Off topic-ish: workshop roof strength??

Shady · 6 Jan 2005

OK folks, I'm in the process of re-modelling the workshop ( a double garage with those w-shaped pre-formed rafter/truss combinations supporting the pitched roof).

The rafters are not the '2 by 4's' of my memory as a kid in dad's loft, but the house/garage is a new build, and they're (I think) something like 1.5" by 3". Question: do you reckon I can put some sheet chipboard on and walk around up there, and load some wood for storage, or not?

I've a belief, from a chat with a structural engineer, that these things are required to be able to support a 1 metre by 1 metre water tank (as the same ones in my house loft do) by law... Given that this equates, when full, to a metric ton of weight on a square metre of area, I reckon I'm ok - but I'm not doing anything until I've confirmed... :roll: My concern is the unsupported span in the middle: the 'w' construction means that there's support at about 1.5 m from each wall, with a gap of maybe 3 m between these two support points.

Rather than paying for advice, I thought I'd see what the board thought first... Any advice/comments gratefully received. (Unscientifically, it can support my 16 stone in a pull up at the centre without flexing appreciably...)

Anonymous · 6 Jan 2005

hi shady,
i am not in the know, but i think a water tank aint going to be moving up and down all the time .16 stone plus storage its alot 2 be putting on a structure which is only designed 2 hold up a roof. have u thought about a
partiton wall beneath or a false ceiling with 16" centres

davidc1075 · 6 Jan 2005

Truss rafters are designed to carry the direct load of the weight of the roof covering(imposed load). The size of the timber used is calculated according to the span using timber of strength class SC3, SC4. The timber used in your garage should be stamped with the stress grading. Building regs have tables which govern the size of the timbers to be used and Trada also publish the span tables. If I remember correctly from college (20 years ago) the roof design should also allow sufficient strength for snow loading. However the truss roof is not designed to carry an imposed load on the ceiling joist. If you look at how a water tank is supported in a modern house with a truss roof it is normally supported on a separate frame normally 6 or 8 x2 joists. This is why, when a loft is converted the ceiling joists which then become the floor joists in the conversion are doubled up with thicker flooring joists.

You could put flooring in the roof space just consider the load that you will put on this. We all board out our lofts and put all our junk up there or is it just me?

What you could do is add further support by adding more substantial joists and for a 3 metre unsupported span Trada recommend 47x147mm joists @400 centres C16 strenght timber for 1.5kN/m2 imposed load. A row of strutting(noggings) must be included for spans over 2.5m. If you need any further technical details let me know as I have the most recent span tables and building regs 11th edition for use when I need to drop off to sleep quickly. Dave

Anonymous · 6 Jan 2005

give that man a cigar

Anonymous · 6 Jan 2005

I saw a program recently with Tommmy Walsh ( I know!) and he used 7" joists to supplement the 4" joists in a loft during a conversion

DaveL · 6 Jan 2005

This aways makes me smile.

We had a loft conversion on a bungalow and they put 7" or 8" joists in between the 4"ones that held the ceiling for the floor upstairs.
We sold that place and bought a house.
On lifting a floor board upstairs I found 4"x2" joists that had the floor on the top and the ceiling on the bottom.