The waterproof sheet is fitted over the decking and fixed only at the perimeter. The building may "move", but 
the sheet remains independent and is not subject to any excessive stress. To ensure the sheet is not lifted and
 blown away by the wind, it is covered with graded gravel or concrete slabs.

Planning considerations

The waterproof sheet is fitted over the decking and fixed only at the perimeter. The building may "move", but the
sheet remains independent and is not subject to any excessive stress. To ensure the sheet is not lifted and blown 
away by the wind, it is covered with graded gravel or concrete slabs.

Planning considerations
Large areas, up to 1000 mē, can be covered quickly with Prelasti sheets but this can have implications on other 
aspects of the roof during the works. Consideration should be given to the following items prior to laying the sheet:

Large roof coverings imply large perimeter lengths, which are unlikely to be permanently sealed during the day of
 installation. Can these be adequately sealed against weather on a temporary basis if necessary? This will also 
apply to roof penetrations such as roof-lights and extraction plant.

The large sheet may cover rainwater outlet points. Provision should be made for rainwater drainage from the membrane
 surface during the works. This may involve cutting of release holes in the sheet over outlet points prior to final 
dressing of the outlet.

It is unlikely that all of the required stone ballast will be placed during the day of installation and therefore
 temporary ballasting will be required at random points around the roof to prevent wind uplift during installation 
(e.g. overnight or at week-ends).

The waterproof sheet is fitted over the decking and fixed only at the perimeter. The building may "move", but the
 sheet remains independent and is not subject to any excessive stress. To ensure the sheet is not lifted and blown
 away by the wind, it is covered with graded gravel or concrete slabs.

Typically, 80 kg/mē of stone ballast will be used on a roof and on large roofs this may amount to many tons of stone. 
How will this be transported to roof level and across the roof to its distribution point? The membrane should be protected 
against the effect of barrows, pallet lifters or other wheeled plant and also against impact loads if the stone is tipped
 from a crane bucket.

If stone or other ballast is to be stored, even for short spells, on the roof ensure that the dead weight of the load 
does not exceed the strength of the roof itself. Stone to be stored should be spread around the roof in small piles.

Procedure

If a protective geotextile fleece is to be used as protection beneath the membrane, place this over the entire roof
 area prior to spreading the roof sheet. Polyester and polypropylene fleeces can be spot welded along their laps with 
a hot air gun to help retain them in place during installation. This is necessary to prevent the fleece from being dragged
 out of position during the membrane laying process.

Spread the sheet over the roof area as described above, taking account of any penetrations during the process.

If an upper layer of protective fleece is being used, spread out a roll of material and apply temporary ballasting at random 
points pending application of the permanent layer of ballast. It is not necessary to spot weld the upper fleece or to lay 
the fleece over all the roof area in one operation.

Apply the stone ballast to the required thickness as specified noting that some parts of the roof may require thicker
 layers of covering than others