Leveling a patio base is obviously an important step in the process of any paver project. Installing a smooth surface for your pavers to lay on is crucial to the overall quality of the final install. This is also why a bedding layer of a finer stone than the base material is installed at a depth of approximately one inch. This is screeded level with a pitch away from any foundation or towards a drainage area where the pavers are installed on top of.
This step of the process can make or break your project. This step comes after the excavation and base preparation step of the process of installing pavers. When it comes to screeding at the proper height, you may want to revisit how to set up a string line for your paver project.
Screeding Pavers: How to Level Ground for Paving
Screeding is the process of dragging back aggregate level to provide a surface to be able to lay pavers. This usually involves screed bars and a level, along with the aggregate used to level. This is how to level pavers for your project and is referred to as the bedding layer in your base.
Screed Rails for Pavers
Screed bars or screed rails are what we use to be able to screed our bedding layer. We use a 3/4″ inner diameter steel black pipe, the same used for oil and gas fittings. These typically come in 10 foot lengths at any hardware store and we have them cut to various sizes. The longer the better, but you will also need some shorter ones for projects like retaining walls or other applications.
These screeding bars follow the slope set by our string line. The string line is set in the initial excavation process to be able to measure down from and ensure that our excavation is at the proper depth throughout the project. This string line will likely need to be reset during your screeding step, but should still be at the same height as it was before if it represented top of paver. To re-establish this string line, you will start at the foundation and level a string line across the foundation from spike to spike to the level that will replicate the level of the pavers. This would have already been accounted for when preparing for your project, but ultimately you need to consider the threshold of your door and the steps (if any) down to top of pavement to be able to establish this string line level.
From there, you can then set your slope to the spikes at the end furthest from the foundation to replicate top of pavement. If you need further help with setting up a string line, click here.
The screed layer refers to the bedding layer. For pavers, this is one of two aggregates:
The screed sand is a concrete sand or ASTM C33. This is used in a traditional base preparation on top of a Granular A material which is a 3/4″ angular crushed stone down to fines. The sand is screeded on top of this base material after it is compacted. The drawback to using concrete sand is that it is difficult to work with when it is raining. It will washout or become uneven with the rain drops and have to be re-leveled. Additionally, weeds can take root much easier in the concrete sand or insects like ants will make use of it. This is the ICPI recommended bedding material because when the pavers are compacted, the concrete sand pushes into the joints from underneath creating interlock of the pavers.
Alternative to concrete sand, a 1/4″ or 3/8″ (ASTM #8 or #9), sometimes called High Performance Bedding (HPB) can be used. This has no fines in it allowing the free flow of water through it with no resistance. We have used it on top of Granular A material much like concrete sand is, but it is optimal for open graded bases and synthetic bases because of its optimal drainage characteristics. It can also be used during rain days. The angular shape of this aggregate prevents wash out. In addition to this, it is a little more difficult for weeds and insects to take hold in this material. This is the aggregate of choice in our installations for the screed layer.
Screed Level: Leveling Pavers
As mentioned, you need to set your paver screed bars to match the slope of your string line which represents top of paver. We start in one corner of the project against the foundation and work our way across the foundation and down the patio to the end of the slope. The screed bar is set below the string line at the measurement of the height of the paver you are installing you can use a tape measure to set this or the paver itself to get the screed bar to the height you need it at. For example, if your paver is 2″ then the top of the screed bar should be 2″ below the string line to allow for that paver to be laid at the height of the string line. Typically we allow for the pavers to be laid approximately 1/16″ to 1/8″ above the string line to allow for minimal settlement following the final compaction.
Start with one end of the screed bar closest to the foundation, set perpendicular to the foundation, and getting it to the right level by either pounding the screed bar into the base material using a dead blow hammer or raising it with a small mound of your bedding material. You can then repeat this for the other end of the screed bar, matching the same level of the string line.
With one screed bar set, we need to then work on leveling out the screed bar across the foundation. The length that you set this one apart from the initial bar depends on how long your level or straight edge is that you will be using to drag back the aggregate. We use an 8 foot level for this, so our second bar is set just under 8 feet from our initial bar. We follow the level of the string line set at the foundation for the one end of this screed bar if possible. For the other end, we can do one of two things. If there is only one slope that our patio follows, meaning the patio is level side-to-side but is following a slope away from the foundation, we can then place our level on the screed bars and level them. Because our initial screed bar was set to the proper slope, leveling the secondary screed bar to the first one will follow that same slope. Alternatively, if your patio has a dual slope, meaning that it slopes both away from the foundation and along the foundation, you then need to set another string line at the foundation that extends beyond the patio at the end and is at the same level of both of these string lines. This should follow along your screed bar that you are trying to set so you can then measure down from the string line and set your screed bar.
Following this, you can then fill the void space between the screed bars with your aggregate while spreading it around with a rake and dragging it back with your straight edge along the screed bars to smooth the surface of this area. Once completed, you can continue this same process both down the patio and across the foundation following your string lines and / or level. Instead of removing both screed bars, you will leave the secondary screed bar in and pull the initial one to then leapfrog over to level and screed.
It works much better to have more than two screed bars when doing this. This would allow you to lay all of your screed bars down the length of the patio for a smaller project to provide you with one smooth surface without breaking it up while also being able to work your way across the foundation. As a screed bar is removed, the bedding material needs to fill the void space left by the screed bar by placing it in and smoothing out the surface.
Depending on the project size, you may opt to screed just a portion of the project and lay it before moving to the next portion. This reduces the likelihood that the screeded area would be walked on by you, someone else, or a pet. If this happens, you will likely need to re-screed or re-level as best you can. Any imperfections in the screed layer will be shown on the final surface. However, with the final compaction of the pavement surface the very minor imperfections will typically level out and correct itself from the vibration of the compactor. Still, this should not be relied upon for anything more than minor imperfections.
Using a rotary laser with a single or dual slope is an alternative solution to setting your screed bars, though the equipment will cost much more than a string line.