Do you know what to do to make sure your porch or deck is stable enough to resist frost heave and other damage due to freeze/thaw cycles? Here are some tips to make sure you build on a firm footing!
Preventing frost heave: it’s all a matter of depth
To protect a deck or porch built on posts or piles from ground freeze in winter, one determining factor is how far those supporting structures extend into the ground. In Quebec’s climate, that depth needs to be at least 1.5 m (5 ft).
The problem is, drilling post holes down that far isn’t easy—especially if you’re relying on the simplest of excavation techniques and you want the hole diameter to be fairly narrow.
The tools available have various limitations: a manual post hole digger or auger (which looks like a shovel with two handles) won’t allow you to go deeper than 1.06 m (42 in.), while the mechanical augers available for rent are usually equipped with a 1.2 m (4 ft) bit. So what solutions are available?
To drill a deeper hole, you have a few options:
- Call a contractor who has the right equipment (i.e., a longer bit);
- Resign yourself to having a bigger excavation job done, using a backhoe;
- Opt for screwable posts.
Concrete pillars, screwable posts… To make the right decision, you will need to consider some basic information.
Concrete pillars: how to maximize stability
Most porches and decks are built on concrete pillars, which are usually made using heavy cardboard formwork—tubes into which the concrete is poured.
It’s important, of course, to build pillars to the correct depth dictated by the local climate and your property’s soil conditions. But there are other details to pay attention to that can improve stability under frost conditions:
- Under each pillar, pour a concrete footing of around 60 cm x 60 cm (2 ft x 2 ft), into uncompacted soil; you can create the footing by building a classic wood form, or by attaching a specially designed plastic cone to the bottom of your cardboard formwork;
- If the area available for digging makes it impossible to install a footing, widen the diameter of the base of the hole when you dig it (“bell-bottom” excavation) and install the cardboard form tube so that its base is about 15 cm (6 in) above the bottom of the hole; this way, the concrete will fill the base when poured;
- Wrap the tube in several layers of thick polyethylene.
Here are a few more good practices for building stable, durable concrete pillars:
- Never pour concrete directly into a drilled hole; the resulting irregular shape will increase the risk of frost heave;
- Only use cardboard tubes with a diameter of 20 cm (8 in) or more;
- Make sure each pillar projects at least 20 cm (8 in) above the ground;
- Strengthen the pillars using reinforcing rods;
- Use concrete with compressive strength of 25 MPa (32 MPa if in a location exposed to chlorides) and 5 to 8% entrained air, which will improve resistance to freeze/thaw cycles; when mixing the concrete on-site, be sure to add exactly the right amount of water to the bag of mix as specified, so that you don’t affect the concrete’s performance.
Screwable foundation posts: several benefits
If you want a footing at a depth of at least 1.8 m (6 ft) that, in principle at least, requires no drilling, you’ll want to go with screwable-post technology.
Screwable foundation posts are particularly appreciated when time is a factor, because they are so much quicker to install. In addition, they will accept loads immediately after being sunk. Screwable posts are also the answer if it is important for you to limit damage to a part of your property that is already landscaped, or when there is no practical path to bring heavy machinery to your build site.
The posts are made of a galvanized steel shaft tipped by at least one helical blade, which first allows the post to be sunk, using a special piece of hydraulic equipment, and then acts to provide the required support once it is in the ground. The required post dimensions are determined by an engineer based on the bearing capacity of the soil and the load to be supported.
In rocky soil, however, it may be difficult to stick to the post spacing plan, or to make sure each post goes into the ground straight. If a post comes up against a large rock on the way down, it may even be necessary to drill the hole, or at the very least make a change (which may be major or minor) to your deck frame design.
Info-Excavation: free, practical advice
Whichever option you choose, though, don’t forget to start by calling the Info-Excavation underground network locating service. It’s free, and it could save you the costly bill that would come if you were to damage an underground line (e.g., natural gas, water, sewer) or cable (e.g., electricity, telecommunications). To reach Info-Excavation, call 514 286-9228 or 1 800 663-9228.
For their help in the writing of this article, we thank François Melançon, professional technologist, and Stephan Beaulieu of Patios et clôtures Beaulieu inc., both members of CAA-Quebec’s network of Approved Residential Suppliers.