The right base for your shed: Irish ground conditions

Why "directly on grass" is not a base

Two reasons people get this wrong: timber-bearer sheds look like they could just sit on grass, and steel sheds with their own metal base look like they don't need a separate one. Neither is true.

• Damp wicks up from grass into the floor, regardless of treatment. The shed floor rots from underneath, you don't see it, and by year 5 it's spongy.
• Ground level is never level, and a shed that sits on uneven ground racks the frame as it settles. Doors stop closing properly inside two seasons.
• Drainage: grass holds water. A shed sitting on grass is a shed sitting in standing water for half the year.

The base does three jobs: keep the shed off the wet ground, keep it level, and let water drain away from the building. Every option below does these jobs to different degrees.

The four base options

Option 1: Gravel + treated timber bearers

The cheapest base that works. Excavate the shed footprint plus a 100 mm overhang, dig down 100–150 mm, lay a permeable membrane, fill with 20 mm crushed gravel, compact in two layers. Place pressure-treated 100 × 50 mm timber bearers across the gravel at 400 mm centres. Shed sits on the bearers.

Why it works: airflow under the floor keeps timber dry, gravel drains, bearers sit on a non-wicking surface. Why it sometimes doesn't: weeds find their way through unless the membrane is good quality, and bearers themselves rot at the gravel contact point eventually.

Materials list (8×6): 0.5 m³ crushed gravel (€60), 5 × 2.4 m treated bearers (€55), permeable membrane (€20), edging pegs (€15). About €150 plus a few hours of labour.

Option 2: Plastic grid base (eco-bearer)

The newer popular option. Interlocking polypropylene grids that you fill with gravel, creating a stable, drained base in roughly an hour. Sheds Direct Ireland, Coola Living and several other Irish suppliers stock these in 8×6 / 10×8 / 10×10 kits.

Why it works: drains better than any other option, very fast to install, no concrete, no skill required. Why it costs slightly more: the kits aren't cheap. For a 10×8 you're typically €150–€220 just for the grid, plus the gravel infill (~€50).

Best on soft ground or sites where drainage is borderline. We'd choose this over gravel + bearers for any garden with heavy clay soil, and most of Ireland has heavy clay soil.

Option 3: Paving slabs on sand

Old-school, still works. Lay a sand bedding layer 50 mm thick over compacted hardcore, set 600 × 600 mm concrete slabs across the footprint, level with a string line and rubber mallet. Shed sits directly on the slabs with a damp-proof course (DPC) strip between slab and floor.

Why it works: dimensionally stable, level, durable. Why it sometimes doesn't: if water pools on top of the slabs, the floor edges still get wet, slabs need to slope very slightly outward (1:60) for drainage. Most amateur slab jobs don't get the slope right.

Worth doing if you've already got reclaimed slabs or your back garden is part-paved already. If you're buying slabs new, a concrete pad is similar money and arguably better.

Option 4: Concrete slab

The proper job. Excavate the footprint plus 50–75 mm overhang on all sides, dig down 200 mm, lay 100 mm of compacted MOT Type 1 hardcore, lay a 1200-gauge polythene damp-proof membrane, place edge formwork, pour 100 mm of C25 concrete, screed level and float-finish.

Costs: about €250–€400 in materials for an 8×6 slab if you DIY. Add €200–€350 if you pay a builder. A day's work for two reasonably competent people.

The 50 mm overhang is the most important detail and the one most amateurs miss: water dripping off the eaves needs to land on the slab and run away from the shed, not track back along the floor edge.

Irish ground conditions: what nobody else mentions

Heavy clay soils

Most of Leinster and east Munster sit on heavy clay. Clay shrinks in summer and swells in winter, which is why a shed on a poorly compacted hardcore layer will heave noticeably across the year. Solution: dig deeper (200 mm hardcore minimum), compact properly, and use a more rigid base type: concrete or full grid kit, not a few bearers on grass.

High water table

If your back garden has standing water in winter, even occasionally, you have a high water table or poor drainage. The fix is not "a thicker shed base". The fix is to install a French drain along the upslope side of the shed footprint, leading water away. €60 of perforated land drain and a half-day with a spade saves the floor of a €1,500 shed.

Frost penetration

Ireland's frost line is shallow (200 mm or less in most counties), so frost-heave isn't the issue it is in continental Europe. But a thin slab on poorly drained ground can crack in a hard winter. C25 concrete with proper hardcore compaction handles this without complication.

Sloped sites

If your garden slopes, level the base by cutting into the slope on the upslope side rather than building up on the downslope side. A built-up edge on the downslope side has to be retained, and that's a separate engineering job most homeowners aren't equipped for.

Common mistakes that ruin a perfectly good shed

1. Base same size as shed. The slab/grid/gravel needs to overhang the shed footprint by 50 mm minimum.
2. No damp-proof course between base and floor. A €5 strip of DPC saves the floor edge.
3. Concrete poured directly on uncompacted soil. Cracks in year one. Always hardcore + DPM first.
4. Skipping the anchor brackets: bolting the shed down to the base is a 30-minute job that prevents storm damage. Don't skip it.
5. Putting the shed door at the lowest point of the slope. Water tracks straight under the door in heavy rain. Plan the door uphill.

Method: how to lay a 100 mm concrete slab for an 8×6 shed

1. Mark out 2.55 × 1.95 m on the ground (8×6 plus 50 mm overhang each side). Drive pegs at the four corners, run string lines, square with the 3-4-5 method.
2. Excavate to 200 mm depth across the marked area.
3. Lay 100 mm compacted MOT Type 1 hardcore. Compact in two layers of 50 mm with a wacker plate or heavy roller: borrowing one is cheap (€40/day from a tool hire).
4. Lay 1200-gauge polythene damp-proof membrane across the hardcore, lapping joints by 150 mm and turning up at the edges of the formwork.
5. Set edge formwork using straight 100 × 50 timber, levelled with a long spirit level.
6. Pour C25 ready-mix concrete (about 0.5 m³ for an 8×6 slab): ordering from a local concrete supplier is much cheaper than mixing bag-by-bag and gives a more durable finish.
7. Screed level with a straight piece of timber, float-finish smooth, cover with polythene for 48 hours to slow-cure.
8. After 7 days, place the shed and bolt down with anchor brackets.

Sources

• Summerhouse24 Ireland: How to lay a concrete base for a shed
• Sheds Direct Ireland: Shed base kits
• Coola Living: UC4 timber base for 8x6 timber garden shed