Grading a Level Pad for a Backyard Game Room

A client called us last autumn after his new backyard game room started to lean. It was a 12 by 16 foot prefab shed he had fitted out with a pool table, a television, a small bar and a couch. He had set it on concrete blocks over the lawn, leveled by eye. Within six months one corner had sunk nearly two inches, the door would not close and his pool table, which needs to be level to within a few hundredths of an inch, was unplayable.

Backyard game rooms, man caves and she sheds are one of the fastest growing small projects we see. Almost all the problems we are called in to fix come down to one thing: the pad underneath was not built for the load.

Why a game room is harder than a storage shed

A garden shed holding a lawnmower and some tools can tolerate a bit of movement. A game room cannot. It carries much heavier and more sensitive loads, and people spend hours in it.

A slate pool table weighs 700 to 1,000 pounds, concentrated on four or six legs.
People moving around, sometimes six or eight at once for a big match.
Furniture and appliances: a bar fridge, couches, sometimes a small kitchen.
Doors and windows that jam with even slight settling.

The client's setup was typical. Most evenings he and a few friends play pool with the football on the screen, one of them usually following the scores and his pool picks on ankertoto. A few friends at once, plus a pool table, puts far more weight on the corners than a lawnmower ever will.

Choosing the right pad

There are three common foundations for a backyard building of this size. The right one depends on your soil, your budget and what goes inside.

Pad type	Good for	Rough cost, 12 x 16 ft
Concrete blocks on lawn	Light storage only	Under 200 dollars
Compacted gravel pad with timber frame	Most game rooms and offices on stable soil	800 to 1,500 dollars
Concrete slab	Heavy loads, pool tables, plumbing, poor soil	2,500 to 4,500 dollars

For a game room with a slate pool table, we usually recommend either a well built gravel pad with a strong floor frame or a concrete slab. Blocks on grass are not suitable.

Building a gravel pad properly

A gravel pad is within reach of a capable homeowner with rented equipment. The steps matter more than the materials.

Strip the topsoil across the full footprint plus a foot on each side. Topsoil is full of organic material that rots and settles.
Grade the subsoil so it is level along the length and falls very slightly, about a quarter inch per foot, away from the house or any other structure.
Compact the subsoil with a plate compactor until it no longer shows footprints.
Lay landscape fabric to keep gravel from mixing into the soil.
Add four to six inches of crushed stone in two layers, compacting each one.
Check level across the whole pad with a laser or long straightedge before the building goes on.

The basic cut and fill principles are the same as for any building site, and cut and fill explained for a first time builder explains them in more detail.

The advice I disagree with: gravel is always enough for a shed

Many shed suppliers tell buyers that a gravel pad is all any shed ever needs. For storage sheds on decent soil, that is usually true. For a heavily loaded game room, especially on clay, it often is not.

Clay soils swell when wet and shrink when dry. A gravel pad on clay moves with the seasons, and a pool table on top of it will need releveling every few months. If your soil is heavy clay, or if the pool table is slate, a concrete slab or concrete piers under the load points is worth the extra cost. If you are not sure what soil you have, a simple jar test or a conversation with a local grading contractor will tell you.

Drainage is part of the pad

Most pad failures we see are really drainage failures. Water that collects around or under a pad softens the soil and causes settling. Make sure:

The surrounding ground slopes away from the pad on all sides.
Roof water from the building is directed away with a gutter and downspout.
The pad is not sitting in a natural low spot where water collects after rain.

If the only available spot is low, raise the pad and add a French drain on the uphill side. Why French drains fail within two years covers how to build one that lasts.

We rebuilt the client's pad in two days: moved the shed aside, stripped the soil, compacted a six inch gravel base and added concrete piers under the pool table's position. The shed has not moved since, the door closes properly and the pool table has stayed level through a full winter. More on preparing small building sites is in our Site Grading section.