What screws to use for pergola, do these plans look ok?

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Forum topic by Dw12345dw posted 09-27-2020 03:35 PM 641 views 0 times favorited 10 replies Add to Favorites Watch
View Dw12345dw's profile


4 posts in 32 days

09-27-2020 03:35 PM

Hi, I’m new here.

I am planning to build a pergola on my pavement had some questions. I am copying this exact design in the photo.

-sizing: 9 width, 18’ length, 10’ height
-using anchor base post to the cement slab

-using flush black joisters for the inside beams, see photo

-using 2×6’s for all of the top framing and inside beams

- would like to use 4×4 for 4 corner posts and 4×6 for two middle posts (18’ length top frame, would cut in half so would be 2×9 ft And connect them at the middle post)

some Questions:

1. Can I use screws to attach the 2×6’s to the posts? If so, how big and long and what type?

2. Can I get away using 4×4 posts? The 6×6 seem So much harder to deal with given their heavy weight, 4×4 will be much easier

3. will the the top outer frame be thick enough to hold weight of the inside beams?

4. What screws to use to attach the joists in the black bracket?

does this make any sense?

10 replies so far

View AndyJ1s's profile


462 posts in 638 days

#1 posted 09-27-2020 07:23 PM

It appears at least the outer posts (away from the house) are sunk in the ground. It also appears, from the shadows on the wall, that the structure is also leaning some (away from the house, and/or toward the camera). While sunk posts provide some rigidity against leaning, post anchors to a concrete slab do not. Therefore, you will need diagonal braces between the posts and the beams they support, in both directions, to keep the structure from swaying.

And it appears to be only about 8 feet high (to the underside of the beams), based on the doorway to the left in the photo.

What is the thickness of the slab on which you wish to build this pergola? A patio slab is probably not sufficient to support the posts for this structure; you may need to cut through the slab and poor footings for the post anchors, flush or proud of the slab. These footings need to be deep enough to penetrate the frost zone in your soil.

I would not consider reducing the size of the posts, particularly since you are raising it to 10 feet. I don’t think 4×4 posts would look right, in relation to the structure, either.

I would bolt the beams to the posts.

-- Andy - Arlington TX

View Madmark2's profile


1714 posts in 1471 days

#2 posted 09-27-2020 07:35 PM

6×6 are overkill, the post load is minimal.

Use 3” screws for attaching 2x lumber. The general rule of thumb is wx the material you’re attaching. 1x is 3/4 actual so twice 3/4 is 1-1/2” screws. 2x is twice that or 3”.

You want exterior coated screws so they don’t rust and weep stains down. Self countersinking is good too. Square or Txx drives are better than #2 Phil. Buy pre-lubed screws or use soap to lube before driving to reduce splits.

There should be a screw specified with the brackets, RTFM. Otherwise use the largest dia screw that will pass thru the bracket hole without catching. Use a screw length between 1/3 and 1/2 the post thickness.

-- The hump with the stump and the pump!

View Dw12345dw's profile


4 posts in 32 days

#3 posted 09-27-2020 07:45 PM

Thank you for your quick replies!!

Can you give me advice if you think the timberlok 4” screw (which claims it is a replacement of 3/8” lag screw) is OK to use for mounting the frame to the posts?
We ended up going with 4×6 posts

We starting using 3/8” lag bolts, 4.4” long but I really would like to use the black timberloks for the rest. Do you have experience with them? They are thinner than 3/8 and 4” long.

View Madmark2's profile


1714 posts in 1471 days

#4 posted 09-27-2020 07:49 PM

I would use bolts/nuts for that connection.

3-1/2” 3/8-16 hex bolt, washers both sides, lockwasher and nut. All galv.

-- The hump with the stump and the pump!

View Dw12345dw's profile


4 posts in 32 days

#5 posted 09-27-2020 07:53 PM

The 3/8” lag bolts I used in the photo above look ok? I should keep with them?
They do make a 3/8” ceramic lag bolt I’m not familiar with ceramic material. Think it’s ok?

If none of these sound good, can you suggest a good black bolt that would be available in the big box stores?

View PCDub's profile


222 posts in 1127 days

#6 posted 09-28-2020 03:37 AM

Timberlocks are pretty darn strong

View AlaskaGuy's profile


6111 posts in 3192 days

#7 posted 09-28-2020 05:17 AM

6×6 are overkill, the post load is minimal.

Maybe the 6×6 are cosmetic. Maybe he didn’t want the pergola to look like ii had birds legs.

-- Alaskan's for Global warming!

View JackDuren's profile


1339 posts in 1842 days

#8 posted 09-28-2020 03:01 PM

6×6 here is typical. I liked it, it meets code and supports any weight I apply…I dug 14”×36” holes for those post. I’m covered on codes everywhere in Missouri…

View Sark's profile (online now)


339 posts in 1243 days

#9 posted 09-28-2020 03:09 PM

It will look better with 6×6 posts. Structurally they aren’t needed. But 18’ long x 10’ high is a big structure for such little end posts. A simpler option is to use a 4×6 for the posts. Cheap and easy, and more substantial than the 6×6.

The longer I’ve worked in woodworking/construction the more I care about about the look of the finished product. You’ll be happier with the final result, I suspect, if it looks substantially anchored to the ground, and not flimsy. Also, consider putting elbow bracing on the posts to give the whole structure more rigidity. Recently I redid a large deck with a covering that had 4×4 posts on 8 foot centers, holding 4×8 beams. The beams and posts were tied together with beefy bolted T’s. But it still had a spindly look that didn’t quite appeal to me.

So I added elbow bracing on all the posts (not needed structurally) and voila, the new look is ever so much more appealing. As I look at your sample picture above, it seems rather plain (to me), though certainly functional and sturdy enough (assuming you’re not in earthquake country). Another way of describing my suggestion is to add some architectural interest to your project, and then everyone will oo and aw over your project. Angled supports off of posts have been used for thousands of years, so that’s an architectural element that creates a subliminal impression. It’s easy to do.

View Dw12345dw's profile


4 posts in 32 days

#10 posted 09-29-2020 11:55 AM

Thanks everyone for your input and help! We decided on using 4×6 as a compromise.
Bumping this up one more time as I need a suggestion for black bolts for the rafters going to the posts.

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