Your Steel Framed Building Sub-Base Guide

Some guidance on correctly installing your base & foundation, ready to build your Steel Framed Garden Room

Firstly, there are a few options for your sub-base, including: The French PEG System, Plinth Foundations, Ground Screws, or the traditional insulated Concrete Base.

Your method may depend on the type of soil your land has, as well as whether you will be fitting a steel-framed base (sold separately to your frame). 

If you have a preferred & reliable method, providing a level sound base, use this. For the purposes of this guidance though, we will be outlying the traditional insulated concrete base - the time-tested successful method.

Step 1: Consult the dimensions & weight of your to-be-installed Steel Framed Garden Building.

Your base will need to be 150mm wider on all sides. The weight may also affect the depth of support you make up.

You will also need to determine the finished floor height of your base, as this will determine the depth of your excavation. We typically advise having your finished base level 150mm above ground level, as per regulations, & to stop rising moisture (you could always build some patio or decking up to your level after).

Step 2: Now you have your info, use this to plan & mark out the required depth & size of your dig. 

To aid this you can use builders squares and/or string lines to ensure you have a square/rectangular profile, as well as measured Pegs to calculate correct depth.

Note: To aid depth calculation, consider the following required make-up: 

- Your Type 1 MOT base (min 100mm depth)

- Your shallow sharp sand layer, & first layer of damp-proof membrane.

- Your insulation layer (minimum 100mm), & top layer of damp-proof membrane.

- Your top layer of cement (minimum 100mm).

Step 3: Excavate your ground to the required base, ensuring as level & plumb base as possible. 

You will likely be using a digger for this, so ensure PPE & trained personnel only.

Note: It would be worthwhile testing for any underground cables & pipes before excavating.

Step 3: This is an optional step, but aides you considerably.

Using some timber, create an upstand or kerb around the perimeter, reinforced with pegs on the exterior where necessary, which will fully house your sub-base. 

This will stop spillage of your hardcore, sand & cement. Additionally, if you take the time to ensure these kerbs are the same height as your finished level, & perfectly plumb - you will have a great level to finish your cement to, ensuring the ideal base for your steel structure!

Step 4: Lay or pour in your Type 1 MOT sub-base. You will need a minimum depth here of 100mm, increasing the depth as required on larger sizes, or based on particularly softer soils.

It is very worthwhile here, levelling your sub-base as much as possible, as this will make next stages much easier.

Use a whacker plate to thoroughly compress the hardcore, helping to stop future settlement.

Use appropriate PPE.

Step 5: Pour & lay your sharp sand blind.

This is only a shallow layer, & is designed to help cover any sharp edges of your hardcore which could penetrate your damp-proof membrane. 

Additionally, it is easier to level a sharp sand layer, making your base even more plumb. It is important to have this layer plumb, as your rigid insulation will lay on top of this, requiring an even weight distribution (no dipping).

Step 6: Lay your polythene or damp-proof membrane onto your sharp sand, gently encouraging out fold lines, & leaving enough around the edges to lap up your previously made kerbs.

Then lay your rigid insulation (we advise kingspan or celotex). This must be fitted tight, staggering your boards where possible for less joint cross hair movement.

Any gaps can be filled with expanding foam (don't allow to bulge out) & your joints taped.

Neatly fold the excess damp-proof membrane onto the top of your insulation.

Step 7: Lay your top layer of damp-proof membrane onto your insulation, as previously done.

Next, using the necessary mix, typically, sharp sand, ballast, cement & water, pour/lay your concrete slab. This top layer of cement should be made as level & plumb to the best of your ability.

To aid this, if your previously made kerbs & support pegs are perfectly plumb at the required height - you can use a long straight edge to glide & scrape over the width of your base, adding or removing excess cement as you go. 

You can trowel to a neat finish as you go, or allow partial curing and trowel then. Also, once cured enough so no sagging will occur, remove your pegs & curbs and trowel/polish your exposed concrete edges.

The minimum thickness should be 100mm, adjusting as needed depending on structure size.

Step 8: Allow to cure for 48 hours before installing your structure. 

If you have an overnight frost predicted, cover your based with the necessary covering, such as hessien lagging.