Flat Roof Preparation Advice

Some advice on preparing your flat roof to achieve best installation end results.

Your EPDM:

For a smoother finish, unwrap the EPDM prior to installation and lay it out somewhere flat, clean and safe (no foot traffic or sharp items that could damage the EPDM). This helps any creases or folds to settle out, making it easier to lay! Conditions for installation should ideally be above 5 degrees when bonding. 

You could also let your rubber settle in position on your finished deck, keeping it dry in the process.

Bare in mind wind uplift can move the EPDM, so if there are high winds, you may need to weigh down your EPDM with safe, soft items, such as a sand bag.

Your timber decking:

Preferably 18mm OSB, if not, plyboard, with a minimum 1 in 80 fall. Boards laid 90 degrees to your roofing joists and laid staggered where possible, for less crosshair movement. 

Your board widths must also meet together on a joist so edges are supported and can’t dip.

Your decking must be completely dry and the temperature ideally above 5 degrees before fitting your EPDM. If there is a risk of rain before you can install your EPDM, we often use either polythene or the EPDM itself to cover the decking – weighing it down against wind uplift appropriately with safe items (no sharp edges) such as sandbags.

Pro tip - optional extra: Use a planer or sander to gently plane any decking joints or wood swarf for a smoother finish and no sharp areas. Use appropriate PPE!


Use the appropriate length fixings, ideally Turbo Gold (available at Screwfix) or our Wupofast fixings. Typically, 4 or 5x50mm Screws are used when securing your 18mm decking directly into your joists.

We advise 4 fixings on the widths, secured into every joist your board runs across. No proud fixings, sink your screws a few mm into your decking, don’t use nails.

Wall chases:

Where your roof meets brickwork: Use a grinder with a 4-inch diamond mortar disc. Grind a wall chase the length of your decking, approx. 25mm deep, ideally along one of the mortar lines at approximately 150mm (don’t go much lower) above the decking height. 

Use the correct PPE! Dust extraction is advisable.

Thoroughly clean any debris out of the chase and clean the area. Then wash the brickwork and wall chase with plain water. For particularly porous areas you can use SBR to prime the area for better adhesion. Once dry, this area is now ready!

Tiled Roof above your decking:

Note: If your tiled roof is below your flat roof, your roof will need to bond onto this area. See EPDM onto Tiles. You should consider building your roof height up to at least 75mm above the top of the pitched roof here

Where your flat roof meets a tiled roof: Your EPDM will need to lay under the tiles and the pitched roofing felt, ideally onto some layboard. If the tiled roof has a valley, hipped area or gable area, the EPDM will need to be dressed under here also – and recemented upon completion.

Carefully remove the first row of tiles and any batons 300mm above your deck height, then tuck the roofing felt under the above row of tiles. Clean the area. You may need to remove some cement if a valley, hip tile and gable tile meets your roof.

If needed, secure what is known as β€˜layboard’ (11mm OSB or plyboard is suitable) to the rafters, starting where your flat roof decking meets the rafters. The depth of the layboard should be approximately 250mm deep, and run the length of the tiled section.

This area is now ready! The roofing felt, tiles and battens (plus any cementing or other works) are then completed after the EPDM install. 

Fascia and gutter:

No battens required!

When fitting new fascia and gutter: For your EPDM roofing trims to get the best fit, you want your roof edges to have a neat 90-degree box finish. Either:

1) Install your decking first, flush with all roof edges (no overhang), then install your fascia. Your fascia will finish flush with the top of the decking. On gutter edges, it is advisable to finish your fascia a few mm beneath the decking to ensure water run-off.

2) Install your fascia before your decking, flush with the top of your roofing joists (not proud/above). Then install your decking flush with the roof edges (sitting on top of fascia boards). You do not need overhangs or roofing battens with our trims!

We sometimes use a planer to gently plane the roof edges for a smoother finish.

When it comes to your guttering, this is best done once you have installed your EPDM drip edge trims. This way you know the ideal height to put them. Start with your first gutter bracket at the highest point, using a spirit level or string line to have a gentle fall towards the lowest point and your downpipe outlet. Gutter bracket spacings maximum of 1m apart!

More information for the installation of your EPDM can be found within the guides. If you can’t find the info you need there, please feel free to get in touch to see if we can help.