Are 8x2s enough? Double up?

[pilsbury]

about to start my mono pitch roof over the garage. The pitch is 40 degrees and the roof will be about 6’ long plain tiled with 4x2 rafters.

The ridge beam will be just under 4m long supported at the ends into the existing brick walls. Will a single 8x2 c24 be enough or should I bolt 2 together?

 

[^woody^]

A ridge beam does not take any load in a mono pitched roof. Unless you're doing something whacky.

 

[pilsbury]

Something whacky? Me? With my considerable building experience?

Your answer has just jolted my mind into a more specific question. So the ridge beam takes no/little load. My assumption is that it is all on the wall plate. All my monopitches on my “over complicated” roof design sit nicely on wall plates except one. The one going over the front of the garage has no wall under it to sit a Wall plate on. I was going to bang an 8x2 timber into walls either side to act as a floating wall plate/joist/bearer (if that is such a term). Will an 8x2 be enough for that? 2 bolted? Span will be 3.2m, roof same size as my original question.

 

[^woody^]

You could bolt a piece of tile lath to the wall, as all that is required is a projection to rest the rafters on. But a 3x2 or 4x2 is more common. Whatever you have really.

For spanning a garage opening, two 8x2's bolted to form a beam would be normal. In your case 6x2's may work. The timbers need to be double not singles though.

 

[pilsbury]

Thanks for that. Out of interest, what forces are acting on my mono pitch ridge beams? If all/most of the downward force is being transferred to the wall plate, what is on the ridge beam? Outward force pushing backwards? Can’t get my head around it.... I’m surprised there is so little weight bearing on it.

 

[^woody^]

There is no ridge beam on a mono pitched roof. The wall takes the load from the top of the roof, and half the span, the other half of the span is loaded on the wall at the bottom of the rafters.

 

[pilsbury]

I might not be explaining myself properly or misunderstanding what you’re saying. Here is a photo of the rear part I’ve done. The front over the garage is the same, but without a wall below. Hence me doubling up the 8x2s to give a solid base/ fake wall plate. As you can see on the back I went full bore and bolted 8x2s together for the “ridge beam” that I set into walls either end.

 

[^woody^]

Thats not quite a monopitched roof, but it's clearer now.

What is holding the ridge beam up, as it surely can't be just the end bearings - ie is a stud wall going to be built off the joists?

 

[pilsbury]

Thats not quite a monopitched roof, but it's clearer now.

What is holding the ridge beam up, as it surely can't be just the end bearings - ie is a stud wall going to be built off the joists?

Sort of. There is going to be a stud wall at the back of the “mono pitch” mainly so I can fix some sort of cladding to close it off. Would these studs be adding support? You said surely it can’t just be the end bearings.... well it kind of was till you mentioned it apart from the stud work which I didn’t think would add structure to the beam.

Have you any suggestions in addition to my stud work back? Your help and thoughts are much appreciated as you may get the impression I might actually not know what I’m doing!

 

[^woody^]

Normally, you would build up a stud wall and this would be off double joists on the flat roof. I'd get some guidance off your building inspector to see if he will accept your method as it is, as I would expect any stud wall to be taking some load, so that would need to be dealt with.

BTW, the way those rafters meet that beam will likely mean that a mono-ridge tile wont cover the top row of tiles because the width at the top is too wide. You may need to make up something on that beam so that the tile batten is fixed further up and above that beam and closer to the back edge of the ridge. You need to allow for the cladding too which will effectively cause the mono-ridge tile to sit further back.

 

[pilsbury]

Good to hear your advice. Also on the tile battens. I think I’m going to end up using regular ridge tiles with flashing at the back. Can’t find monopitch ridge tiles that are old like my plain tiles. They’re plenty wide but I shall bear it in mind. Kind of feeling my way and making it up as I go along! Some might say a recipe for disaster but I recon I’m just over half way and so far no calamities have come to light.

Would you believe the building inspector came today to look at the roof. Said it was all fine - however he did ask what I’d be putting on the back of my monopitch to close it in. When I suggested plastic cladding he said some cement board behind to offer fire protection seeing as I’m within 6m of the boundary. Nothing about the ridge beam itself. But what his level of structural knowledge is I don’t know. He may be clueless working off a tick sheet.

 

[^woody^]

Generally, inspectors want things doubled up when walls are supported off floors, and in your case roofs.

But years back, walls were happily built not just of single joists but even just floorboards.

So if he has accepted it, then it may be due to having knowledge, rather than lack of it.

 

[^woody^]

Btw, that's looking quite a nice neat job.

 

[pilsbury]

Btw, that's looking quite a nice neat job.

Thank you.

With regards to you other comments and the building inspectors lack of comment on it, I might just double the joist underneath and stud off that. I know the tiles weigh a lot but everything feels rock solid. With the whole building process I can’t help but feel everything I’m doing is so over engineered

 

[^woody^]

Just put noggins every 400mm between the two joists under where the stud wall will be. That will share any load and be as effective as a double joist.

Your joints look really tight, so it will end up nice and sturdy.