Build­ing a per­gola that is safe and beau­ti­ful

The Palm Beach Post - Residences - - News - Tim Carter

Ques­tion: I need to build a per­gola to sep­a­rate a small pa­tio from my drive­way. I’m try­ing to cre­ate a pri­vacy screen and as much shade as pos­si­ble. The is­sue is I have only a tiny strip of ground to work with and the per­gola needs to be more like a fence than a ta­ble with four legs, if that makes sense.

An­swer: Let’s first talk about the struc­tural chal­lenges all per­go­las face. Yours is of par­tic­u­lar con­cern be­cause it’s go­ing to be a tall fence as you al­ready un­der­stand. Your pri­mary con­cern should be hor­i­zon­tal wind load. The to­tal weight of all the ma­te­ri­als used to build a typ­i­cal per­gola can be in the hun­dreds of pounds. You don’t want it fall­ing on some­one at a later date.

While you may not in­tend to have vines on the per­gola, some fu­ture home­owner may de­cide to grow flow­ers and fruit on the struc­ture. Not only does this veg­e­ta­tion add hun­dreds of pounds, but the dense growth also in­creases the wind load. Your safety and that of all who sit by or un­der the per­gola are sub­ject to how well you obey the laws of physics.

All lum­ber used needs to be strong enough to re­sist crack­ing or snap­ping un­der the force of the wind or snow loads. You live where wet heavy snow can coat the per­gola, so plan ac­cord­ingly. All the fas­ten­ers need to be pre­mium hot-dipped, gal­va­nized or stain­less steel.

If you de­cide to use modern treated lum­ber, the fas­ten­ers and all metal fram­ing con­nec­tors must be ap­proved for the newer treated lum­ber that has a higher cop­per con­tent. Fail­ure to do this will cause ad­vanced and rapid cor­ro­sion of the metal from gal­vanic re­ac­tion when the per­gola gets wet.

You’ve got to be very con­cerned about strong winds that can blow your per­gola over. A tra­di­tional per- gola that might have as few as four posts can blow over, but it’s harder to do if the four posts are bolted se­curely to con­crete piers.

You need to bury your posts much like util­ity poles are in­stalled. My guess is your per­gola is go­ing to be about 10 feet high once it’s all done, so I’d want to see the posts buried at least 4 feet into the ground.

I’d coat the wood that’s go­ing to be in the ground with a read­ily avail­able cop­per naph­then­ate so­lu­tion and sur­round the posts with an­gu­lar crushed gravel that’s the size of large grapes. This type of gravel in­ter­locks and acts much like con­crete, but it pro­vides for great drainage once the wa­ter leaves the soil in the spring.

It’s all about scale at the end of the day so you need to use big posts. You may get by with 4-by-6 posts, but, trust me, 6-by-6 posts would be bet­ter.

Think about can­tilever­ing the top of the per­gola a lit­tle bit. The hor­i­zon­tal rafters don’t have to be equal on each side of the main beams or the posts. I’d not ex­ceed a 3:1 ra­tio be­cause it will add a ro­ta­tional force that could cause the per­gola to tilt from the off­set weight of the over­hang.

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