Re-engineering trusses for attic storage.

[Big Pete]

Hi all, firstly, a brilliant site, hopefully I get a bit of time here!!! You seem a very helpful, knowledgeable bunch, so I would request a bit of advice. Heres my problem. I have a large garage (Floor dimensions are about 8m by 6m, which has web trusses exposed in the roof. These trusses are constructed from 90 x 35mm pine, and span about 5.9m from wall to wall, where they sit on 200 x 75mm bearers. They are about 2m high at the centre. I would like to put in a floor for additional storage space, probably yellow tongue sheets, but the trusses are in the way. I was thinking I could install collar ties accross the top, and vertical supports about 1.75m from the centre vertically down to the bottom chord, then remove all the webbing, leaving me an area of about 3.5 x 8m clear to floor. I would also install additional spans in between each truss, as the spacing between trusses is about 870mm. The roof is sheet, and the garage is about 25 years old. So, my questions, is this plan feasable?? Will what I propose reduce the structual integrity?? Or is there a better way?? Appreciate any comments, good or bad. Thanks.

[journeyman Mick]

Pete,
the only way to do this is to engage the services of an engineer to redesign your roof structure.

Mick

[jackruss]

Hi Big Pete,

Hey I've always wanted to do it, "Welcome to the Board!"

what you ask is basically impossible :mad: , with keeping what you have.

Short answer, is NO don't touch the trusses as it will not only "reduce the structual (sic) integrity" but probably remove it all together.

A truss by definition, is designed to take the vertical load, via the web and transfer it to the outside wall.

The rafters, at 90 x 35 are carrying a relatively small load down and out thru the bottom chord. I would also say you are at 600 centres for the truss. Which is right at the tolerance ( for a standard truss - yes I know its a generalisation)

If that roof was designed using stick construction, ( straight rafters with under purlins ( the bits running perpendicular to the rafters half/third way up), the rafters would be close to 200x50, with beefy under purlins carrying the load to a quite thick hanging beam( which is similar to the height of your current ceiling) You then frame the ceiling rafters, which is designed to take a floor.

Also the pitch of your roof is an issue too, at 2 metres head height in the centre I would say you are around the 22.5 deg roof pitch, which also means you have to beef up the rafter as opposed to a steeper roof pitch ( again a generalisation, but trying to give as much information as possible.)

No all that said, what do you want to put in the roof. Yellow tongue is VERY heavy, and knowing that the roof is not designed for a live load ( people walking around), if you want to use it for storage, I would put down the lightest base material you can find that wont overly bow, ( say 9mm MDF), and just use the area for light storage, say Christmas tree and lights, the gaps side ways between the trusses, would still be full height, and can fit most average boxes.

But again at the end of the day, trussed roofs are made to be the most efficient use of timber, the cheapest and easiest building technique, and definitely don't like being modified in any way. But you can sneak some extra load on top of the bottom chord for light storage, I wouldn't be storing a car engine up there that is for sure.

My $2.50

JR