Reinforcing a flat garage roof

Factual Questions
1

OK, time to pick some brains.

I’ve got a flat garage roof, roughly 24x24 feet. It’s mostly made of ceiling joists running the same direction but not completely so. The thing has obviously been expanded at least once and not very expertly.

It sags a bit. Water pools on it. Three years ago we had a new torchdown layer put on so at least it’s water tight. However, being as I live in Denver, I’m worried that one day my roof will collapse under snow weight.

I’d like to add a couple beams at, roughly, the 8’ and 16’ points with some jack posts on either side and near the middle (actually, somewhere between 30 and 40% across the span. Then I could tap in some cedar shakes as shims where necessary and, at least, feel better about the snow.

Is there a good way to know just how much beam I need? Tripled 2x10’s? 2x8’s? Do I need something from the engineered beam list?

I could just go with 2x12’s, I supposed, the largest I can get from my local big box lumberyard but they’re heavy and I’d rather not use them unless I have to.

FWIW, code for snow load in Denver is 30".

Any thoughts?

TIA!

2

This diagram may help.

https://lh5.googleusercontent.com/-2y2KV6WRIpw/UlbKDVpAmhI/AAAAAAABXEI/lyoW1-ofAo4/w725-h732-no/Slide+10102013+93720+AM.bmp.jpg

3

Is that the current configuration? Making a diagram of what you’re proposing might help.

4

Yeah, that’s current.

Minimally I want to replace that marked beam. It’s too thin, doesn’t attach properly at the ends nor middle.

I guess I’m looking for advice on what to replace it with and if that should be sufficient or if I should consider more bracing.

I’m hampered, of course, by the spot the car has to go in. I can’t drop a post halfway between that first tripled 2x6 and the right-side wall.

5

I’m not sure we have ample info. You have stated the size of the support beams but not the size of the joists they support. If for example, some of the joists are undersized 2X4’s that appear to be spaced 24" on center (OC), I’m uncertain if replacing the support support beam would solve your issues or concerns. Ideally, the joists should be 16" OC and probably 2X6’s since they appear to span about 16’ at the top and bottom of your diagram.

If the joists are not undersized and spaced correctly, I understand your concern about trying to avoid working with heavy 2X12’s but the engineered wood you mentioned (such as laminated veneer lumber [LVL]) is also heavy as hell.

Here’s a thought. What about increasing the size of your current 2X6 tripled post so that it can carry the weight of the of your vertical and horizontal beams on the diagram? Then I would replace both beams with a minimum of triple 2X8 or 2X10 beams. It looks like the longest open span for both the vertical and horizontal beams (based on your diagram) is about 16’ and that should carry the weight of the roof. However, when it comes to snow load, I have no idea how much 30" of snow weighs.

You may wish to have a contractor have a look for a no obligation estimate and get some additional ideas from that individual.

6

Here is a PDF with LVL load tables. An LVL beam is going to be heavier than a dimensional lumber beam, but way, way stronger.

LVL’s are pretty cheap for their effectiveness, I really would not bother with 2x12’s. Simply replacing the existing beam with LVL’s of similar dimensions is going to significantly strengthen the roof.

Most suppliers are going to have the software to figure it out if you bring them the drawing. You aren’t asking for a roof plan or engineer’s letter so I don’t see a problem with that.

You’ll need a framing nailer and compressor.

7

Just run a steel I-Beam underneath and perpendicular to the existing joists and be done with it.

8

And please don’t get a free estimate with no intention of hiring a contractor. We work hard for our money and you are paying for experience. Guys work long days, go home and spend their evening doing an estimate, one out of ten of which turns into a paying contract.

9

This is a quick and economical solution. Also consider putting a sloped roof on. It will cost much more, but probably worth it in the long run. It can go up very quickly using pre-made trusses and it will give you some extra storage space, and no more worries about leaks and collapsing under a snow load. In my experience here in the Northeast where there’s plenty of snow and rain I’ve never seen a flat roof in residential construction that didn’t leak, sag, and eventually get replaced with a traditional roof.

10

Sorry for the hijack, but I have a quick question about LVLs. Can you drill 3/4" holes in them to run wiring (Romex)? I know it’s not a problem with standard 2X12s.

11

Some info on that subject. Holes of 1" diameter or less shouldn’t be a problem when properly located.

12

I second the sloped roof option. Some of the houses in my neighborhood were built with flat roofs with hot tar & gravel. Seems like they are constantly getting them worked on. Several have installed a sloped roof with shingles for a permanent fix.

13

Least work and best cost/benefit = steel. Build a concrete block support at either end. Best long term solution is to fit a pitched roof. This will also have the bonus of storage space in the loft.

14

I have and estimate for a new roof but the $10,000 cost is a bit out of reach right now.

I need to invest $6000 in a new sewer line first. I’m looking for a solution that makes me feel better about the flat roof for the next couple years. Long term the whole roof will probably be replaced.

Thanks for the LVL table, FluffyBob. I’ll call around a bit on a steel beam, too.

Masonry post? Anything wrong with using a screw-type iron jack post?

15

Jack Posts offer good support. Especially if there’s a specific point that is starting to sag. They are often temporary fixes. But its not unusual for them to be left in place for a long time time. Even boxed in with wood to be more decorative.

The only downside is the support relies entirely on the concrete. If its thin then the post might punch through. But that would require a lot of weight pressing down. Like a two or three story house. A garage roof shouldn’t be a problem at all.

16

put a steel plate under the jack.

Is the jack post in the diagram permanent? Put another one next to it under the beam that’s tacked on. Or move the jack and apply a spanning cap that covers the area where it all joins together. It doesn’t appear by the diagram that there is a great deal of utility gained by having an open space with the extension.

17

The post marked in the diagram is a tripled 2x6 post, it takes the weight of the tripled 2x6 beam to the ground.

My thought is to replace the marked partial beam that’s in place now, it’s just two 2x8’s with one of the two nailed into the other beam’s end. I want to replace it with a tripled 2x12 or 2x10 beam.

16 foot boards will take the joint to the same place as the other post now where I can place a new jack post alongside the first.

Jack posts at the wall will carry the other side of the beam.

A couple long bolts through the 2x12 beam into the end of the 2x6 beam will keep the beam from twisting.

I agree with the steel plate idea but I have no clue where to get them.

Oh, and yeah, the post can be permanent if possible. That way I can adjust at a future date.

18

If the post is permanent then you don’t need anything other than a plate and a big piece of wood on top that spans what you’ve got. You could build that up with 4x4’s. That’s assuming everything hanging on the walls are well secured. You can always reinforce those connecting points if needed. A steel plate can be purchased at any place that sells industrial metal. Such places usually have a whole section of scrap pieces to choose from. A lumber store might even sell plates specifically for jack posts.

19

Building code in the US (at least used to be - gawd, I hate being so old I have to precede simply everything with that) available in a little red book - it even had a fly sheet with the space to write in the name of your county, the appropriate signatures and seals, reading "Adopted this (date) as building code for ___________.

In it, you find simply everything about snow loads, type of lumber required (note: probably NOT HD crap for anything structural - they don’t even dry anything larger that 2x4, and green lumber twists, warps, and splits), and max spans without support.

I also tells you how far you can move a drain without installing a new vent, what guage of wire to use for when you decide to run a wire out there, etc.

Find a copy - it will pay for itself.
And go a long way toward satisfying the building inspector when your pissed-off neighbor rats you out.

Look under yellow pages for “steel supply” = they deal in 1" plate as their bread and butter, and can cut you off a couple of chunks.

Please - bolts do NOT stop green lumber from twisting and splitting - esp in a place like Denver.

Get the book, read thoroughly, and start thinking “engineered beams” - wooden I-beams with solid wood caps and OSB web (a quick study in physics will tell you why it is 300% better than solid lumber, esp junk lumber from a big box.

Yes, they cost money - not nearly as much as this bizarre patchwork you seem to be contemplating.

OK - a freebie - a beam resists bending because of the distance between the top part (which must compress) and the bottom (which must stretch). As long as those two bit a quality material, the stuff that keeps them apart is irrelevant.
This is also why, when drilling a load-bearing beam, you alwaya drill through the center - the structurally neutral point.