Gazebo Rafter Span 2

Yrok said:

Roof sheathing will likely be 1/2" OSB (unless something else is suggested)

Click to expand...

I'd suggest 5/8" thick at a minimum. Also, if the OSB will be visible from the bottom, that's probably not very appealing aesthetically (not that that's a structural concern).

Yrok said:

I live outside of Huntsville, AL so snow isn't really a concern (4 inches is basically a blizzard), but we do get strong winds occasionally.

Click to expand...

In that case, the controlling loads are likely either roof live load or wind load, as calculated from ASCE 7.

Yrok said:

I currently have the rafters modeled as 4X6s which is my primary question.

Click to expand...

Yrok said:

Are 4X6s or 2X6s sufficient for the rafters? Would 2X8s be better?

Click to expand...

Once roof loads have been calculated, wood framing would be designed per the NDS code. There are lots of ways to accomplish a code compliant structure using different member sizes, wood species, grades, spacings, etc. For something exposed like this there are often aesthetic considerations in addition to the typical strength/serviceability checks.

Yrok said:

Any other design thoughts/suggestions?

Click to expand...

I don't see how there's any lateral stability in the direction to/away from the house. Lacking that, this seems like a rather dangerous design.

Definitely hire a local engineer to help with this. This is a big, freestanding wind sail that you don't want collapsing on people.

Eng16080 said:

I'd suggest 5/8" thick at a minimum. Also, if the OSB will be visible from the bottom, that's probably not very appealing aesthetically (not that that's a structural concern).

Click to expand...

Thanks for the suggestion, I was already considering using thicker sheathing.I also intend to cover the rafters on the underside with thin planking to improve the look

Eng16080 said:

In that case, the controlling loads are likely either roof live load or wind load, as calculated from ASCE 7.



Once roof loads have been calculated, wood framing would be designed per the NDS code. There are lots of ways to accomplish a code compliant structure using different member sizes, wood species, grades, spacings, etc. For something exposed like this there are often aesthetic considerations in addition to the typical strength/serviceability checks.


I don't see how there's any lateral stability in the direction to/away from the house. Lacking that, this seems like a rather dangerous design.

Click to expand...

The beams i suggested for the sides in my original post would remedy this, correct? I thought the rafters and roofing would provide sufficient stability but it sounds like that was a bad assumption.

Eng16080 said:

Definitely hire a local engineer to help with this. This is a big, freestanding wind sail that you don't want collapsing on people.

Click to expand...

Yrok said:

The beams i suggested for the sides in my original post would remedy this, correct? I thought the rafters and roofing would provide sufficient stability but it sounds like that was a bad assumption.

Click to expand...

I don't think so. I would suggest adding knee braces in the front/back direction. Currently they're only shown left/right.

I would suggest HIRING a structural engineer to help you.
This site isn't for homeowners getting free engineering services online.

Fasten a plate to the house framing and then put in whatever decorative braces you want.

JAE said:

I would suggest HIRING a structural engineer to help you.
This site isn't for homeowners getting free engineering services online.

Click to expand...

The website is called Eng-Tips. How does asking for an opinion on design not fit that? I didn't ask anyone to do any math or make any guarantees, just am I missing anything since structural stuff isn't my background. Tired of going into every post on this site and seeing this same comment in response to every question.

Tomfh said:

Fasten a plate to the house framing and then put in whatever decorative braces you want.

Click to expand...

Thanks for the suggestion but the goal was to avoid any attachment to the house. We have a pain in the neck HOA that likes to throw their weight around. I try to stay off their radar as much as possible.

Yrok said:

The website is called Eng-Tips. How does asking for an opinion on design not fit that?

Click to expand...

Advice to other practitioners is free; opinions to lay persons bear risk and therefore cost money.

Yrok said:

Tired of going into every post on this site and seeing this same comment in response to every question.

Click to expand...

I suspect there’s a very good reason for that. Engineers should command a fee for their services, wouldn’t you agree?

Yrok said:

The website is called Eng-Tips. How does asking for an opinion on design not fit that? I didn't ask anyone to do any math or make any guarantees, just am I missing anything since structural stuff isn't my background. Tired of going into every post on this site and seeing this same comment in response to every question.

Click to expand...

It's called Eng-Tips not Do-All-The-Engineering-For-You. If you had some questions about how to determine wind loading or how to make a particular connection that would be worth discussing.

You seemingly don't even know how to design a simple span rafter, this shows that you haven't engineered anything on this project you've just drawn a pretty picture in Sketchup. Hire an engineer.

Agree with the others.

The site has always been for engineers to assist other engineers.

For something as rustic and stick built like this, you're better off talking to a few local gardening structure builders.

Something this close to your house, but not attached to it is risking damage to your property or windows when if falls down in a gale in 10 years time.

Stiffness of a beam is basically BH^3. So depth is far more important than width.
Check minimum slope for your roof covering
Screw everything down.

I agree this is a bit non engineering so prob best to ask to move it to the "hobbies" forum.

Harbringer80 said:

It's called Eng-Tips not Do-All-The-Engineering-For-You. If you had some questions about how to determine wind loading or how to make a particular connection that would be worth discussing.

You seemingly don't even know how to design a simple span rafter, this shows that you haven't engineered anything on this project you've just drawn a pretty picture in Sketchup. Hire an engineer.

Click to expand...

I did a lot of engineering and research to come up with what is here working within constraints based on the location. For instance, I calculated the dead load of the roof based on the weight of materials used (even taking into account wood species), estimated the live load based on my area, and even calculated the snow load though it's not really a concern where I live. I took this information and used a beam deflection calculator to estimate the deflection of the rafters. I read up on rafter spacing and based my design on products that can be purchased commercially. I also had to keep the posts from blocking the windows while trying to maximize the amount of space covered by the roof. However, my background is not structural/construction, so I was simply asking for a sanity check from people with a more appropriate background. I would have preferred to just buy something commercially, but our HOA requires the structure to be made of wood and the roof to be shingles which really narrowed the available selections with a lot of what is available being cost prohibitive. As far as the "pretty picture", I do not have access to CAD software and used Sketchup as it was free and easy enough to figure out on my own.

Yrok said:

The website is called Eng-Tips. How does asking for an opinion on design not fit that? I didn't ask anyone to do any math or make any guarantees, just am I missing anything since structural stuff isn't my background. Tired of going into every post on this site and seeing this same comment in response to every question.

Click to expand...

In my post above, the intention was to provide you with basic design feedback which should hopefully be of some value. Yes, you should definitely hire a structural engineer as we've all said, and when you do, at least you'll have a better idea of some of the things that might come up in discussions with them. Again, the main issue I see here is a lack of lateral bracing in the front/back direction. An alternative approach, as mentioned by Tomfh is to connect to the house, which will solve some of the lateral stability issues while also creating some new challenges. I understand that that solution doesn't suit you though.

The structural engineering of even a simple structure like this is not simple, and the risk in giving advice to non-structural engineers is that there will be a ton of things that they don't know which will likely be overlooked. On this forum just this week is another discussion about the difficulties in designing a simple knee brace connection like you show in your sketch. This stuff is really hard to do right. I wouldn't take the suggestions to hire an engineer as malicious. And aside from that, there's still plenty of meaningful feedback here.

Sometimes it's faster to design an entire house than a "simple" gazebo.

Eng16080 said:

the risk in giving advice to non-structural engineers is that there will be a ton of things that they don't know which will likely be overlooked

Click to expand...

I’d like to expound on this by sharing that some state boards explicitly crack down on what could be construed as “teaching” structural engineering to lay persons. We had that issue in Texas, specifically. Their reasoning: it’s dangerous to give someone only 2 of the 2,000,000 tools they need to design safe structures. We had to cut some ASCE-specific content out of our seminars (which were targeted to lay persons, not other engineers).

JAE said:

The site has always been for engineers to assist other engineers.

Click to expand...

Well I'll bite then. For the left/right direction, is anyone else applying a notional load or subjective 500# or so lateral load in these cases? Because the wind load on less than 12 square feet does not add up to anything meaningful.

A simple google search for "wood gazebo kits" yields many results. I think your best bet is to purchase one of those (after confirming local loading / permitting requirements) and install per manufacturer's instructions.

OP is clearly downvoting people for saying this is not a free engineering website

The bit that would concern me is that lack of strength in the front to back direction and the reliance on the moment capacity of your posts at the base. Treated pine isn't going to last ten years when buried in concrete. Back to front you also have a continuous load from the roof weight. I would look at making it a ridge to equalise forces. Even then there is a force unless you tie the front and back together.