People,

I'm currently in the process of putting some boards down in the loft to make it into some usable storage space. This process has been easy in the centre section but either end of the loft I have a problem.

As can be seen in the pics the 'floor' joists have been fitted with some diagonal beams that meet at the end of the centre joists. Laying the flooring here will be quite time consuming as a lot of pieces will have to be cut with the diagonal to fit alongside these beams.

http://img11.photobucket.com/albums/v35/simondean/house/loft1.jpg
http://img11.photobucket.com/albums/v35/simondean/house/loft2.jpg

Now I can't see what stresses are placed on the joists in this direction and why the beams have even been fitted. They're not particularly strong, and seeing as the house is only 15 years old the only reason I can think of for their existence is maybe that the whole roof was assembled on the ground before being lifted by crane and fitten in one go. These beams would then hold the joists in place and give the roof some 'square' to it as it was lifted into place.

Am I right, or are they actually an integral part of the loft structure? Would I be able to remove them to make my flooring easier to fit?

Any advice very appreciated

Thanks,

Simon.

When loads are placed on a horizontal beam, the beam will obviousley bend in the direction of the load, another effect is the beam will deflect and twist. I would say those timbers are in place to eliminate any twist in the joists, and hopefully any bending, its one of those structural tie it all together jobs.

If I were you, I would not fix loft boarding directly to the top side of the joists, as it can be tricky with wiring - you would have to notch the top of the joists to take the upstairs lighting cables, potentially weakening the joists..

When I boarded my loft I spanned all the joists with 1"x2" timbers, these timbers need to be approx 12" apart, is also best to double up where the top boards will meet, in the end its worth it as you can set the centres for fixing the boards down, this makes the whole job easier.. You can get a 3.6 metre lenth of 1x2 tannelisd timber for about £2.50.

You will probly find your loft hatch will only allow you to get 2ft with of board in anyway, so the 1x 2 have to be fixed to suit., also best to double up the 1x2 where boards will meet.

If you span the joists with 1X2 you will be able to remove the diagonal braces that are in the way, as the 1x2 will do the same job.

If you can afford it uses 2x2 instead, better still...

see below, hope this helps.

http://www.maj.clara.net/upload/roof.jpg

:D

Thanks for the advice and thorough reply mj. Will review it all again tonight with a view to doing what you suggest. Have a virtual beer on me ;)

Simon.

[slurp]....[burp]...ahhhh, cheers. :)

watch out as most of the time the roof joists are not load baring so will result in cracks or the ceiling coming down :eek:

whats on the underside of the joists?

looks like plywood from here, never seen that before.

My guess is they were there to stop teh roof flexing when it was put on, but without a closer look I'd hesitate to reccomend you remove them.

Instead, why not get a load of sawn timber the same thickness as the diagonals, pack the remaining joists up with them, and lay over the lot?

putting bearers over the whole lot is a nasty job. Avoid it by using 3/4" plywood for the flooring instead of floorboards.

I looked into doing this too - was advised here to put 'H' brackets on the existing joists and put in more substantial load carrying joists in. The cracked ceilings in the bedrooms kinda back this theory up - and you see the new cracks (just from me messing about up there) through the wallpaper!

D

without the bracing the roof would collapse and thats what happened originally when these pre-assembled roof sections came about. They just put them up with no extra bracing and in high winds the roof collapsed.

Cliff

1. If the diagonal bracing is removed and replaced by fully screwed down flooring, then part of the flooring is doing the job of the diagonal bracing. Wouldn't work if the flooring was laid but not fixed to the joists. You still have diagonal bracing to the pitched and 'vertical' members of the roof truss, so it will be reasonably stable, just don't do the job on a windy day:( Normal disclaimers apply.

2. TBH you will not get much usable space from the area under the eaves in the photograph. Why not just use this for storing empty cardboard boxes and the like, which have little weight?

3. <WARNING> Do not board out right to the eaves and then seal the connection. You need to leave a gap to allow circulation of air through the loft space.

good advice speedking .

I would also add that these TRADA roof trusses are not designed to take much weight for any period of time they are categorically NOT joists as they perform a totally different job to joists.

this is a common mistake:-
those things you call "joists" are working mainly to tie the roof together and stop the rafters "spreading" apart.
I had a customer in a bungalow once complain about cracks in her walls and when i went to look there were several plaster cracks like sagging lines at the top of her non loadbearing partition walls it turns out she had loaded the loft with stacks of books in boxes and the bottom of the truss was resting on the non load bearing wall and causing the cracks.

Told her to shift the books double quick.

so the "joists" are not joists ???, I assume that joists are only between floors and not between loft and upper floor. Are they not as strong as joists ????

Cliff

in a nutshell VTR yes.
joists are designed to hold a specific load at the centre of the span and as such are much thicker and taller.

The bottom of the truss forms a triangle of forces to convert the roof load on to the outside walls.
think of the bottom of the truss more as a kind of tie beam it will support a man standing on them but if you load them too much at the centre they will deflect and distort and evetually break this is one of the big problems for modern houses when you want to do a loft conversion.

actually cliff the problems with early TRADA roof trusses wasnt just lack of bracing it was lack of ventilation and the resultant rotting of new green timbers but the amount of bracing and ventilation now required by the NHBC is getting a little excessive.

So bottom of loft are called trusses and between floors are joists, right?

so how thick are joists and there must be a weight limit on them.

This is quite interesting

Cliff

I take it you have project in mind then Cliff LOL.

The truss is the whole W shape in a triangle unit forming the rafters which join at the apex and the "bottom" of the truss is what you might call the joist of the roof these are spaced usually a 400 mm centres.

ordinary timber joists span up to i think 5.2m and range from inch and a half thick to 3 inch and usually 9 inch deep depending on the span ( dont quote me as i refer to tables usually or specify a 9" by 3" as this does for most spans).

edited when i could be arsed to look at nhbc tables for joists

[Edited by mattstant - 1/13/2004 4:01:21 PM]

It is a bit dangerous to call them joists. The bottom member is a tie. Only designed to resist a horizontal tension or 'pulling' force, and the weight of the ceiling/insulation.

A joist is designed to support a load.

Most materials are strong in tension. The tie could be replaced by a thin wire, but the load carrying capacity in the vertical direction would be negligible.

As most roofs contain water tanks that constitute quite large vertical loads the joists are designed to carry those as well as the tension. Or the joists are at closer centres, or the tank load is spread by a supporting frame.

The capacity of the ties to carry load is depebndent on the cross section, the material, the magnitude, type and posisition of the loads, and the support and fixing details. A complex topic.

I Take it you havent read ALL my posts then Speedking

as i said after youre last post
they are categorically NOT joists as they perform a totally different job to joists.


[Edited by mattstant - 1/13/2004 4:47:07 PM]

what is the correct name for them then ????

Hmmmm bit embarrassing this as they are commonly refered to as "Ceiling joists".
Or more properly "Tie beams" within the truss itself which i think structurally is more appropriate.

Fascinating insight chaps. I see what you're getting at now.

To get back to the original question, I've left the diagonal beams in place, but put 1" x 2" beams spanning across the ties in the places where I can, then I've put the boarding on top of that.

I understand what has been said about it not supporting a lot of weight, but it should be able to take my scalextric layout under there. ;)

Thanks for the info people.

Simon.

Does the truss refer to the W-shape member, tie beam and rafter ??. I assume the rafter is the wood that has the felt/tiles on it

Cliff

by george i think he's got it :D:D

Thats it exactly Cliff my 3 years HNC weren't wasted then :D:D

SD you could have left the bracing out entirely if you are replacing it with chipboard as the chipboard would be doing a better job of bracing than the 2*1

oh and scalextric should be fine just look at what youre water tank is standing on ????.

My example of the books in the loft on the previous page was a litle extreme being a bungalow it had a very wide span making it more susceptible to point loads at the centres.
And the lady in question was a lecturer with a huge collection of books pile right up to the rafters.

[Edited by mattstant - 1/14/2004 10:45:17 AM]

By the way just taken on board the point MJ made about wires DO NOT drill holes in ANY of the truss members to feed cables through you can drop the wires dwon to run along side the ceiling joists but knowing what a mess sparkys leave behind (only jesting MJ :D:D)the wire could go everywhere and you could end up rewiring your loft.
I first thought MJ's solution was OTT but to avoid re routing wires it does make some sense just depends how much wiring you have up there.

cheers matt - You sound like you know your onions in all things loft related - I just found fixing battens at the right centres makes the job 10 times easier when it comes to fixing the boards down - its an added bonus that the lighting wiring can still sit on top of the "joists" with no additional holes or notches etc.

:)

No probs MJ i have been designing and building houses for nearly 20 years now so along the way i have picked up the odd tip or two:D.
I am supposed to be designing three blocks of 10 flats as i write.

Just to clarify - My water tank sits on a platform which is on beams that rest on the 'top' of the front and back bedroom walls (if you see what I mean), and doesn't sit on the 'floor' of the loft at all, so the ties/joists/whatever ;) don't need to hold it up.

My wiring up there is very simple so I've not got that to worry about.

I understand about taking of those tie bars from the floor, but was worried as the boarding is only 4' by 1' so in theory they wouldn't hold it all together too well as they wouldn't span the same distance as the tie bars. Wouldn't gaps just open up between the boarding?

I'd love to have the money to have a house built to my specification to be honest - I wouldn't have any of this money saving stuff, I tell you. :)

Simon

SD It is quite likley that if you removed the offending bracing there would be no ill effects to your roof.

BUT due to the way things are today in terms of litigation indemnities and ever increasing restrictions on what you can and cant do to your own property I cant recommend you do.

What makes me laugh is that there are probably millions of houses only a few years older than yours with negligble amounts of bracing
that will still be standing for years to come.
often extra bracings are insisted on by Building/NHBC inspectors In new housing due to an isolated case of failure where they could have helped prevent it.

Or in other words some numptie who didnt do the job properly costs everyone else money as well.
I still maintain that the boarding will do a better job than the original bracing as you are covering much more of the trusses and fixing to the trusses in far more locations.
A quick word to your local building inspector and even a visit may cost you nothing but peace of mind