Your Steel Framed Building Flat Roof Guide

A guide on installing your flat roof to your Steel Framed Garden Building.

Our advice covers the make-up of your flat roof, once your Steel Framed Structure is built, & your exterior is completed (or at least the membrane, battens & timber) - see Exterior Finishes Guide. We also cover the basics on the installation of your Elevate EPDM roof covering, check the dedicated guides here for more detailed info. 

If you are installing your own roof covering (such as felt or fibreglass), use the relavent guidance - though we believe Elevate EPDM is the superior system.

This guidance is designed to conform with Part L of building regulations, including the correct depth & makeup of insulation. If you are not insulating your flat roof, you can skip the necessary steps - yet you must consider other necessary steps to provide a thermal barrier or ventilation to stop cold bridging & condensation build-up.

Step 1: You should first consider the exterior finish once your flat roof is completed (as stated above). 

If you want your finished exterior wall product to carry all the way to the top of your finished roof level, your flat roof should finish flush with your timber battens, not your choice of finish (else you will create a kick).

If you are creating a bespoke soffit overhang, or will install a form of fascia which overlaps your exterior wall finish - your flat roof should finish flush with the exterior finish, or at the required overhang for your soffit finish. 

Step 2: Next you should consider the direction of water run-off from your flat roof. This is typically from front (door) to back, so you gutter system is hidden.

You should installed graded timber or 'firings', creating a fall from your highest desired point, to your desired gutter edge. 2 inch to 0 is typically used.

Your firings should run along each of your structure roof beams, or at a maximum of 600mm centres, secured with appropriate fixings. They should finish at your roof edge based on step 1's advice.

Note: Elevate EPDM is guaranteed against sitting water, so this step is not critical, just advised.

Step 3: Install your first layer of timber boarding, we advise 8x4ft 18mm OSB. Your boards should run along the length of your structure, across the beams/firings, not parallel to them.

Stagger your boards along the length where possible to limit cross hair movement. Use 4 fixings on the width & into each beam your board runs across using appropriate fixings.

Boards should finish at the desired roof edge (timber battens, exterior finish, or desired overhang).

Step 4: Lay your damp-proof membrane onto the OSB, smoothing out any creases. Leave 200mm overhang around all edges.

You can now lay your insulation. 150mm is the regulated depth & we advise rigid 8x4ft boards from either Celotex or Kingspan.

Stagger your boards as before, and keep tight - you can use expanding foam on any gaps (don't overfill). Then tape your joints with foil backed tape.

Ensure you boards don't overhang, as your insulation can slide easier than boards.

Step 5: Fold the polythene or dampproof membrane up and onto the face of the insulation. You can tape in place or trap with your top layer of OSB (next).

Now install your top layer of boarding (again we recommend 18mm OSB). Stagger your boards as before and ensure correct board edge position. A spirit level can help ensure you have achieve a vertical level at your desired roof edge finish.

Keep 4 fixings on the width & 5-6 rows of this evenly spaced along the length of your board.

Step 6: Before you bond your rubber membrane, you should now complete any remaining exterior finishing at this stage, such as cladding to the top, or fascia, consulting Step 1s diagram.

If you were carrying your exterior finish all the way to your roof finish, complete your cladding from your battens all the way to the top (not proud).

If you are overlapping a fascia board, install that now, flush with top of your top decking (not proud - certainly not on your gutter edge).

If you planned and created an overhang and soffit, install any downlights, install soffit & fascia boards at this stage (fascia flush with top layer of boarding).

Step 7: For a better finish on your EPDM, use a planer or sander to gently plane your timber joints & any wood swarf. Then clean/brush down your roof thoroughly.

Lay out your Elevate EPDM membrane to settle for at least 1 hour if not already done so, then follow the steps as seen in Laying your EPDM.

If you have a bespoke, modular Steel Framed Building & have a roof protrusion, such as a skylight or vent, you should consult the following guides for more information: (additionally, ensure your polythene is lapped up any verticals to stop cold bridging).

Note: There are many guides in our EPDM section you may find helpful.

Laying your EPDM around verticals & obstacles such as skylights

Triple Glazed Flat Roof Installation Guides

Step 8: If you are installing any skylights or other verticals such as vents, complete them now using the relevant guides.

Otherwise you can install your roofing edge trims, as seen below. We also advise installing your guttering system after your roofing edge trim has been installed.

EPDM Roofing Edge Trims - Gutter Edge

EPDM Roofing Edge Trims - Kerb Edge

Congratulations, your roof is completed & water-tight for at least 20 years!