A properly mounted pergola will stay true to shape and safely secured for life. Making sure the posts don’t move over time will give you peace of mind that your investment will stand the test of time.
Even though a pergola is heavy and difficult to move once assembled, it has to be anchored securely to the ground for a number of reasons. Primary among them is safety. A poorly anchored pergola can be moved by high winds if there is a canopy or cover on it as this adds to the wind load the pergola could see in a storm or high winds. Secondary is to ensure long term stability to keep the pergolas shape. Wood over time can bend, bow and twist with the seasons and the frost. With proper anchoring, your pergola will keep its shape for many years.
The following is a review of the different surfaces pergolas mount to and the fastening options for each of them:
Mounting Pergolas to Deck Surfaces
A properly constructed deck provides ample strength to anchor a pergola. However there are a few points to make to ensure you get the most strength you can with a little more effort.
Any pergola post can be anchored to a wood surface with 90 degree strap brackets. There are a few additional considerations before screwing into the post and into the deck. First, be sure you are screwing into the deck’s beams, not the deck’s boards. If a beam is not located where you want a post, put a 2X6 flat under the deck boards such that it runs across a number of deck boards. Then screw through the deck boards into the 2X6 underneath. This spreads any lift loads over a number of boards instead of one. Using the deck posts/railings as anchors are recommended because they go right down to the footings or the sub structure. Please make sure that the screws used are long enough to reach the support beams underneath.
Mounting Pergolas to Poured Concrete Patio
Hammer set fasteners for concrete are easy to set and are very strong. It is important to ensure the poured concrete is of sufficient mass to be used with these fasteners. Most contractors like to see 6” of depth to use this method. If you are going to pour a concrete pad for your patio, make sure you specify 6” depth of pour. With that in place, anchoring is a simple matter of hammer drilling the holes for the fasteners, hammering the fasteners into place, and bolting down the mounting brackets for the pergola posts.
Mounting Pergolas to Paving Stone Patio
These are the most troublesome and frustrating surfaces to mount to. Although tempting to take an easy route, you should not fasten posts to paving stones. Paving stones do not provide sufficient anchorage for a pergola that will be subject to wind loads and the seasonal forces that will move the structure over time. You have to remove the stones where the posts are located and sink footings according to your local code. In the northeast, that is typically 48” to a concrete poured footing. There are other footing options we will discuss (helical screws, earthen screw, etc) but in any event, a footing is needed to mount properly to paving stone surfaces. Note that the image above represents the footing that should be used under the paving stones.
Mounting Pergolas to Ground ‘Foundation’ Anchoring
Previously, you would dig a hole for each post and pour a concrete footing at the bottom of each hole for each post to ‘set’ in. Once the concrete had set, you would build the pergola on the posts. Today, there are more efficient options to set footings such as helical piers or footings. If you are a do it yourselfer, the helical pier may be a bit difficult to set because equipment is needed to drill down to the 48” depth most require for code compliance. However, your contractor can screw these piles right into the ground with an auger type attachment on their backhoe. The only caution is if there is a potential for construction debris like old bricks and wood to be buried.
Mounting Pergolas to Ground ‘Location’ Anchoring
Similar to the helical pier but with much less strength is the earthen screw. If your pergola is sitting on ground and you want to be sure it does not blow away, these can do the job of securing to the posts into the ground. However, they will not be part of a proper foundation like a helical pier or poured concrete footing. The same goes for steel pole anchors. They provide lateral location for posts but do not prevent frost motions or anchor from negative wind (lift) loads. If your pergola has a canopy or cover on it, stick to the foundation type anchors.