Building a flat roof with joists running across the slope

[public]

Hi all,

I have a block-built outbuilding, 3m x 5.5m, which has a flat roof with a 5 degree slope running down its length. The current roof is corrugated asbestos cement, supported on joists running across the shorter span, at about 1m centres. The joists are embedded in narrow slots in the inner face of the wall.

I want to replace the roof with a solid roof. Everything I've read so far details the construction of such a roof based upon the joists running down the length of the slope, rather than across the slope. Now for my building, having the joists running that way would mean I need

5.5m lengths of 3"x9". If I ran the joists the other way (as they are currently) I could use smaller joists (2"x6"), or greater centres, either of which makes the whole construction cheaper.

The difficulty I see with running the joists across the slope, rather than down its length, is securing them. I can't reuse the existing joist slots - they are too narrow, and widely spaced. Instead, I would have to position the joist on top of the walls (actually quite desirable, since this gives me more headroom inside the building). If I attached the joists directly to the top of the wall, then they would be off vertical by 5 degrees, so I would have to position blocks of timber to fit into the gaps between the joists to provide sideways support. Alternatively, I could run a timber bar along the top of the wall, and then cut vertical notches in that timber to support the joists in an upright position. I would then have to cut the top off each joist, to provide a 5 degree slope to which I could attach the plyboard deck.

Is building a roof with the joists running across the slope feasible, or should I conform to the practice of running the joists down the length of the slope?

thanks,

dan.

[longshot]

in steel construction the joists often run across the slope.

[Kickstart]

you can run across the slope, I'm interested in your measurements though, your obviously on the metric system but you refer to wood sizes in inches. Is this common practice through out the building industry in all metric orientated countries also ?

kickstart

[public]

I don't really think of 2x3 as meaning 2"x3" - its just a convenient nickname for a particular size. I learnt about these things from my Dad and other older members of my family, and they all use the `nicknames' to refer to the different sizes, so its easier when talking to them about 2x4, 2x3, and so on. I would always use the metric size for buying, using, measuring, etc.

Are there any references on the web about building with joists across the slope?

dan.

[Mike]

In Canada we use metric, however wood is sold via 2x4,2x3,2x6 etc and most construction workers still use inches and feet as this is what they were taught.

[marson]

there should be no problem with joists running across the slope. i would fasten a plate on top of the block with concrete anchors, toenail the joists to this, and run a row of blocking above the plate. if you are concerned about a tornado coming along and taking the roof off, you can add some sort of hurricane ties from the joist to the plate.

[Kickstart]

If your concerned about tornados you should have a tornado shelter, If one hits directly, anchors and tie downs wont help anyway. Use your span table for spacing and sizing

kickstart

[DT]

[DT]

Sorry about the blank reply :-)

I would run a header (what you call a timber bar) along the tops of each wall. This would normally be a doubled joist, if you are using 2 x 6 rafters, then the header would be (2) 2 x 8's. It is fully supported, so you could get away with 2 x 6's, but I like to use a larger size. If you want a higher ceiling, use a wider header, 2 x 10's or whatever.

Layout your rafters on 16" centers. Attach them to the header at each end with metal joist hangers set vertically, there is no need to notch anything. You can indeed bevel the top edge of each rafter to 5 degrees, this is a nice touch and the correct way to do it, although I suspect most people would just leave them and support the roof on the corner of each rafter. Bevel the rafters first, before you position the hangers and put things together.

Fasten your roof panels to the rafter being sure that all ends of the roof panels fall directly over the center of a rafter, so each panel is supported. Note that the first rafter spacing will have to be less than 16", so the roof panels will start at the very edge of the roof and end up on the center of a rafter.

-- Dennis

[DT]

One more thing - when I mentioned the rafters on 16" centers, that is because it is standard for construction in the US with 48" x 96" x 1/2" thick plywood or OSB sheets. If your plywood panels are metric lengths, obviously you need to set the rafter spacing to accomodate your panels. If your panels are thicker than 1/2" (13mm) then you may be able to use wider spacing on the rafters. Check with the building center where you buy your materials for customary spacing in your area.

-- Dennis