Summerhouse Bases

A base for a building you actually spend time in

A summerhouse is a room, not a store. IBRAN grids give a firm, level, free-draining base that keeps the floor dry and the building square, and looks like a considered part of the garden rather than a slab.

Why the base matters more than the shed next to it

A summerhouse asks more of its base than any other garden building, because of what happens inside it.

People sit in it for hours. There are rugs, cushions, books, a sofa, sometimes a desk and a laptop, often power and lighting. All of that is unforgiving of damp in a way a lawnmower is not. A shed floor that is slightly damp underneath is a shed. A summerhouse floor that is slightly damp underneath smells musty within a season and ruins soft furnishings within two.

It also gets looked at. A summerhouse is usually placed as a focal point at the end of a garden, so the base is visible from the house all year. A grey concrete apron reads as construction; a clean gravel surround reads as landscaping.

And it carries more weight than people expect. Furniture, occupants, a wood burner, a water butt on the corner, sometimes a small deck. Spread unevenly, over a base that settles unevenly, and the building slowly goes out of true.

A grid base handles all three: confined stone gives a rigid platform that does not settle in patches, the surface stays fully permeable so moisture drains away and air moves under the floor, and gravel finishes as a deliberate part of the garden rather than an intrusion into it.

Where this works well

Summerhouses and garden rooms
Home offices and studios
Log cabins and chalets
Gazebos, pergolas and garden bars
Hot tub and spa surrounds
Decked areas and raised platforms
Approach paths and thresholds
Replacing a failed slab or paved base

At a glance

Sub-base	100-150mm compacted MOT, more for larger cabins or heavy contents
Infill	6-14mm graded blend, angular and clean, filled level with the cell tops
Total dig	Around 140-190mm, depending on grid depth and sub-base
Appearance	Finished in decorative aggregate if wanted, matched to the garden
Drainage	Fully permeable. No standing water against the building, no damp floor
Accessibility	A densely packed graded fill gives a firm approach suitable for wheeled access
Design life	25 years, subject to documented installation
Material	100% recycled polypropylene, made in the UK

Keeping the inside dry

This is what separates a summerhouse that is pleasant in March from one that is only usable in July.

Damp gets into a garden room from underneath far more often than through the roof. Moisture rises from the ground, condenses against a cold floor and has nowhere to escape, and the first sign is usually a smell rather than a stain.

A permeable base breaks that cycle. Water drains down through the stone instead of sitting under the building, and air moves through the void beneath the floor rather than stagnating in it.

Build on bearers so there is a clear ventilated gap, keep the base extending beyond the walls so roof runoff lands on stone, and keep planting cut back from the sides. Where the building is insulated and used year round, a damp proof membrane in the floor build-up is worth discussing with the manufacturer, because the better insulated the room, the more it matters that moisture cannot get in from below.

Making the base part of the garden

This is the section that does not exist for a shed, and it is where a gravel base earns its keep visually.

Take the base out well beyond the building footprint and it stops being a base and becomes a setting: a gravel apron the building sits within, wide enough for a chair outside the door, a table, a couple of pots. It reads as intentional, and it gives you somewhere dry to stand that is not lawn.

Decorative aggregate works perfectly here, so the surface can be matched to existing paths, planting or the house. Local stone usually looks better than imported and costs less to deliver.

Where the summerhouse adjoins decking or a patio, run the grid base up to it and finish flush, so there is no step and no gap collecting leaves. Where it meets lawn, an edging strip gives you a clean line and doubles as the mowing edge.

Which grid depth?

The number on a grid is a decision about how deep the confined stone layer is. Summerhouses are heavier than sheds and are occupied, so err slightly deeper than you would for a store.

IBRAN®-X40 is the sensible default for most summerhouses and garden rooms: enough depth for the building, the furniture and the people in it.
IBRAN®-X30 for smaller structures, gazebos and pergolas, or where finished levels have to match an existing patio or threshold closely.
IBRAN®-X50 for large log cabins, buildings with a wood burner or heavy internal fit-out, or where a hot tub is involved.

Hot tubs deserve their own thought: filled and occupied, they are far heavier than anything else likely to sit in a garden. Check the tub's stated loaded weight and design the sub-base for it specifically.

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How it goes in

Check the manufacturer's base requirements first. Larger cabins often specify a minimum base size, a bearer spacing or a particular level tolerance, and it is easier to build to it than to discover it on delivery day.
Mark out the base generously: at least 100mm beyond the building on every side as a minimum, and considerably more where you want a usable apron.
Clear turf, roots and soft soil to firm ground, and check the ground falls away from the building position.
Excavate to your total depth: sub-base plus grid depth, typically 140-190mm.
Lay a woven geotextile across the dig, lapping joins by at least 300mm and taking it up the sides.
Add the sub-base and compact it in layers. 100-150mm of MOT, compacted properly, is what stops an occupied building settling unevenly over the years.
Blind with a thin layer of grit sand and screed it level. Check with a straight edge and spirit level in both directions, and check the diagonals of the marked area match.
Push-fit the grids together, cutting the perimeter to line with a hand saw.
Fill the cells with a 6-14mm graded blend, level with the cell tops, brushing in so no cell is left hollow.
Set the building on bearers, check square and level, then fix down.

Full step-by-step detail is in our installation guides.

Services and the things you forget

A summerhouse is usually the one garden building that ends up with power, and occasionally water, and both are considerably easier to deal with before the base goes down.

Run a duct under the base while it is open, even if you have no immediate plans for electrics. An empty conduit costs almost nothing at this stage and saves lifting a finished surface later, which is the point at which most people give up and run a cable across the lawn instead.

Think about where the water goes, too. A summerhouse roof concentrates runoff at its eaves or downpipe, and a permeable base handles ordinary rainfall comfortably but not a downpipe discharging in one spot for years. Direct it into a butt, a soakaway, or at least away from the corner of the building.

Any electrical work should be done by a qualified electrician and will usually need to comply with Part P.

Thresholds, steps and access

A summerhouse is entered and left constantly, and the threshold is where a base either works or annoys you daily.

Take the base out further on the door side than anywhere else, so there is a proper standing area rather than a step straight onto grass. If the building has a veranda or decking, run the base underneath and beyond it.

Where the garden room is used year round or by anyone with limited mobility, a densely packed graded fill gives a firm surface that takes a wheelchair or a walking frame, which loose gravel does not. Keep the finished level flush with the threshold rather than leaving a lip, and run the approach path in the same surface so there is no transition to catch a wheel.

Sloping gardens

Summerhouses are often sited at the end of a garden, which is frequently the part that is not flat.

For a gentle slope, dig into the high side rather than building up the low side: material you have added is always less stable than ground you have not disturbed, and an occupied building will find any weakness over time.

On a steeper slope you will need a retained edge, and it is worth getting that right rather than improvising, because it is holding back the ground your building sits on. Where the drop is significant, consider whether a deck platform is a better answer than a very deep cut, and take advice on anything structural.

Get it right first time

The one thing worth knowing: build the base bigger than you think you need.

Almost nobody regrets a generous base. Plenty of people regret a tight one, and it is the single most common thing to wish you had done differently, because it is the one thing that cannot be changed once the building is up and the planting has gone in around it.

An extra half metre of gravel apron costs very little at construction and completely changes how the building is used: a chair outside the door, a table in the sun, somewhere dry to stand in March, room to get round the outside to treat the timber. Stop the base at the walls and you have a building you go inside; extend it and you have a place in the garden.

Ask our engineer

Questions about depths, base sizing, sloping ground or hot tub loading? Ask below, or get in touch directly.

Common Questions

Things that frequently come up when carrying out this project.

Is a gravel grid base suitable for a summerhouse or garden room?

Yes, and it suits an occupied building particularly well. Confined stone gives a rigid platform that does not settle in patches under furniture and people, and the surface stays fully permeable so moisture drains away rather than collecting under the floor.

It also finishes as a considered part of the garden rather than a concrete apron in view of the house.

How do I stop a garden room getting damp?

Damp usually comes from underneath rather than through the roof: moisture rises from the ground, condenses against a cold floor and has nowhere to escape. A permeable base lets water drain down through the stone instead of sitting under the building, and bearers keep a ventilated gap so air moves through.

Extend the base beyond the walls so roof runoff lands on stone, and keep planting cut back from the sides.

How deep should a summerhouse base be?

Around 140mm to 190mm in total: 100mm to 150mm of compacted MOT sub-base plus the grid depth, with more for larger cabins or heavy contents.

Compact the sub-base in layers rather than tipping and raking it, because that is what stops an occupied building settling unevenly over the years.

Which grid depth do I need for a summerhouse?

The 40mm grid is the sensible default for most summerhouses and garden rooms: enough depth for the building, the furniture and the people in it. Use 30mm for smaller structures, gazebos and pergolas, or where levels must match an existing patio.

Use 50mm for large log cabins, buildings with a wood burner or heavy fit-out, or where a hot tub is involved.

How big should the base be?

At least 100mm beyond the building on every side as a minimum, and considerably more where you want a usable apron. An extra half metre of gravel costs very little at construction and completely changes how the building is used: somewhere to put a chair, a table, or just somewhere dry to stand.

It is the most common thing people wish they had done, and it cannot be changed once the building is up.

Can I run electrics to the summerhouse?

Run a duct under the base while it is open, even if you have no immediate plans for power. An empty conduit costs almost nothing at that stage and saves lifting a finished surface later.

Any electrical work should be carried out by a qualified electrician and will usually need to comply with Part P.

Will a gravel base take a hot tub?

It can, but design for it specifically. Filled and occupied, a hot tub is far heavier than anything else likely to sit in a garden, so check the manufacturer's stated loaded weight and build the sub-base and grid depth against that figure rather than against the building.

The 50mm grid with a deeper compacted sub-base is the usual answer.

Is the base wheelchair accessible?

A densely packed graded fill gives a firm surface that takes a wheelchair, walking frame or pushchair, which loose gravel does not.

Keep the finished level flush with the threshold rather than leaving a lip, and run the approach path in the same surface so there is no transition to catch a wheel.

What gravel should I finish it with?

A 6mm to 14mm graded blend, angular and clean, filled level with the cell tops. Decorative aggregate works well here so the surface can be matched to existing paths, planting or the house, and local stone usually looks better than imported and costs less to deliver.

Avoid rounded shingle, which shifts underfoot, and anything with fines, which binds and stops the base draining.

Can I build a summerhouse base on a slope?

Yes. For a gentle slope, dig into the high side rather than building up the low side, since added material is always less stable than undisturbed ground and an occupied building will find any weakness.

On a steeper slope you will need a properly retained edge, and where the drop is significant it is worth considering whether a deck platform is a better answer and taking advice on anything structural.