Reinforcing Loft Beams

[Buckie1967]

 

[Buckie1967]

I am in the process of making the entrance hatch to my loft bigger and have purchased a nice TB Davies Luxfold loft ladder which I aim to install.

I have only been in my house for a year and note there are only 3" x 2" beams which run the width of the loft with a 2' span. In essence as you see from my diagram there are three distinct sections covering the width of the loft, all around 7' in length so approx 21' in total.

Now due to practicality and height restrictions I am only interested in boarding out the middle section. As you will note in the photo the previous occupant has already placed boards down in this middle section and I put some lights up. Now below this section is our bedroom which has a crack running down the middle and I suspect it is due to people going into the loft occasionally and the fact the beams are only 3" x 2" which will cause movement as they will no doubt flex. There are 13 beams running the length of the loft.

Whilst opening up the entrance slightly for my new ladders and loft hatch I was thinking about building the 3" x 2" beams up by adding some 4" x 2" on top of them (as per Tony 1851's diagram above) however ONLY in the middle section where I aim to put the boards down (highlighted area on my diagram shows where I intend to introduce the 4" x 2" timber) Even if I wanted to add longer lengths I am prevented from doing this due to the joints at the base of the truss webs that run either side.

There is an argument to suggest all I would achieve by doing this is adding to the weight of the existing 3" x 2" beams and creating more strain on them. I feel I will actually be reinforcing this middle section of beams and ensuring there is less flex which will benefit from less movement being transferred to the ceilings below and hopefully preventing cracking. It would also make a deeper recess so that I can add more insulation before putting the chipboard flooring back. I aim to get my bedroom ceiling re skimmed next year but want to address this issue first to ensure I eliminate possible movement and reducing the chance that the crack will come back once it has been re-plastered.

We only intend to store stuff up there.......I am not building a gym.........those days are long gone.

Any advice welcome.

Read more: https://www.diynot.com/diy/threads/reinforcing-loft-beams.581825/#ixzz7D9atAMr9

 

[Deleted member 174758]

Tony's original drawing looks to be referring to conventional ceiling joists, NOT Fink trusses.

Even with conventions joists I have never had such a strengthening methodology either suggested or approved by a S/E. Yes, I have been told to bolt through from above a couple of times (with steel flitch plates top and bottom), but more often the instruction is to sister the joists with ones if the correct depth and either way the stregthening is always over the full length (or nearly so) of the original joist (as in @tony1851's sketch) and not over part of the length as you want to do.

The problem, though, is that you are comparing apples with oranges - you want to add a piece to a manufactured truss (a Fink truss, W-truss) which is normally not permitted - the standard advice in the industry is that unless they are specifically designed from the off to take the extra loading you envisage then Fink trusses should never be cut, modified, have extra loading applied or have extra bits tacked on in the way you are suggesting without either the truss manufacturers approval or a structural engineer's drawings and calcs specific to your situation

 

[Buckie1967]

Oh I see. I need to modify and strengthen in effect the bottom chord of a fink truss roof. In fact what else I was planning on doing was to cut into one of the bottom chords to enlarge the opening for the new loft hatch. I was hoping to do this by using trimmers and doubling up the existing beams either side. The existing opening is squeezed between two bottom chords so the opening is very narrow.

 

[Buckie1967]

....and I have just read another thread regarding cutting the bottom chord on a fink truss roof and it is a no no without the manufacturers guidance or a SE's recommendation as to how to go about it.

I would simply go between the spans (2ft) without having to cut into one of the bottom chords but the opening is simply too narrow

 

[Deleted member 174758]

You currently have cracking in the ceiling which is often an indication of overloading, as you have surmised - and now you intend to put more load onto trusses which aren't designed for it. Screwing random odd bits of timber on top of the joist is no way to strengthen it, and because the bottom chord in a Fink truss is actually a stressed element you could potentially do more damage than good. Lastly, Tony's example is very obviously NOT for a Fink truss bottom chord - it is for a conventional joist, presumably where there is no realistic prospect of getting a full length sistering joist into position - so a completely different scenario and not applicable in your case

 

[Buckie1967]

Yes.....what you fail to recognize is nothing is "obvious" to a layman. That's why we post on these sites for help and support. Thanks for your responses.

In relation to the crack in the ceiling which I recognize may be due to overload in the loft, my intention is obviously not to compound this existing problem but to try and address it, hence why I am here asking questions about the best way to reinforce the loft floor purely for storage purposes.

I have contacted an associate of mine who is a Building Control Inspector from my local council and he has steered me towards a local Roof "Truss" company who I have engaged with and responded as follows:

"We unfortunately cannot engineer or offer remedial details for cutting or amending any trusses that were not designed on our roof software.
The cutting of a truss means the trusses either side of the new opening will become supporting trusses for the cut truss. In theory this can be done by bulking up the trusses either side with timbers and bolted ply gussets. We would normally go to our software Engineers, but they won’t touch any trusses that were not originally designed with their software."

I have also sent an inquiry to my Structural Engineer that I have previously used recently.

One observation by me is in my diagram above I have outlined my new opening which is marked in red which is dissected down the middle by bottom chord 2. I had planned to build a square solid frame out of say 4" x 2" timber. I would then integrate this between the section of the bottom chord that I intended to remove. The loft hatch would in turn be fitted inside this square frame that would be made in a size that would accommodate the loft hatch unit. The frame would also be securely attached to both bottom chords 1 & 3. If both ends of the bottom chord number 2 were then securely attached to this frame, with say metal plates, then surely this bottom chord would retain some strength? In effect this frame would act as a substitute for the section that was removed.

It's not as if I intend to cut a section out of one of the chords and leave the two exposed ends unattached to anything, which I agree would then render it having no structural benefit.

This is probably a common problem and identifying what type of roof structure is within your loft is no doubt the first challenge. I think it is also quite common and understandable that folks would like to utilize their loft space for storage purposes.

 

[RonnyRaygun]

It's unlikely any structural engineer will give you the go ahead to cut the bottom chord of a truss. Your average consulting structural engineer won't carry out any design on strengthening existing trusses with additional timber and will leave that to the specialists (who it sounds like also don't want to get involved). If I was asked to come up with a design that allowed you to cut the bottom chord of a truss, it would probably involve quite a lot of steelwork.

If you didn't want to cut any of the trusses, you are unlikely to do any harm to them by strengthening part of the bottom chord as per Tony's diagram. As you suggest, you'll increase the stiffness of the central section which should help to reduce the deflection. Note that the bottom chords (and therefore the trusses themselves) will only have been designed to carry 25kg/m2 storage load, so even with strengthening, you shouldn't exceed this load, as it'll add load to the struts and top chords too. From what you've described though, it doesn't sound like you plan to use the loft for anything other than standard storage.

 

[Deleted member 174758]

Maybe I should point out that the original posting which Tony's drawings came from was made in 2012 and concerned strengthening a conventional Victorian joisted ceiling and NOT a Fink truss, so not a technique which was being put forward for the Fink truss scenario at all

 

[RonnyRaygun]

Maybe I should point out that the original posting which Tony's drawings came from was made in 2012 and concerned strengthening a conventional Victorian joisted ceiling and NOT a Fink truss, so not a technique which was being put forward for the Fink truss scenario at all

I understand that, but if the OP isn't intending to increase the loadings onto the truss, then he's not increasing the stresses on them, just increasing the depth of the central part of the bottom chord, which, so long as the new upper member is glued and screwed sufficiently, will help to reduce deflection in that portion of the truss.

I doubt a truss manufacturer would give anyone the go ahead to do it, but it's highly unlikely to negatively affect the performance of the truss.

 

[Buckie1967]

Thanks for responses.
What I have discovered is that whoever built the roof must have started at the wall (lower chord number 13) and worked towards the chimney stack (lower chord number 1).

What has resulted is the gap between chord 1 and 2 is not the normal 2ft span but reduces to around 18"!!! This is why the existing hatch is very narrow....if the width was a normal 2ft then I could fit a 550 wide ladder between these bottom chords numbers 1 & 2 without the need for any fettling.

I have clearly purchased loft ladders too wide for my loft as they are 700 wide. I intend to sell these, purchase a 550 wide set, and go into my bedroom and create a new opening which will most likely be between bottom chords 3 and 4. This will allow me to centre them precisely between the chords. I also intend to still build the middle section up using 2 x 4's as per Tony's diagram albeit that this is for reinforcing a traditional old style loft with actual supportive beams rather than bottom chords like what you get in a truss design.

These loft hatches are quite deep and I would need to attach them to something deeper than 3" anyway so adding the extra 4" gives me a better surround to attach the hatch too. For a nice finish I really want the top of the frame of the hatch to be flush with the flooring within the loft. Also, this will allow me to top up the roof insulation into the extra 4" that I would be creating.

We are having the bedroom ceiling skimmed and the landing ceiling skimmed next year (as they are currently artex'd with hairline cracks) and therefore all the mess created with repositioning this loft hatch can be resolved by my plasterer. I obviously don't want the cracks to come back and hopefully reducing the flex on the bottom chords by adding the 4 x 2's should imo help.