2x4's between the strongbacks as that won't give me much insulation space.

If I put it on top of the strong backs that gives me some space for insulation but the ply will flex if it is supported only every five feet. Should I add some 2x6's inbetween or more strong backs?

(post #95332, reply #19 of 30)

Asennad, after reading your latest post its very obvious....You haven't a clue!

Those vertical 2xs are not simply "left over from the build" as you so aptly put it. They were definately put there by someone for a purpose. The fact that you can't visulaize what the purpose is ... is reason enough to hire someone competent and let them do the attic work instead of you.

And for the record, it doesn't matter if you sheet your joists with 1/2 inch thick ply, 3/4 ply, 2x decking, or any other type of flooring....the flooring all gets nailed or screwed down to the TOP of the joists. The size of the joist determines how much "insulation cavity"  space you can have beneath your floor your case, a 2X4 is 3-1/2 inches wide, so in this instance, your insulation cavity underneath your new proposed flooring will give you 3-1/2 inches for insulation.

The thickness of the flooring material ( 1/2 inch ply vs 3/4 ply etc.) is what gives you a more solid feel ( ie...strength)...and this in turn depends on your joist spacing. Thickness of flooring does nothing for giving you a larger insulation cavity.


If you want to put down flooring in your attic  and want more insulation underneath that floor....then you need to INCREASE YOUR JOIST SIZE. Sistering 2X6 joists next to your current 2x4 joists will simply give you a new floor height of 5-1/2 inches as opposed to the current 3-1/2 inches. Or,  you could sister in 2x8s instead of 2x6s and get 7-1/4 inches... or use 2x10s for 9-1/4 inch cavity...etc. etc....

 Now if you run 2x6 STRONGBACKS instead of sistering....your strongbacks sit on top of your 2x4s, rather than beside them ( as when sisitering). Strongbacks would give you an extra 5-1/2 inches ON TOP OF your 2x4's current 3-1/2 inches, for a total height of 9 inches that you can insulate.

However, as you surmised, just placing a 2x6 strongback "here & there" will not give you sufficent OC spacing to enable you to install flooring on top.The idea of using a few strongbacks is IMHO just an attempt to save you some money by allowing you to utilize your 2x4 joists ( instead of sistering a bunch of 2x6s). Unless you install the strongbacks in a regular floor spacing configuration, ( such as either a 16 inch OC or even 24 inch OC) you cannot floor your entire attic overtop a few strongbacks....period. 

As for that galvanized pipe running into your could very easily be a plumbing vent pipe...or it could be  a vent pipe for a gas appliance...more likely a plumbing line vent...but as earlier pointed out, it could be one of a number of things and like it or not, may still be in operation.

The fact that you have no clue what it is, or where it leads to, or whether or not it still may be "live" is simply another indication that you have absolutely NO Business doing any structural work in your attic what so ever.

Use that new attic ladder of yours to haul your butt down out of there ...before you fall down out of there and bring the house down with you!


(post #95332, reply #20 of 30)

Recall a "Great Moments" from the late 1980's where 2 physicists were remodelling, still somewhat recall the phrase they used in the article "that puppy was sure in there" about their comments as they cut 'an un-needed, what's it there for'  tension pipe out of the attic just before the ceiling sagged!

(post #95332, reply #22 of 30)

I'm with ya on that one!


(post #95332, reply #21 of 30)

Oh yea, I almost forgot..... you stated your intentions about using the strongbacks, but then asked the following which I'm para-phrasing here:

" I'm going to take the good advice presented to me and install 2X6 strongbacks running the length of the attic...........the only other issue I have is what type of flooring material to use.....if I put it on top of the strongbacks the ply will flex if supported only every 5 feet..... "Should I add some 2x6s in-between ( the 2x4s) or (add) more strongbacks?

Asennad...I'm truly sorry for picking on you like this, I shouldn't be doing it's my question to you? If you add strongbacks ( which sit on top of your 2X4 joists and run perpendicular to can you ALSO at the same time install 2x6 "sisters" to the 2x4s, since a 2x6 is larger than a 2x4? Do you see the logic here?

 The installed strongback attached to the original 2x4 joists will not allow enough "headroom" for the 2x6 joist to be sistered into the 2x4...unless you notch the 2x6 at each strongback location. If you did install 2x6 sisters along with installing several strong accomplished nothing as far as providing a support base for your intended flooring; because the 2x6 sister is in a different "plane" or height zone than that of your strongbacks. The ply flooring can only be nailed to the strongbacks in this proposed configuration, so sistering 2x6s does not give it a nailing base to nail adding additional strongbacks would give the ply more support.... so if you added more strongbacks you would be doing yourself and your floor some good.

The fact that you simply asked this question...about using 2 different systems at the same time in order to provide a better nailing base for your intended flooring, simply suggests that you really haven't grasped the structural concepts of what role a joist plays and how it affects the structural integrity of everything connected to it.

Please let someone else do this work for you.


(post #95332, reply #23 of 30)

Thanks for the kind words of encouragement. The conclusions you made about what I had in mind, although wildly inaccurate, were certainly interesting.

I have now installed several 2x6 strong backs in my attic spanning the supporting walls of the rooms below.

I have also added 2x6’s in between the strong backs (obviously parallel to the SB’s and NOT sistered to the existing 2x4’s ) to support the ply floor.

I never said that the vertical boards attached to the rafters were 2x’s . They are very rough half inch boards randomly placed and if the were placed there to serve some support purpose then the roof would have collapsed a long time ago.

As far as the pipe goes, I am still wondering about it’s original purpose. It was not attached to anything and the chimney is non-functional. I don’t think that the section of house it was in has ever had plumbing and I wouldn’t think a pipe used for combustion venting would be in direct contact with flammable material.

(post #95332, reply #24 of 30)


You are right, my visualization was definately inaacurate...and I am glad things are working out for you. Yes I can understand how 2x6 blocking can be placed "in between" the strongbacks and be parallel to the existing 2X4 joists; infact it's best to rest the blocking directly overtop a  joist...makes nailing easier.

I guess because the word "blocking" was never used, and only"joist", "sistering" and "strongback" were the terms being thrown around, I jumped to the wrong conclusion that you were talking about sistering...when what you really meant was blocking. 

As for the random vertical boards attached to your rafters.... from the only photo that you presented depicting these boards, it appears that your rafters have collar ties attached and the vertical 1x hanging down from the collar tie, could easily be just that, a hanging support designed to be attached to your 2X4 joist to keep the ceiling joist from sagging. It may or may not be attached... It may have been at one time but has since been cut loose for whatever reason...I have no way of knowing cause the photo of the vertical hanger shows it ending into the loose insulation and no attachment point can be seen. As for said supports being random...the photo you posted looks as if the hangers are spaced on every other collar tie...which to me would not be "random"but, of course, it's really hard to tell for sure because your photo is only depicting the last few feet of the attic's run .  You are correct in that a 1x would not provide much support for your roof's ridge line; but I doubt this was the intention. The fact that your ceiling joists span 15 feet and are only 2x4s and are holding up plaster and lath, it makes more sense that the 1xs were trying to support the ceiling joists below.

As for your galv pipe situation...I believe you stated said line to be approx 3inch in diameter?  I've come across pipe this size that was used for plumbing vent lines. I've found such vents "buried" below the roof sheathing; simply venting into the attic only in hopes that the air would be carried out through the gable end vents. Yes, this is totally wrong...but in lieu of this finding, it is not too far a strecth to think someone routed the galv vent pipe into the chimney, rather than fitting it up through the roof in proper fashion.  Other than a vent for something, this pipe may have been attached to a TV antenna of long ago?

Anyway, sorry for the mix-ups.



(post #95332, reply #25 of 30)

davo is 100 percent correct about the the vertical boards running down from the ridge to support long spans in the ceiling joists. it was common place in them days to do that, and even though it was one by.  one by in them days was stronger than most two by we have today. check the locations of the joists Ill bet they do not rest on a partition at these points  

(post #95332, reply #16 of 30)

no. when I said mid span I was referring to mid span of the rafters as jaybird pointed out but with a 12/12 pitch I don't think you need to worry about it

(post #95332, reply #17 of 30)

dialup pix

Arguing with a Breaktimer is like mud-wrestling a pig -- Sooner or later you find out the pig loves it. Andy Engel

A Pragmatic Classical Liberal, aka Libertarian.

I'm always right!
Except when I'm not.

Use a "composite beam" method (post #95332, reply #26 of 30)

I found this yesterday, and dismissed the conversation as no longer in progress. I am back, to see what happens with my post. I also want to know why I don't see graphics posted in threads, which discourages my participation. I find that I can simply copy in a post from my Google blogspot blog. Some will not bother with a link. I apologize for the duplication.


In my work in attics, it is sometimes structurally necessary that I provide substantial decking of the attic floor. 

Here are three examples in a Picasa Web Album. In the first two, I have repaired broken floor joists where they must support needed attic ladders. In the third, I have provided load bearing capability over 2x4 truss bottom elements otherwise flimsy. In all three, the strength of a composite beam is assured only where 2x elements at bottom (ceiling of rooms below) and at top (attic deck surface), are not free to rotate. No deck, less-strong floor, and dangerous navigation with trampling of insulation. 

Here is a graphic illustration of the composite beam application with 2x4 trusses:




I will pay no attention  to silly rulemakers who know and teach nothing of any innovation. Search at Google "Energy Trust of Oregon". "Fraction of attic floor decked." Find 2012 Weatherization Specifications. Find AT 2.4:

Sadly, there is no forum for public input to rules. I can only try to influence, by this kind of public expression. Here then, let me ask: What is a "cavity to be filled"? I will link this by email to Energy Trust, and see what they have to say, on the broad topic. I invite them to respond by comment here.

This post was imagined, in dealing with structural analysis of composite beams, as presented in another public forum I have joined, DIY Chatroom:

Perhaps readers here will see value in composite beam construction, and might comment. I think floor reinforcement by "sistering" has no validity, where added lumber couples that found, with rotational freedom and unlikely bearing on supports below.

I wish for academic and professional comment. Lacking that, I can attempt my own defense. The ultimate would be deflection testing of sample beams, which is within my capability, but would best happen with support of an interested college or university. For now, note that beam stiffness and strength are in linear proportion to thickness, and cubic to height. Sistering at best doubles strength. A composite 2x10 is likely at least four times stronger than a 2x6, and more than ten times stronger than a 2x4. A composite 2x12 of the same 2x lumber is strengthened by a further factor of two. What wonderful and easy control of strength! Testing and questioning are hardly justified. Does it matter that you might increase 2x4 strength by times thirty? Maybe it is just the added insulation depth that matters. Often it is sufficient to just do all that the situation allows for strength and insulation, as in availability of headroom. Sistering is best effort, only if space for making the floor thicker is absolutely unavailable.

Well, (post #95332, reply #27 of 30)

You've succeeded in confusing me.

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


Rock Solid (post #95332, reply #28 of 30)

I know that floors reinforced as tall beams become rock solid.  I feel it as I walk and work, in about a dozen homes where this has been applied so far. Floors deflect MUCH less at a given load. I think the only unknown is how a composite beam might fail under extreme loading. Knowing that, one may make informed better choice of plywood web quality and thickness, and of sufficient screw patterns. I will be very pleased to do load testing to failure in some academic experiment. Aren't we all eager to learn new things, and to do better? Isn't it nice to find cause to have a decked thick attic floor?


In response to your comment, I have added another photo to my album of examples, as attached here. Your questions might be answered with the contrast of an application in a crawl space.

IMG_2954.JPG2.75 MB

so, (post #95332, reply #29 of 30)

what did you do there?

add to the bottom of the joists and sandwich the sides w/ply?

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


2x8 Joist Repair In Crawl Space (post #95332, reply #30 of 30)

I still am unable to post photos with this editor. I instead copy and paste from an expansion of my blog post:

Here is the photo.



And, here is a cross-section detail. This beam bears more load than any other in my crawl space, yet suffers a carve-out passing the bathtub drain. I go way beyond compensating for the divot. Surely the reinforced beam is as strong as a joist of 1 1/2 x 12" cross section, better strength is by the ratio (12/7.25), cubed. That is, by times 4.5.

I worried some about that beam before my remodel. I won't anymore.

(post #95332, reply #4 of 30)

Stacking two 2x4s does not equal a 2x8 for strength.  Doesn't even equal a 2x6.  Save yourself the trouble, just stand 2x6s (or 2x8s) alongside the existing and screw them together, then cover with ply.  Use #2 SYP, not SPF, and you'll be fine.


Whenever you are asked if you can do a job, tell'em "Certainly, I can!"  Then get busy and find out how to do it.  T. Roosevelt

I'm sorry, I thought you wanted it done the right way.

(post #95332, reply #7 of 30)


No, no, no, no,......Do not attempt to install a 2X4 on top of another 2X4 to make a Ed Hilton pointed out this is no good at all! There are other words I could use to describe that attempt but I will remain kind.

No, do what Ed said and be done with it...attach ( sister) 2X6 joists to the 2x4s and attach your flooring on top the 2X6s. As for insulation, rent a macine and blow in cellulose insul in the remaining cavities. The 2X6 is plenty strong to span the 10 foot; infact a 2X6 is perferable to me over a 4X4, and what width you lose for insulation, you make up for in height by using a 2x6, so your insulation area should be a "wash" in the end. Blown-in cellulose is a good insulator, and will pack nicely, and is cheap in cost to boot.

No...again, forget my earlier post, and forget the idea of somehow "salvaging" your existing 2x4s....sister in 2X6s and be done...period.


(post #95332, reply #5 of 30)

From Souther Pine Span tables:


Visually inspected grade, for 2x4 on 16" centers span is 9' 4".  For 2 x 6 on 16" centers,  14' 7" span.

Your span is not a problem,  but the depth is a big consideration.

Depending on your climate,  you will want much more depth for adequate insulation.  For example,  for blown in celulose,  you'll need around 14 or 15 (settled) inches for R-40. 

Help yourself further in saving heating bills by air sealing all the little holes and air leaks up there first.  Then insulate. There is much discussion on this site about "how to".






Quantum materiae materietur marmota monax si marmota monax materiam possit materiari?

(post #95332, reply #11 of 30)

Iv done a lot of remodels on old places like yours .I'm working on one that was built in 1905 right now. anyways if your ceiling joist are 2 by 4 then your rafters are most likely 2 by 4 as well. with the strong back at mid span you can kill two birds with one stone. as per jaybirds drawings if you were to increase the 2 by six to say 2 by 10 then the 2 by 4  flat part of the strong back becomes the sole plate for a pony wall to take the sag out of the roof.       string line the bottom of the rafters. jack up  each of the rafters off the 2 by 10. cut and install studs nailing to sole plate and through the 2 by 10 into the studs. now your ceiling is flat and so is your roof

Edited 5/28/2004 2:44 am ET by TECONAIL

(post #95332, reply #12 of 30)

I like the idea of using these strong backs but I have a few questions. Fortunately I got a new digital camera but it took me the last couple of days to get it up and running. So I have posted some pictures.

I am trying to understand how the strong backs strengthen the joists. If the strong backs run perpendicular to the current joists will they provide extra support for the joists? It seems to me that they would be strongest if run the same direction as the joists.

Yes the rafters are 2x4 however the roof line is still good. I wouldn’t mind reinforcing it as you suggest.

So what I understand that you are saying, is to build a wall running the length of the house to support the peak of the roof. Is that right?

“with the strong back at mid span …..increase the 2 by six to say 2 by 10 then the 2 by 4 flat part of the strong back becomes the sole plate for a pony wall to take the sag out of the roof. String line the bottom of the rafters. jack up each of the rafters off the 2 by 10. cut and install studs nailing to sole plate and through the 2 by 10 into the studs. now your ceiling is flat and so is your roof’

What I am considering is using 2x6 strong backs and should the necessity arise that I need to reinforce the roof rafters later, then I would building two supporting structure along the sides of the attic. Come to think of it would it need to be along the peek of the roof?

In another post it was suggested I need 14 inches of space to insulate my ceiling properly for this climate - Toronto Canada. That’s seems like a great deal of space. Would the current 4 inch rafters and 6 inch strong backs be sufficient for insulation space?

Edited 5/31/2004 3:59 pm ET by ASENNAD